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Acta Materialia
Journal Prestige (SJR): 3.263
Citation Impact (citeScore): 6
Number of Followers: 308  
  Hybrid Journal Hybrid journal (It can contain Open Access articles)
ISSN (Print) 1359-6454
Published by Elsevier Homepage  [3184 journals]
  • Editors for Acta Materialia
    • Abstract: Publication date: 15 October 2019Source: Acta Materialia, Volume 179Author(s):
  • in situ High Energy X-ray Diffraction Measurement of Strain and
           Dislocation Density ahead of Crack Tips Grown in Hydrogen
    • Abstract: Publication date: Available online 18 September 2019Source: Acta MaterialiaAuthor(s): Matthew Connolly, May Martin, Peter Bradley, Damian Lauria, Andrew Slifka, Robert Amaro, Christopher Looney, Jun-Sang Park The deformation fields near fatigue crack tips grown in hydrogen and in air were measured using high-energy x-ray diffraction. A larger magnitude of elastic strain was observed in the hydrogen case compared to the air case. The magnitude of elastic strain was quantified through an effective crack tip stress intensity factor. The dislocation profile ahead of the crack was probed via x-ray line broadening and electron back-scatter diffraction was used to assess the crack path (intergranular vs. transgranular). Ahead of the crack tip grown in hydrogen, an order of magnitude lower dislocation density, compared to a baseline density far from the crack, was observed. This decrease in dislocation density was not observed in the air case. These differences are discussed in terms of two leading hydrogen embrittlement mechanisms, Hydrogen Enhanced Localized Plasticity (HELP) and Hydrogen Enhanced Decohesion (HEDE). We have observed a decrease in transgranular cohesion (transgranular HEDE), as well as an increase in intergranular fracture. The measurements of dislocation activity support a model of a decrease in intergranular cohesion (intergranular HEDE) which is likely facilitated by the HELP mechanism. This suggests that the increase in fatigue crack growth rate is due to a sum of the two effects of hydrogen, in which the crack grows faster in the transgranular fracture mode and faster due to an increase in a new mode of intergranular fracture.Graphical abstractImage 1
  • Revealing the deformation mechanisms of nanograins in gradient
           nanostructured Cu and CuAl alloys under tension
    • Abstract: Publication date: Available online 16 September 2019Source: Acta MaterialiaAuthor(s): J.J. Wang, N.R. Tao, K. Lu A gradient nanostructured (GNS) surface layer was induced on coarse-grained (CG) Cu and CuAl alloys by means of surface mechanical grinding treatment. The GNS/CG Cu-4.5Al sample subjected to tensile tests yields at a higher strength and fails at a higher uniform elongation (∼42%) in comparison with the GNS/CG Cu and Cu-2.2Al samples. The microstructures of the GNS/CG samples before and after tension at different strains were systematically investigated by transmission electron microscope. It is revealed that grain coarsening dominates the plastic deformation of nanograins in the GNS/CG Cu sample while the propensity of deformation twinning in nanograins increases in the GNS/CG CuAl samples. The experimental results suggested a transition of deformation mechanism of nanograins from grain coarsening to the partial dislocation associated deformation twinning in the GNS/CG Cu and CuAl alloys with increasing Al solute concentration. The obvious activation of deformation twinning accommodates the large tensile plasticity of the surface nanograins in the GNS/CG Cu-4.5Al sample. This work demonstrated that the partial dislocation associated deformation twinning is an effective deformation mechanism to retard the strain localization and to improve the tensile ductility of nanograins.Graphical abstractImage 1
  • Role of surface oxide layers in the hydrogen embrittlement of austenitic
           stainless steels: a TOF-SIMS study
    • Abstract: Publication date: Available online 14 September 2019Source: Acta MaterialiaAuthor(s): Chika Izawa, Stefan Wagner, Martin Deutges, Mauro Martín, Sebastian Weber, Richard Pargeter, Thorsten Michler, Haru-Hisa Uchida, Ryota Gemma, Astrid Pundt Hydrogen environment embrittlement (HEE) of low-nickel austenitic stainless steels (AISI 300 series) with different chemical compositions was studied focusing on the impact of the steels surface oxides, grain sizes and dislocation arrangements. The susceptibility of the steels to HEE is judged with respect to the relative reduction of area (RRA), where the HEE susceptibility is lower for larger RRA values.For many AISI 300 steels a linear trend is observed correlating RRA and the probability of strain induced martensite formation in tensile tests. Some steels, however, depart from the general trend, revealing greater HEE resistances.A careful examination of possible factors influencing HEE of the investigated steels reveals that high RRA values are linked to a specific type of oxide layer, namely the “high constant level oxide”, as categorized by TOF-SIMS evaluation. Thus, this type of oxide layer may be able to lower the steels HEE susceptibility. Other types of surface oxides, grain sizes and dislocation arrangements in the matrix of the particular AISI 300 steels appear to be of secondary importance.Graphical abstractImage 1
  • Effect of solutes on strength and ductility of Mg alloys
    • Abstract: Publication date: Available online 13 September 2019Source: Acta MaterialiaAuthor(s): D.F. Shi, M.T. Pérez-Prado, C.M. Cepeda-Jiménez This work investigates the origin of the simultaneous increase in strength and ductility that takes place in Mg polycrystals alloyed with Al and Zn solutes. With that purpose, twelve polycrystalline binary Mg-Zn and Mg-Al alloys, with up to 2 wt. % of alloying additions and average grain sizes comprised between 3 and 42 μm, were prepared by casting, hot rolling and annealing and were tested at room temperature and quasi-static strain rates. Electron backscattered diffraction-assisted slip trace analysis was then utilized to characterize slip activity, and the latter was related to the grain size, to the texture, and to the topology of the grain boundary network. Basal slip was found to be the dominant deformation mechanism in all the binary alloys, irrespective of composition and grain size. Alloying additions were observed to have little influence on texture development but acted as strong modifiers of the topology of the grain boundary network developed during processing. In particular, they reduced the connectivity of grains that are well oriented for basal slip, preventing intergranular slip localization and, in turn, leading to considerable strengthening of basal slip. Solutes act also as enhancers of diffuse slip within individual grains. It is proposed that the simultaneous increase in strength and ductility of Mg alloys by the addition of solutes must be understood as a multiscale phenomenon resulting from the coupling of solute-dislocation interactions at the atomic scale with alterations of the topology of the grain boundary network at the mesoscale.Graphical abstractImage 1
  • Efficient Use of Multiple Information Sources in Material Design
    • Abstract: Publication date: Available online 13 September 2019Source: Acta MaterialiaAuthor(s): Seyede Fatemeh Ghoreishi, Abhilash Molkeri, Raymundo Arróyave, Douglas Allaire, Ankit Srivastava We present a general framework for the design/optimization of materials that is capable of accounting for multiple information sources available to the materials designer. We demonstrate the framework through the microstructure-based design of multi-phase microstructures. Specifically, we seek to maximize the strength normalized strain-hardening rate of a dual-phase ferritic/martensitic steel through a multi-information source Bayesian optimal design strategy. We assume that we have multiple sources of information with varying degrees of fidelity as well as cost. The available information from all sources is fused through a reification approach and then a sequential experimental design is carried out. The experimental design seeks not only to identify the most promising region in the materials design space relative to the objective at hand, but also to identify the source of information that should be used to query this point in the decision space. The selection criterion for the source used accounts for the discrepancy between the source and the ‘ground truth’ as well as its cost. It is shown that when there is a hard constraint on the budget available to carry out the optimization, accounting for the cost of querying individual sources is essential.Graphical abstractImage 1
  • Atomistic Investigation into Interfacial Effects on the Plastic Response
           and Deformation Mechanisms of the Pearlitic Microstructure
    • Abstract: Publication date: Available online 13 September 2019Source: Acta MaterialiaAuthor(s): Matthew Guziewski, Shawn P. Coleman, Christopher R. Weinberger Atomistic modeling is used to investigate the mechanical response and deformation mechanisms at 5 K temperature within the commonly reported orientation relationships between ferrite and cementite within pearlite: Bagaryatskii, Pitsch-Petch, Isaichev, and their associated near orientations. For each orientation, compressive and tensile simulations were performed in the transverse and longitudinal directions for a range of ferrite to cementite volume ratios. Important mechanical properties such as peak stress, flow stress, and the activated slip systems in both lamella are reported. Significant variation in mechanical response is found between the various orientation relationships. In the transverse direction, the responses are well described by composite theory; longitudinal loading requires further consideration of the strain compatibility of the interface. Plasticity within the ferrite is found to initiate from the interface and is well described by Schmid factors; slip and failure in the cementite are affected by slip transfer mechanisms across the the interface between the lamella. Simulation results are used to create a simple model for predicting deformation behavior in pearlite, allowing for greater understanding of the plasticity and failure mechanisms within the various reported orientations, and raising the possibility of the targeted creation of specific microstructures based on the intended mechanical loading.Graphical abstractImage 1
  • Effect of Ag addition on the precipitation evolution and interfacial
           segregation for Al-Mg-Si alloy
    • Abstract: Publication date: Available online 12 September 2019Source: Acta MaterialiaAuthor(s): Yaoyao Weng, Lipeng Ding, Zezhong Zhang, Zhihong Jia, Boyang Wen, Yingying Liu, Shinji Muraishi, Yanjun Li, Qing Liu The effect of Ag addition on the precipitation evolution and interfacial segregation for Al-Mg-Si alloys was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), atom probe tomography (APT) and density functional theory (DFT) calculation. At the early aging stage, Ag atoms could enter clusters and refine the distribution of these clusters. Then, Ag atoms preferentially segregate at the GP zone/α-Al and β"/α-Al interfaces at the peak aging stage by the replacement of Al atoms in FCC matrix. With prolonging aging time, Ag atoms generally incorporate into the interior of β" precipitate, facilitating the formation of QP lattice (a hexagonal network of Si atomic columns) and the local symmetry substructures, Ag sub-unit (1) and Ag sub-unit (2). At the over-aged stage, the Ag sub-unit (1) and Ag sub-unit (2) could transform to the β′Ag (i.e. β′Ag1 and β′Ag2.) and Q′Ag unit cells, respectively. All the precipitates at the over-aging stage have a composite and disordered structure due to the coexistence of different unit cells (β′Ag1, β′Ag2, Q′Ag and β′) and the non-periodic arrangement of Ag atoms within the precipitate. In the equilibrium stage, the incorporated Ag atoms in the precipitates release into the α-Al matrix as solute atoms or form Ag particles. In general, Ag atoms undergo a process of “segregate at the precipitate/matrix interface → incorporate into the interior of precipitate → release into the α-Al matrix” during the precipitation for Al-Mg-Si-Ag alloys. Besides, Ag segregation is found at the interfaces of almost all metastable phases (including GP zone, β″, β′/β′Ag phase) in Al-Mg-Si-Ag alloys. The Ag segregation at the β′/α-Al interface could increase the length/diameter ratio of β′ phase and thus promote the additional strengthening potential of these alloys. These findings provide a new route for precipitation hardening by promoting the nucleation and morphology evolution of precipitates.Graphical abstractImage 1
  • Swamps of Hydrogen in Equiatomic FeCuCrMnMo Alloys: First-principles
    • Abstract: Publication date: Available online 12 September 2019Source: Acta MaterialiaAuthor(s): X.L. Ren, P.H. Shi, W.W. Zhang, X.Y. Wu, Q. Xu, Y.X. Wang High-entropy alloys (HEAs) merit promising applications in nuclear reactors. A FeCuCrMnMo HEA with low activation under neutron irradiation was studied, which was first focused on the search of an equilibrium state by a hybrid method combining the Metropolis Monte Carlo method and density functional theory (DFT). As a transmutation byproduct in fission reactions or fuel for fusion reactions, the evolution of hydrogen, in the HEA was then investigated through a systematic analysis of H solution and diffusion using DFT and molecular dynamics simulations. Rooted in the unique distortion of HEA lattices, i.e., the destroyed translational symmetry of the energy landscape, tetrahedral and octahedral interstitial positions have no significant difference in the priority of H residing. Diffusion of H, as a guest atom, also presents a sluggish effect. The dramatic increases and decreases in potential energy generate a great number of insurmountable barriers pervading the matrix and largely suppressing the mobility of H. However, this effect originates not only from the difference between the potential energies of interstitial positions as observed with host atoms, which increases the fluctuation of migration barriers and decreases the effective atomic jump frequency, but also from the destabilization of interstitial positions for H residing. This blocks the diffusion channels of H and further decreases the atomic jump probability in Einstein’s equation. The present investigation provides fundamental insight into H behavior in HEAs and clues for the application of HEAs as materials of tritium permeation barriers or resistance to hydrogen irradiation.Graphical abstractImage 1
  • Recoverability of large strains and deformation twinning in martensite
           during tensile deformation of NiTi shape memory alloy polycrystals
    • Abstract: Publication date: Available online 11 September 2019Source: Acta MaterialiaAuthor(s): Yuchen Chen, Orsolya Molnárová, Ondřej Tyc, Lukáš Kadeřávek, Luděk Heller, Petr Šittner Superelastic NiTi wires were deformed in tension up to gradually increasing total strains at various temperatures, recoverable strains were evaluated and lattice defects left in the microstructure of deformed wires were analyzed by TEM. The recoverable strains evaluated in tensile tests: i) are surprisingly large - exceed 10 % strain at low temperatures T ≤ 20 °C, ii) are not significantly restricted by large plastic deformation up to 55% at low temperatures, iii) display local maxima in dependence on total strain in tensile tests at low temperatures, iv) decrease with increasing test temperatures and iv) increase with increasing total strain in tensile tests at high temperatures T ≥ 50 °C. Besides slip dislocations, key lattice defects created by the tensile deformation beyond the martensite yield point are deformation bands containing {114} austenite twins or R-phase. No other austenite twins were found statistically relevant.It is concluded that the plastic deformation of NiTi wires is initiated by deformation twinning in oriented martensite, accompanied by dislocation slip in austenite and/or martensite in extent depending on the test temperature and total strain. The deformation twinning is found to play an ambiguous role in NiTi deformation. When it proceeds at low temperatures (T ≤ 20 °C) beyond the yield point, it raises recoverable strain up to 13 %, while at high temperatures (T ≥ 50 °C) when it proceeds already within the plateau range as intermediate step of the stress induced B2=>B19’=>B2T martensitic transformation, it restricts the recoverability of transformation strains. The repeated deformation twinning in oriented martensite enables low temperature deformation of NiTi wires with specific microstructure up to ∼50 % strain and causes severe microstructure refinement during the mechanical and thermomechanical processing of NiTi.Graphical abstractImage 1
  • The Effects of Ultra-Fine-Grained Structure and Cryogenic Temperature on
           Adiabatic Shear Localization in Titanium
    • Abstract: Publication date: Available online 11 September 2019Source: Acta MaterialiaAuthor(s): Zezhou Li, Shiteng Zhao, Bingfeng Wang, Shuang Cui, Renkun Chen, Ruslan Z. Valiev, Marc A. Meyers The deformation at low temperatures (173 K and 77 K) in ultrafine-grained (100 and 500 nm) titanium is investigated and its effect on adiabatic shear localization is established. In comparison with coarse-grained titanium, the strength of ultrafine-grained titanium is higher due to the classic Hall-Petch effect while the strain hardening approaches zero. Our results show that shear localization in dynamic deformation is also altered. The width of the shear band of coarse-grained titanium decreases from 30 to 18 μm (by 40%) with decreasing the initial deformation temperature to 77 K. In contrast, for 100 nm titanium, the width of shear band decreases more significantly, from 4 μm at room temperature to 1 μm (a 75% decrease) at 77 K. This difference is attributed to the combined effects of the decrease in the thermal conductivity and specific heat capacity, and the increase in thermal softening rate. These changes in the width are consistent with the predictions of the Grady and Bai-Dodd theories. Ultrafine- and nano-recrystallized grains are observed inside the bands which are dependent on initial grain size and initial deformation temperature. The dislocation evolution is evaluated for the different conditions using a Kocks-Mecking formulation; the rotational dynamic recrystallization mechanism responsible for forming the ultrafine/nanosized grains (40 to 200 nm) is successfully modeled incorporating the differences in initial temperature and grain size. Our results and analysis are important in enhancing the understanding of the structural evolution processes under high strain-rates and cryogenic temperatures.Graphical abstractImage 1
  • Resistance to amorphisation in Ca1-xLa2x/3TiO3 perovskites – a bulk
           ion-irradiation study
    • Abstract: Publication date: Available online 10 September 2019Source: Acta MaterialiaAuthor(s): Sebastian M. Lawson, Neil C. Hyatt, Karl R. Whittle, Amy S. Gandy The changes induced from 1 MeV Kr+ and 5 MeV Au+ ion irradiation at room temperature have been utilised to determine the impact of cation vacancies on the radiation damage response on bulk Ca1-xLa2x/3TiO3 perovskite structured ceramics. Perovskite systems have long been considered as candidate waste forms for the disposition of actinide wastes and doping with multi-valent elements such as Pu may lead to cation deficiency. Based on GAXRD and TEM analysis, two regions of resistance/susceptibility to amorphisation have been confirmed with reference to CaTiO3. Increased resistance to amorphisation has been observed for 0.1 ≤ x ≤ 0.4, with an increased susceptibility to amorphisation for x ≥ 0.5. It is proposed that these processes are induced by enhanced recovery from radiation damage for 0.1 ≤ x ≤ 0.4, and reduced tolerance for disorder/the increasingly covalent nature of the A-O bond for x ≥ 0.5. Lattice parameter analysis of the x = 0 and 0.5 samples showed a saturation in radiation damage induced volume swelling at 4.7 ± 0.1 % and 1.8 ± 0.1 %, respectively, while the saturation limit for the b parameter was lower than the respective a and c orthorhombic parameters. In the x = 0.2 and 0.4 samples, amorphisation was not observed, however the b parameter was found to swell to a lesser extent than the a and c parameters. Swelling was not observed for the ion irradiated x ≥ 0.6 samples.Graphical abstractImage 1
  • Rearrangement of interstitial defects in alpha-Fe under extreme condition
    • Abstract: Publication date: Available online 9 September 2019Source: Acta MaterialiaAuthor(s): A. Chartier, M.-C. Marinica In this study, by theoretical means, we reveal the main mechanisms that underpin the microstructure evolution driven by the formation of self-interstitial atoms (SIAs) clusters in body centered cubic iron under extreme conditions. Using Frenkel pairs accumulation simulations we point the complex interplay between the two families of interstitial defects, the dislocation loops with Burgers vectors and ½ and the tridimensional C15 clusters. We reconcile the previous sparse understanding of microstructure evolution that put in opposition various mechanisms of defects formation by showing that both ½ loops self-interactions and C15 clusters transformations produce loops. Moreover, we exhibit the fact that these tri-dimensional clusters can form under irradiations with only the Frenkel pair accumulation that mimics electron irradiation and not only in high-energy cascades as it was previously stated. Finally, we show that the tridimensional C15 clusters even precede production of loops under irradiation.Graphical abstractImage 1
  • Corrigendum to Multiscale investigations of nanoprecipitate nucleation,
           growth, and coarsening in annealed low-Cr oxide dispersion strengthened
           FeCrAl powder [Acta Mater. 166 (2019) 1–17]
    • Abstract: Publication date: November 2019Source: Acta Materialia, Volume 180Author(s): Caleb P. Massey, Sebastien N. Dryepondt, Philip D. Edmondson, Matthew G. Frith, Kenneth C. Littrell, Anoop Kini, Baptiste Gault, Kurt A. Terrani, Steven J. Zinkle
  • Fracture behavior and deformation mechanisms in nanolaminated
           crystalline/amorphous micro-cantilevers
    • Abstract: Publication date: Available online 7 September 2019Source: Acta MaterialiaAuthor(s): Y.Q. Wang, R. Fritz, D. Kiener, J.Y. Zhang, G. Liu, O. Kolednik, R. Pippan, J. Sun In order to quantify the fracture toughness and reveal the failure mechanism of crystalline/amorphous nanolaminates (C/ANLs), in-situ micro-cantilever bending tests were performed on Ag/Cu-Zr and Mo/Cu-Zr C/ANLs in a scanning electron microscope over a wide range of cantilever widths from several microns to the submicron scale. The results demonstrate that the fracture behavior was strongly influenced by sample size and constituent phases, respectively. The Ag/Cu-Zr micro-cantilevers failed in a ductile manner, with fracture toughnesses higher than the Mo/Cu-Zr samples that exhibited brittle failure. Both materials also displayed different cantilever width-dependences of fracture toughness. The Ag/Cu-Zr beams showed a fracture toughness that increases with the cantilever width, mainly due to a size-dependent constraining effect on the deformation of the crystalline phase. For the Mo/Cu-Zr beams, the fracture toughness decreased gradually to a low plateau as the cantilever width exceeded ∼1500 nm, which can be rationalized by a transition in stress condition. The underlying fracture mechanism of the Ag/Cu-Zr micro-cantilevers was identified as the interconnection of microcracks initiated in the amorphous Cu-Zr layers, compared to a catastrophically penetrating crack propagation in the Mo/Cu-Zr samples. The discrepancy in size-dependent fracture behavior between the two material systems is discussed in terms of plastic energy dissipation of ductile phases, crack tip blunting, crack bridging and the effect of strain gradient in the plastic zone on crack propagation.Graphical abstractImage 1
  • Hydrogen pickup during oxidation in aqueous environments: the role of
           nano-pores and nano-pipes in zirconium oxide films
    • Abstract: Publication date: Available online 7 September 2019Source: Acta MaterialiaAuthor(s): Jing Hu, Junliang Liu, Sergio Lozano-Perez, Chris Grovenor, Mikael Christensen, Walter Wolf, Erich Wimmer, Erik V. MaderABSTRACTOxidation of metals by water generates hydrogen which can enter the solid causing serious degradation of its mechanical properties and may also influence the corrosion rate. The present work focuses on hydrogen pickup during the corrosion of zirconium alloys in an aqueous environment. Transmission electron microscopy using Fresnel imaging on three different samples of oxidized Zr has been used to study the type, distribution, concentration and connectivity of nano-porosity as a function of depth through the oxide layer. Extensive interconnected nano-pipes are found in the non-protective outer part of the oxide, while in the protective barrier layer closer to the metal-oxide interface, continuous nano-pipes turn into individual nano-pores. Ab initio calculations show that molecular hydrogen is formed spontaneously by the reaction of water with oxygen vacancies in zirconium oxide. Molecular dynamics simulations reveal that these H2 molecules can diffuse rapidly through nano-pores and nano-pipes as small as 0.5 nm in the oxide layer. Calculations demonstrate that molecular hydrogen dissociates spontaneously on surfaces of suboxides found experimentally at the metal-oxide interface. Oxygen vacancies in ZrO enable the ingress and diffusion of H atoms with an energy barrier of approximately 65 kJ/mol. Further diffusion of hydrogen through oxygen-saturated α-Zr metal is fast, leading to the formation of thermodynamically stable zirconium hydrides. Thus, formation and diffusion of molecular hydrogen through nano-pores in the bulk oxide and ingress of H atoms via suboxides is a possible mechanism of hydrogen pickup in any metal or alloy covered by an oxide scale that contains nano-porosity.Graphical abstractImage 1
  • Critical role of atomic-scale defect disorders for high-performance
           nanostructured half-Heusler thermoelectric alloys and their thermal
    • Abstract: Publication date: Available online 7 September 2019Source: Acta MaterialiaAuthor(s): Ho Jae Lee, Kyu Hyoung Lee, Liangwei Fu, GyeongTak Han, Hyun-Sik Kim, Sang-Il Kim, Young-Min Kim, Sung Wng Kim Atomic-scale defects are essential for improving thermoelectric (TE) performance of most state-of-the-art materials by simultaneously tuning the electronic and thermal properties. However, because the plural atomic-scale defects are generally inherent and disordered in nanostructured TE materials, their complexity and ambiguity on determining TE performance remain a challenge to be solved. Furthermore, the thermal stability of atomic-scale defects in nanostructured TE materials has not been studied much so far. Herein, we report that the atomic-scale defect disorders are indispensable for high TE performance of nanostructured Ti1–xHfxNiSn1–ySby half-Heusler alloys, but gradually degraded at over 773 K, deteriorating the TE performance. It is found from the thermal annealing of nanostructured Ti0.5Hf0.5NiSn0.98Sb0.02 alloys that the annihilation of Ti,Hf/Sn antisite defects primarily reduces atomic-scale defect disorders and largely contributes to the increase of lattice thermal conductivity. Moreover, it is verified that the Ni interstitial defects mainly dominate the electronic transport properties, leading to the enhancement of power factor. Direct atomic structure observations clearly demonstrate the inherent Ni interstitial defects and the thermal vulnerability of Ti,Hf/Sn antisite defects. These results provide an important guide for the application of half-Heusler alloys with highly disordered atomic-scale defects.Graphical abstractImage 1
  • Local Structural Investigation of Hafnia-Zirconia Polymorphs in Powders
           and Thin Films by X-ray Absorption Spectroscopy
    • Abstract: Publication date: Available online 6 September 2019Source: Acta MaterialiaAuthor(s): Tony Schenk, Andris Anspoks, Inga Jonane, Reinis Ignatans, Brienne S. Johnson, Jacob L. Jones, Massimo Tallarida, Carlo Marini, Laura Simonelli, Philipp Hönicke, Claudia Richter, Thomas Mikolajick, Uwe Schroeder Despite increasing attention for the recently found ferro- and antiferroelectric properties, the polymorphism in hafnia- and zirconia-based thin films is still not sufficiently understood. In the present work, we show that it is important to have a good quality X-ray absorption spectrum to go beyond an analysis of the only the first coordination shell. Equally important is to analyze both EXAFS and XANES spectra in combination with theoretical modelling to distinguish the relevant phases even in bulk materials and to separate structural from chemical effects. As a first step toward the analysis of thin films, we start with the analysis of bulk references. After that, we successfully demonstrate an approach that allows us to extract high-quality spectra also for 20 nm thin films. Our analysis extends to the second coordination shell and includes effects created by chemical substitution of Hf with Zr to unambiguously discriminate the different polymorphs. The trends derived from X-ray absorption spectroscopy agree well with X-ray diffraction measurements. In this work we clearly identify a gradual transformation from monoclinic to tetragonal phase as the Zr content of the films increases. We separated structural effects from effects created by chemical disorder when ration of Hf:Zr is varied and found differences for the incorporation of the substitute atoms between powders and thin films, which we attribute to the different fabrication routes. This work opens the door for further in-depth structural studies to shine light into the chemistry and physics of these novel ferroelectric thin films that show high application relevance.Graphical abstractImage 1
  • Size Effects in the Martensitic Transformation Hysteresis in Ni-Mn-Sn
           Heusler Alloy Films
    • Abstract: Publication date: Available online 5 September 2019Source: Acta MaterialiaAuthor(s): Yijia Zhang, Julia Billman, Patrick J. Shamberger Understanding the effect of small characteristic length scales on phase transformations requires microscopic observations to identify mechanisms which may influence the progression of the transformation. Here, we report thickness-dependent hysteresis in electrochemically deposited Ni-Mn-Sn Heusler alloy films, as observed by optical microscopy. This approach allows for analyzing size dependent phase transformation behavior within individual grains from films with decreasing thickness. Hysteresis is not correlated with grain size, but increases with decreasing film thickness following a power law relationship. This behavior is attributable to internal friction-induced energy dissipation at the film/substrate interface in microscale alloy films.Graphical abstractImage 1
  • In situ study on fracture behaviour of white etching layers
           formed on rails
    • Abstract: Publication date: Available online 4 September 2019Source: Acta MaterialiaAuthor(s): A. Kumar, A.K. Saxena, C. Kirchlechner, M. Herbig, S. Brinkmann, R.H. Petrov, J. Sietsma Failure in engineering materials like steels is strongly affected by in-service deleterious alterations in their microstructure. White Etching Layers (WELs) are an example of such in-service alterations in the pearlitic microstructure at the rail surface. Cracks initiate in the rails due to delamination and fracture of these layers and propagate into the base material posing severe safety concerns. In this study, we investigate the microscale fracture behaviour of these WELs. We use in situ elastic-plastic fracture mechanics using J-integral to quantify the fracture toughness. Although usually assumed brittle, the fracture toughness of 21 – 25 MPa√m reveals a semi-brittle nature of WELs. Based on a comparison of the fracture toughness and critical defect size of WELs with the undeformed pearlitic steels, WELs are detrimental for rails. In the micro fracture tests, WELs show crack tip blunting, branching, and significant plasticity during crack growth due to their complex microstructure. The fracture behaviour of the WELs is governed by their microstructural constituents such as phases (martensite/austenite), grain size, dislocation density and carbon segregation to dislocations and grain boundaries. We observed dislocation annihilation in some martensitic grains in the WELs which also contributes to their fracture behaviour. Additionally, the strain-induced transformation from austenite to martensite affects the crack growth and fracture.Graphical abstractImage 1
  • Effect of hardening on toughness captured by stress-based damage
           nucleation in 6061 aluminum alloy
    • Abstract: Publication date: Available online 3 September 2019Source: Acta MaterialiaAuthor(s): Tom Petit, Jacques Besson, Claire Ritter, Kimberly Colas, Lukas Helfen, Thilo F. Morgeneyer A deterioration of fracture toughness, especially of the tearing modulus, with aging time and associated strength increase is observed for aluminum 6061 and reproduced here numerically thanks to a stress-based damage nucleation criterion.A correlative multiscale analysis by scanning electron microscopy, atom probe tomography as well as 3D X-ray laminography shows that coarse particles and the characteristic damage mechanisms do not depend on aging time: the fracture mechanism is typically ductile and transgranular as shown by electron backscatter diffraction analysis of sections of compact tension specimens containing interrupted cracks. Large Mg2Si inclusions fracture at very low plastic strain, and defects nucleate at large (Fe,Si)-rich inclusions with increasing plastic deformation. Only the hardening nanoprecipitation increases with aging time: aging favors the precipitation of nano-size Mg2Si precipitates which causes hardening of the matrix so that damage nucleation at coarse inclusions becomes easier - thus leading to a decrease in toughness. Indeed, larger clusters and a substantially higher area fraction of iron based intermetallic particles are found on the fracture surfaces of the longest aging time CT samples compared to the shortest aging time samples.Based on these observations, a Gurson-Tvergaard Needleman type model is proposed to simulate the tearing tests using Finite Elements. It uses damage nucleation kinetics which depend on the maximum principal stress, since a classical strain-based nucleation is not sufficient to reproduce the deterioration of the tearing modulus.Graphical abstractImage 1
  • Grain Refinement Mechanism of Nickel-Based Superalloy by Severe Plastic
           Deformation - Mechanical Machining Case
    • Abstract: Publication date: Available online 3 September 2019Source: Acta MaterialiaAuthor(s): Zhirong Liao, Mikhail Polyakov, Oriol Gavalda Diaz, Dragos Axinte, Gaurav Mohanty, Xavier Maeder, Johann Michler, Mark Hardy This paper studied the formation mechanism of white layer of a next generation nickel-based superalloy formed under severe plastic deformation induced by a mechanical material removal process. A graded microstructure of the white layer in the nickel-based superalloy has been revealed for the first time, which is composed of (i) a “dynamic recrystallisation” layer formed by nanocrystalline (∼200 nm) grains at the vicinity of the surface and (ii) a “dynamic recovery” layer with subgrain microstructures extending further into the subsurface. The mechanism of surface grain refinement was identified based on the results obtained via crystallographic and chemical analysis, as well as in-situ micro-mechanics experiments in the scanning electron microscope. It is found that in the top surface layer not only grain refinement but also the γ’ phase dissolution occurs, changing drastically from the bulk material. Furthermore, it is shown how the high plastic strain and cutting temperature along the subsurface causes grain refinement in the white layer and grain elongation in the subsurface. The γ’ precipitates in the recrystallisation layer are dissolved during the machining process, while the ultra-high cooling rate suppresses the further precipitation of this phase, resulting in the supersaturation of γ grains or minimized γ’ precipitates in the top surface layer. Hence, the grain refinement does not result in an increase of mechanical stiffness but a deterioration of mechanical properties due to the dissolution of the strengthening phase γ’, which leads to a lower strength and increased ductility. Machining is generally treated as a cold-working process. However, according to our findings hot-working with dynamic recrystallisation and recovery, as well as phase evolution, occurs in the white layer of nickel-based superalloys.Graphical abstractImage 1
  • Rapid solidification of Nd1+XFe11Ti compounds: phase formation and
           magnetic properties
    • Abstract: Publication date: Available online 31 August 2019Source: Acta MaterialiaAuthor(s): F. Maccari, L. Schäfer, I. Radulov, L.V.B. Diop, S. Ener, E. Bruder, K. Skokov, O. Gutfleisch The effects of compositional variations and different annealing regimes in Nd(Fe,Ti)12 alloys were studied in terms of phase formation and magnetic properties analysis. NdxFe11Ti (x=1.05, 1.10, 1.15, 1.20) alloys were produced by rapid solidification through suction casting technique. The effect of Nd content and post annealing were investigated in the temperature range of 700-1200°C. Single 1:12 phase samples were obtained at temperatures between 1150-1200°C for compositions with Nd concentration of 1.15 and 1.20. Intrinsic magnetic properties and magnetization reversal were studied for 1:12 single phase samples, revealing uniaxial anisotropy with anisotropy field (HA) of 1.08T and saturation magnetization of 137Am2kg-1 at room temperature. In addition, the demagnetization mechanism in bulk polycrystalline samples was analyzed by means of Kerr microscopy under applied magnetic fields. Magnetization reversal process starts at the twin boundary, which acts as a nucleation center for the reversal domain, and coupling between adjacent grains is also observed. These may be part of the reasons for the observed low coercivity in the NdFe11Ti systems. The findings of the present study leads to a better understanding of the relation between magnetic properties and microstructure, and can open new strategies to obtain coercivity in this 1:12 phase system and, possibly, in the corresponding nitride.Graphical abstractImage 1
  • Experimental and computational analysis of binary Fe-Sn ferromagnetic
    • Abstract: Publication date: Available online 30 August 2019Source: Acta MaterialiaAuthor(s): Bahar Fayyazi, Konstantin P. Skokov, Tom Faske, Ingo Opahle, Michael Duerrschnabel, Tim Helbig, Ivan Soldatov, Urban Rohrmann, Leopoldo Molina-Luna, Konrad Güth, Hongbin Zhang, Wolfgang Donner, Rudolf Schäfer, Oliver Gutfleisch Ferromagnetic Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were synthesized using the reactive flux technique. Derived from single crystal x-ray diffraction and Transmission Electron Microscopy (TEM), a new structural model is proposed for the Fe5Sn3 crystals - the threefold twinning of an orthorhombic unit cell with (3+1) dimensional space group Pbcm(α00)0s0. The spontaneous magnetization (Ms) and the anisotropy constants K1 and K2 of Fe3Sn, Fe5Sn3 and Fe3Sn2 single crystals were determined in a wide temperature range using M(H) dependencies and a modified Sucksmith-Thompson technique. Ms and K1 were also evaluated in the framework of Density Functional Theory (DFT) and an overall good agreement was observed between the calculated and experimental results. Furthermore, a critical evaluation of different analytical models for the assessment of magnetocrystalline anisotropy was performed, which are restricted to the analysis of uniaxial magnetic domain patterns, and it is shown that such high-throughput techniques can lead to unrealistic results. Finally, a DFT high-throughput screening of the Fe-Sn phase diagram was used to identify Fe-Sn based phases with potential to be stabilized upon alloying, and their magnetization and magnetocrystalline anisotropy were evaluated. The results show that a similar strong anisotropy as observed in Fe3Sn may also be found in other Fe-Sn based phases, having higher potential to be used as hard magnetic material.Graphical abstractImage 1
  • Shear deformation determined by short-range configuration of atoms in
           topologically close-packed crystal
    • Abstract: Publication date: Available online 30 August 2019Source: Acta MaterialiaAuthor(s): Yongchao Zhang, Kui Du, Wei Zhang, Beining Du, Dongqing Qi, Wenqing Li, Miao Song, Liyuan Sheng, Hengqiang Ye Dislocation behaviors, which determine mechanical properties of materials, are generally believed to be controlled by long-range lattice translational order. For intermetallic compounds, however, their structures are often closely related to atomic environments, with the short-range configurations possibly deviating from the long-range translational order. Thus, how dislocations move in these crystals is obscure. Here, we resolve the shear deformation, dominated by atomic-environment polyhedra, in a representative topologically close-packed phase with (Cr,Ni,Al)2Nb stoichiometry, using aberration-corrected scanning transmission electron microscopy combining with atomic-resolution energy dispersive X-ray spectroscopy. Dislocations there have Burgers vectors deviating from slip planes and surprisingly move by switching between two different slip planes. Long-range diffusion and local composition variation assist the dislocation motion, as demonstrated by high temperature quasi-static and room temperature impact deformations. These discoveries demonstrate the defining role of short-range configuration in the deformation of complex structured intermetallics and shed light on the novel behavior of dislocations.Graphical abstractImage 1
  • Structural Perspective on Revealing Energy Storage Behaviors of Silver
           Vanadates Cathode in Aqueous Zinc-Ion Batteries
    • Abstract: Publication date: Available online 30 August 2019Source: Acta MaterialiaAuthor(s): Shan Guo, Guozhao Fang, Shuquan Liang, Minghui Chen, Xianwen Wu, Jiang ZhouABSTRACTExploitation and improvement of electrode materials mainly rely on the understanding of electrochemical reaction mechanisms. Here we provide a comprehensive perspective of zinc ions storage behaviors in silver vanadates (e.g. Ag0.33V2O5, Ag1.2V3O8, Ag2V4O11, β-AgVO3, Ag4V2O7), which exhibit electrochemical redox multi-mechanisms. Ag0.33V2O5 with stable tunnel structure and low mole ratio of Ag/V demonstrates a combination of reversible displacement/intercalation reaction with good cyclic stability. Ag1.2V3O8 and Ag2V4O11 with layer structure and higher mole ratio of Ag/V show a reversible insertion/extraction reaction accomplished by an irreversible displacement reaction to form a highly conductive Ag0 matrix, leading to the high rate performance. The chain-like β-AgVO3 and isolated island-like Ag4V2O7 with unstable structure and the highest mole ratio of Ag/V reveal irreversible phase transition mechanism to form the amorphous matrix. The crystal structure is the decisive factor in the basic electrochemical properties, providing a new insight into battery storage mechanism.Graphical abstractWe provide a comprehensive perspective of zinc ions storage behaviors in bimetallic cathode materials (e.g. Ag0.33V2O5, Ag1.2V3O8, β-AgVO3, Ag2V4O11, Ag4V2O7), which exhibit electrochemical redox multi-mechanisms. This work provides a new structural insight into energy storage mechanism in aqueous zinc-ions battery system.Image 1
  • Mechanism of the α-Zr to hexagonal-ZrO transformation and its impact on
           the corrosion performance of nuclear Zr Alloys
    • Abstract: Publication date: Available online 29 August 2019Source: Acta MaterialiaAuthor(s): Junliang Liu, Hongbing Yu, Phani Karamched, Jing Hu, Guanze He, Daniel Goran, Gareth M. Hughes, Angus J. Wilkinson, Sergio Lozano-Perez, Chris R.M. Grovenor Displacive transformations have been widely reported in metals, alloys and ceramics, but rarely reported to be important in the aqueous corrosion of alloys. We report here our analysis of the formation of the hexagonal-ZrO suboxide during the aqueous corrosion of α-Zr alloys and propose this to be a paraequilibrium displacive transformation with the rate controlled by oxygen diffusion. Two orientation relationships were identified between α-Zr and hexagonal-ZrO, (0002)α−Zr∥(1¯011)h−ZrO and [2¯110]α−Zr∥[101¯2]h−ZrO or (0002)α−Zr∥(224¯1¯)h−ZrO and [2¯110]α−Zr∥[11¯01]h−ZrO, with the first one more commonly observed. No specific orientation relationships between either hexagonal-ZrO and monoclinic-ZrO2 or α-Zr and monoclinic-ZrO2 were identified, which suggests that the formation of often-reported bulk oxide texture during aqueous corrosion is not related directly to the texture of the metallic substrate. These results provide a guideline for understanding the mechanisms of crystallographic evolution during oxide growth on commercial zirconium alloys, and also demonstrate the capability of transmission Kikuchi diffraction to investigate orientation relationships in nano-scale materials.Graphical abstractImage 1
  • a +basal+screw+dislocations+in+hexagonal+titanium+alloys&rft.title=Acta+Materialia&rft.issn=1359-6454&">Influence of simple metals on the stability of 〈 a 〉 basal screw
           dislocations in hexagonal titanium alloys
    • Abstract: Publication date: Available online 29 August 2019Source: Acta MaterialiaAuthor(s): Piotr Kwasniak, Emmanuel Clouet Basal slip acts as a secondary deformation mode in hexagonal close-packed titanium and becomes one of the primary mechanisms in titanium alloyed with simple metals. As these solute elements also lead to a pronounced reduction of the energy of the basal stacking fault, one can hypothesize that they promote basal dissociation of dislocations which can then easily glide in the basal planes. Here, we verify the validity of this hypothesis using ab initio calculations to model the interaction of a screw dislocation with indium (In) and tin (Sn). These calculations confirm that these simple metals are attracted by the stacking fault existing in the dislocation core when it is dissociated in a basal plane, but this interaction is not strong enough to stabilize a planar configuration, even for a high solute concentration in the core. Energy barrier calculations reveal that basal slip, in the presence of In and Sn, proceeds without any planar dissociation, with the dislocation being spread in pyramidal and prismatic planes during basal slip like in pure Ti. The corresponding energy barrier is higher in presence of solute atoms, showing that In and Sn do not ease basal slip but increase the corresponding lattice friction. This strengthening of basal slip by solute atoms is discussed in view of available experimental data.Graphical abstractImage 1
  • Processing-induced secondary phase formation in Mo-substituted lanthanum
           tungstate membranes
    • Abstract: Publication date: Available online 29 August 2019Source: Acta MaterialiaAuthor(s): Ke Ran, Wendelin Deibert, Hongchu Du, Daesung Park, Mariya E. Ivanova, Wilhelm A. Meulenberg, Joachim Mayer The compositional homogeneity of a technically relevant hydrogen separation membrane, La5.4W0.8Mo0.2O12-δ (LWO-Mo20), was studied using comprehensive transmission electron microscopy (TEM) techniques. The membrane is predominantly composed of dense LWO-Mo20 grains with a defect fluorite structure. In addition to the primary phase, the observed secondary phase (SP) grains were identified as La2/3(Mg1/2W1/2)O3, with the W sites partially occupied by Mo, Fe and Al. Part of the SP grains were incorporated into single LWO-Mo20 grains through smart orientations, in which massive structural defects at the interface of the LWO-Mo20 and SP grains are efficiently avoided. Slight elemental disorder is limited within a few atomic layers. In contrast, the LWO-Mo20 grains share barely common features with neighboring SP grains, and are unstable under electron beam irradiation. The formation of the SP was tracked back to the traces of impurities in the precursors. Excluding such impurities is technically challenging and unacceptable in terms of cost. Hence, our results here show an opportunity to remedy these impurities through engineering the SP into individual primary grains, in which even a significant cost reduction could thus be realized.Graphical abstractImage 1
  • Solute drag and dynamic phase transformations in moving grain boundaries
    • Abstract: Publication date: Available online 29 August 2019Source: Acta MaterialiaAuthor(s): Y. Mishin A discrete model and the regular solution approximation are applied to describe the effect of grain boundary motion on grain boundary phase transformations in a binary alloy. The model predicts all thermodynamic properties of the grain boundary and the solute drag force, and permits a broad exploration of the parameter space and different dynamic regimes. The grain boundary phases continue to exist in the moving grain boundary and show a dynamic hysteresis loop, a dynamic critical line, and other features that are similar to those for equilibrium phases. Grain boundary motion strongly affects the relative stability of the phases and can even stabilize phases that are absolutely unstable under equilibrium conditions. Grain boundary phase transformations are accompanied by drastic changes in the boundary mobility. The results are analyzed in the context of non-equilibrium thermodynamics. Unresolved problems and future work are discussed.Graphical abstractImage 1
  • Hierarchical n-Point Polytope Functions for Quantitative Representation of
           Complex Heterogeneous Materials and Microstructural Evolution
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Pei-En Chen, Wenxiang Xu, Nikhilesh Chawla, Yi Ren, Yang Jiao Effective and accurate characterization and quantification of complex microstructure of a heterogeneous material and its evolution under external stimuli are very challenging, yet crucial to achieving reliable material performance prediction, processing optimization and advanced material design. Here, we address this challenge by developing a set of hierarchical statistical microstructural descriptors, which we call the “n-point polytope functions” Pn, for quantitative characterization, representation and modeling of complex material microstructure and its evolution. These polytope functions successively include higher-order n-point statistics of the features of interest in the microstructure in a concise, expressive, explainable, and universal manner; and can be directly computed from multi-modal imaging data. We develop highly efficient computational tools to directly extract the Pn functions up to n = 8 from multi-modal imaging data. Using simple model microstructures, we show that these statistical descriptors effectively “decompose” the structural features of interest into a set of “polytope basis”, allowing one to easily detect any underlying symmetry or emerging features during the structural evolution. We apply the Pn functions to quantify and model a variety of heterogeneous material systems, including particle-reinforced composites, metal-ceramic composites, concretes, porous materials; as well as the microstructural evolution in an aged lead-tin alloy. Our results indicate that the Pn functions can offer a practical set of basis for quantitative microstructure representation (QMR), for both static 3D complex microstructure and 4D microstructural evolution of a wide spectrum of heterogeneous material systems.Graphical abstractImage 1
  • Growth and coarsening kinetics of gamma prime precipitates in CMSX-4 under
           simulated additive manufacturing conditions
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Benjamin Wahlmann, Florian Galgon, Andreas Stark, Sören Gayer, Norbert Schell, Peter Staron, Carolin Körner Additive manufacturing of superalloys offers new opportunities for alloy design but also poses significant processing difficulties. While the γ′ phase is responsible for the excellent high-temperature resistance of these alloys, it also induces cracking by precipitation hardening during manufacturing. Using small-angle X-ray scattering, we characterized the dynamic precipitation, dissolution, coarsening, and morphological evolution of the γ′ phase in situ during simulated additive manufacturing conditions. For this purpose, a CMSX-4 cylinder was subjected to cyclic heat treatment with heating and quenching rates up to 300 K/s. A specialized setup employing aluminum lenses to focus the X-ray beam was utilized to extend the q-range to small scattering vectors up to 0.035 nm-1. It was shown that the γ′ phase precipitates extremely fast without any measurable undercooling but remains below the equilibrium fraction throughout the process. Coarsening is readily measurable over timespans of only several seconds. A fraction of the γ′ phase that was dissolved during heating reprecipitated by forming new particles instead of growing on already existing precipitates. The findings provide new insight into the dynamic behavior of the γ′ phase during additive manufacturing and may prove valuable in designing new superalloys and processing strategies for additive manufacturing.Graphical abstractImage 1
  • Atom probe tomography study of an Fe25Ni25Co25Ti15Al10 high-entropy alloy
           fabricated by powder metallurgy
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Zhiqiang Fu, Andrew Hoffman, Benjamin E. MacDonald, Zhenfei Jiang, Weiping Chen, Maalavan Arivu, Haiming Wen, Enrique J. Lavernia In this study, transmission electron microscopy (TEM) and atom probe tomography (APT) were utilized to investigate the microstructure and phases in an Fe25Ni25Co25Ti15Al10 high-entropy alloy (HEA) prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The bulk Fe25Ni25Co25Ti15Al10 HEA was characterized by a high tensile strength of 2.52 GPa and contained a minor bcc phase (17.7 vol.%), together with a primary fcc phase (82.3 vol.%) containing hierarchical nanoprecipitates. The bcc phase was a B2-type NiAl phase that contained substantial amounts of Co, Ti and Fe; it also exhibited Fe and Co rich nanoprecipitates with an average diameter of 1.11± 0.33 nm. The fcc phase consisted of a γ Fe-(Co,Ni)-based solid-solution matrix (A1), and coherent primary γ’ (Ni,Co)3-(Ti,Al)-based intermetallic precipitates (L12). A1 structured secondary γ* precipitates were found coherently embedded in the L12-γ’ precipitates. We propose that the formation of the secondary γ* precipitates was largely driven by the unique chemical composition of the γ’ precipitates which accommodate substantial amounts of Fe, Al and Ti, coupled with the nonequilibrium processing route used in our studies. Surprisingly, a novel type of Al-Ti-O oxide was identified via APT. A Ti(C,N) compound containing ∼12.27 at.% Ni was also detected by APT, rather than a simple TiC. Our analysis suggests that the Al-Ti-O oxide likely formed during MA, whereas the Ti(C,N) phase formed during sintering. In addition, a CALPHAD (Calculation of Phase Diagrams) approach was utilized to assist in understanding the underlying phase formation mechanisms. The notable high strength of 2.52 GPa in the Fe25Ni25Co25Ti15Al10 HEA support the hypothesis that phase formation mechanisms play an important role in the mechanical performance of HEAs fabricated by powder metallurgy.Graphical abstractImage 1
  • In-situ quantitative TEM investigation on the dynamic evolution of
           individual twin boundary in magnesium under cyclic loading
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Bo-Yu Liu, K.Eswar Prasad, Nan Yang, Fei Liu, Zhi-Wei Shan Quantification of dynamics of individual twin boundary (TB) migration such as the velocities and corresponding stresses, is of critical importance for understanding the deformation behavior of magnesium alloys. By conducting in-situ cyclic loading experiments on submicron magnesium pillars inside transmission electron microscope (TEM), the dynamics of individual TB migration and the associated twinning-detwinning phenomena are systematically investigated. It is found that the TB can migrate forward and backward under each cyclic loading paths, corresponding to the twinning-detwinning cycles. The TB morphology changes constantly during its migration. Surprisingly, the stress required for TB migration is found to be higher in compression than in tension, and the TB migration velocity in compression is slower than in tension. Such asymmetry is proposed to be associated with different defect environment on either side of TB and the TB structure per se. The considerable amount of energy absorbed during the TB migration is believed to account for at least part of the good damping properties of Mg. Our results are also expected to benefit the modeling of deformation twinning behavior in Mg and other HCP metals.Graphical abstractImage 1
  • Model for Ratchet Growth in PBX 9502
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): R.B. Schwarz Polycrystalline solids composed of crystals with anisotropic thermal expansion coefficients exhibit ratchet growth (RG), a phenomenon characterized by a cumulative and irreversible volume expansion that develops upon exposing the material to cyclic excursions in temperature. We developed a statistical model for RG. The model attributes RG to the formation of intergranular cracks caused by tessellated internal stresses that develop during the thermal excursions. It is postulated that different sets of internal cracks form upon heating and cooling the polycrystalline solid. The model reproduces RG measurements in pressed TATB and PBX 9502 energetic materials and suggests an explanation for why the amplitude of RG generated by a heating excursion is larger than that generated by a subsequent cooling excursion of the same amplitude.
  • Bond-Order Bond Energy Model for Alloys
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Christian Oberdorfer, Wolfgang Windl We introduce a novel way to parameterize alloy energies in the form of a bond-order bond energy model. There, a bond order function models the transition between competing phases and switches their respective bond energies on and off. For the case of the Ni-Cr-Mo alloys investigated here, which assume face- or body-centered cubic structures, we propose a sigmoidal switching function fitted to the c/a ratio of “Bain-like” cells. With that, the model does an excellent job in describing the DFT-calculated alloy energies. We also show that the average atomic charge density can vary considerably as a function of composition, which can significantly modify alloy bond energies from simple expectation. The fitted bond energies can among others be used to determine phase diagrams, where we find excellent agreement with previous assessments in the solid range for Ni-Cr and Cr-Mo phase diagrams once the necessary entropy terms are added. They also allow quantitative, composition-dependent calculation of chemical potentials, which we use to determine vacancy formation energies in binary NiCr alloys for configurations that have been energy-minimized with Monte Carlo simulations. We show that the resulting regime of negative formation energies is a sign for thermodynamic instability of the underlying crystal and lies in the two-phase concentration range in the phase diagram, resulting in a holistic picture that unites defect and phase stability through the fully quantitative link between stoichiometry and chemical potentials enabled by the proposed bond energy model.Graphical abstractImage 1
  • Achieving high strength and ductility in traditionally brittle soft
           magnetic intermetallics via additive manufacturing
    • Abstract: Publication date: Available online 28 August 2019Source: Acta MaterialiaAuthor(s): Tomas F. Babuska, Mark A. Wilson, Kyle L. Johnson, Shaun R. Whetten, John F. Curry, Jeffrey M. Rodelas, Cooper Atkinson, Ping Lu, Michael Chandross, Brandon A. Krick, Joseph R. Michael, Nicolas Argibay, Donald F. Susan, Andrew B. Kustas Intermetallic alloys possess exceptional soft magnetic properties, including high permeability, low coercivity, and high saturation induction, but exhibit poor mechanical properties that make them impractical to bulk process and use at ideal compositions. We used laser-based Additive Manufacturing to process traditionally brittle Fe-Co and Fe-Si alloys in bulk form without macroscopic defects and at near-ideal compositions for electromagnetic applications. The binary Fe-50Co, as a model material, demonstrated simultaneous high strength (600-700 MPa) and ductility (35%) in tension, corresponding to a ∼300% increase in strength and an order-of-magnitude improvement in ductility relative to conventionally processed material. Atomic-scale toughening and strengthening mechanisms, based on engineered multiscale microstructures, are proposed to explain the unusual combination of mechanical properties. This work presents an instance in which metal Additive Manufacturing processes are enabling, rather than limiting, the development of higher-performance alloys.Graphical abstractImage 1
  • Thermodynamics of an austentic stainless steel (AISI-348) under in situ
           TEM heavy ion irradiation
    • Abstract: Publication date: Available online 27 August 2019Source: Acta MaterialiaAuthor(s): Matheus A. Tunes, Graeme Greaves, Thomas M. Kremmer, Vladimir M. Vishnyakov, Philip D. Edmondson, Stephen E. Donnelly, Stefan Pogatscher, Cláudio G. Schön The stability of the face-centred cubic austenite (γ-Fe) phase in a commercial stainless steel (AISI-348) was investigated through in situ transmission electron microscopy (TEM) with heavy ion irradiation at 1073 K up to a fluence of 1.3 × 1017 ions⋅cm−2 (corresponding to a dose of 46 dpa). The γ-Fe phase was observed to decompose at a fluence of around 7.8×1015 ions⋅cm−2 (3 dpa) when a new phase nucleated and grew upon increasing irradiation dose. Scanning transmission electron microscopy (STEM) with energy dispersive X-ray (EDX) spectroscopy and multivariate statistical analysis (MVSA) were used to characterise the irradiated specimens. The combination of such experimental techniques with calculated equilibrium phase diagrams using the CALPHAD method led to the conclusion that the new phase formed upon irradiation is the body-centred cubic Cr-rich α' phase. At the nanoscale, precipitation of M23C6 (τ-carbide) was also observed. The results indicate that ion irradiation can assist the austenitic stainless steel to reach a non-equilibrium state similar to a calculated equilibrium state observed at lower temperatures in which, under conventional conditions, is suppressed due to kinetic restrictions.Graphical abstractImage 1
  • Sub-surface measurements of the austenite microstructure in response to
           martensitic phase transformation
    • Abstract: Publication date: Available online 27 August 2019Source: Acta MaterialiaAuthor(s): Ashley Bucsek, Hanuš Seiner, Hugh Simons, Can Yildirim, Phil Cook, Yuriy Chumlyakov, Carsten Detlefs, Aaron P. Stebner In this work, we measure the microstructure response of the austenite phase during martensitic phase transformation tens of micrometers beneath the surface of a bulk single crystal nickel-titanium shape memory alloy. Using an emerging dark-field X-ray microscopy (DFXM) technique, the austenite phase fraction, relative misorientation, and elastic lattice plane strain are measured in the interior of the microstructure with a spatial resolution of 108 nm. The results show that some defects consistently induce forward transformation and delay reverse transformation, while other defects consistently impede the propagation of both forward and reverse transformation fronts. We also show that the austenite undergoes an orientation splitting wherein the austenite near the transformation front is constrained from rotating and the austenite far from the transformation front is free to rotate. Finally, we measure interfacial strain fields at the transformation front that extend tens of micrometers into the material. We use an analytical model to show how these strain fields can be explained by a lack of kinematic compatibility between the austenite and martensite phases at the austenite-martensite interface.Graphical abstractImage 1
  • Precipitate evolution and strengthening behavior during aging process in a
           2.5 GPa grade maraging steel
    • Abstract: Publication date: Available online 26 August 2019Source: Acta MaterialiaAuthor(s): Mengchao Niu, Gang Zhou, Wei Wang, M. Babar Shahzad, Yiyin Shan, Ke Yang Development of precipitation strengthening steels with ultrahigh strength and high ductility requires thorough understanding of nanoscale precipitation mechanisms. In this study, atom probe tomography (APT), HRTEM and first-principles calculations were used to reveal an interesting co-precipitation mechanism of Ni3Ti and Mo-rich nanoparticles in a 2.5 GPa grade maraging steel. The Ni-Ti rich clusters preferentially nucleate from the supersaturated solid solution and grow into Ni3Ti with extension of aging time, meanwhile the rejection of Mo atoms leads to heterogeneous precipitation of Mo-rich nanoparticles adjacent to the Ni3Ti particles and finally forms a core-shell structure along with Ni3Ti phase. Calculations of interaction energy between alloying elements in different aging process exhibit that the preferential formation of Ni-Ti rich clusters is due to the low interaction energy between Ni and Ti atoms, however, the Ni-Ti cluster is only a transitional phase, and when stable Ni3Ti is formed, Mo atoms is rejected from Ni3Ti to form a core-shell structure along with Ni3Ti precipitates. Finally, four modified theoretical prediction models are introduced to describe the yield strength as a function of microstructure and precipitates characteristics of the experimental steel.Graphical abstractImage 1
  • Two-stage rejuvenation and the correlation between rejuvenation behavior
           and the boson heat capacity peak of a bulk metallic glass
    • Abstract: Publication date: Available online 25 August 2019Source: Acta MaterialiaAuthor(s): Hongbo Zhou, René Hubek, Martin Peterlechner, Gerhard Wilde The influence of severe plastic deformation on the rejuvenation behavior and the boson heat capacity peak of a Pd40Ni40P20 metallic glass is investigated for as-cast and pre-annealed states. Deformation by high-pressure torsion leads to a two-stage rejuvenation behavior, quantified by the structural enthalpy and fictive temperature, in coincidence with a strongly enhanced excess heat capacity at low temperatures, quantified by the boson peak height. Correlations between structural enthalpy, fictive temperature and the boson peak height were analyzed. The results show that the boson peak is not only related to the structural enthalpy but associated with the so-called “overshoot” of the glass transition. A universal link is established between the fictive temperature and the boson peak height, even under the joint influence of deformation and heat treatment. The significance of this link is that it sheds light on the structural connection between high-temperature relaxation and low-temperature excess vibrational density of states.Graphical abstractImage 1
  • Temperature-dependent nucleation kinetics of Guinier-Preston zones in
           Al-Cu alloys: An atomistic kinetic Monte Carlo and classical nucleation
           theory approach
    • Abstract: Publication date: Available online 22 August 2019Source: Acta MaterialiaAuthor(s): Hiroshi Miyoshi, Hajime Kimizuka, Akio Ishii, Shigenobu Ogata Guinier-Preston (GP) zones, which form in Al-Cu alloys, exhibit characteristic patterns of Cu-rich, disk-shaped atomic clusters along the {100} planes. GP zones have received much attention not only for their precipitation-hardening effect on the Al matrix, but also from fundamental interest in the physics and chemistry of the nanoscale organization of solute atoms. In this study, we established an atomistic kinetic Monte Carlo (kMC) modeling technique for exploring the nucleation and formation processes of GP zones in Al-Cu alloys by constructing an effective on-lattice multibody potential for a dilute Al-Cu-vacancy system by density functional theory calculations. Our model can describe the clustering of Cu atoms via successive exchanges with vacancies and reproduce the characteristic planar nanoprecipitates along the {100} planes in a manner consistent with the crystallographic nature of the GP zones in the early-stage aging of Al-Cu alloys. The time evolution and critical nucleus size of Cu clusters were characterized at various temperatures based on the kMC results. Consequently, we predicted the existence of an optimum temperature (i.e., nose temperature) for the formation of Cu clusters at which the cluster growth was maximized, which was attributable to the interplay between the critical nucleation barrier and the diffusion rate. In addition, the critical nucleus size and temperature for cluster formation were examined based on classical nucleation theory along with the developed multibody potential. These provided an insight into the competition between the enthalpic and entropic effects on the formation of GP zones in the Al-Cu system.Graphical abstractImage 1
  • On the chevron morphology of surface martensite
    • Abstract: Publication date: Available online 22 August 2019Source: Acta MaterialiaAuthor(s): Annick P. Baur, Cyril Cayron, Roland E. Logé It is established that the annealing twin boundaries of austenite act as nucleation sites for the martensitic transformation. In the present study, we observe that in surface martensite the transformation only takes place for twin boundaries that are approximately vertical in respect with the sample free surface. This phenomenon is shown to be related to variant selection. The criterion for surface martensite formation introduced in a previous study is used here to account for the selection of the twin boundaries that promote the transformation. This criterion correctly captures the phenomenon and explains the highly symmetrical chevron morphology observed in surface martensite.Graphical abstractImage 1
  • Isolation of a Ferroelectric Intermediate Phase in Antiferroelectric Dense
           Sodium Niobate Ceramics
    • Abstract: Publication date: Available online 22 August 2019Source: Acta MaterialiaAuthor(s): Hangfeng Zhang, Bin Yang, Haixue Yan, Isaac Abrahams Switchable ferroelectric/antiferroelectric ceramics are of significant interest for high power energy storage applications. Grain size control of this switching is an interesting approach to controlling polarization and hence dielectric properties. However, the use of this approach in technologically relevant ceramics is hindered by difficulty in fabricating dense ceramics with small grain sizes. Here an intermediate polar ferroelectric phase (P21ma) has been isolated in dense bulk sodium niobate ceramics by grain size control through spark plasma sintering methods. Our findings, supported by XRD, DSC, P-E (I-E) loops and dielectric characterization, provide evidence that the phase transition from the antiferroelectric (AFE) R-phase, in space group Pnmm, above 300 °C, to the AFE P-phase, in space group Pbma, at room temperature, always involves the polar intermediate P21ma phase and that the P21ma → Pbma transition can be suppressed by reducing grain size.Graphical abstractImage 1
  • Crystal orientation effect on fretting fatigue induced geometrically
    • Abstract: Publication date: Available online 21 August 2019Source: Acta MaterialiaAuthor(s): Qi-Nan Han, Shao-Shi Rui, Wenhui Qiu, Xianfeng Ma, Yue Su, Haitao Cui, Hongjian Zhang, Huiji Shi The effect of crystal orientation on fretting fatigue induced crack initiation and dislocation distribution is studied by in-situ SEM observation and electron back-scattered diffraction (EBSD) in this paper. Cracks and slip lines are observed in the fretting contact area of Ni-based single-crystal (NBSX) superalloys. The in-situ SEM observation captures different crack and slip line behaviors under different crystal orientations. The EBSD analysis results show obvious misorientation and orientation deviation in the fretting contact area. For both crystal orientations, the geometrically necessary dislocation (GND) density distributions in the contact area are obtained by using Hough-based EBSD methods. The peak position of grain reference orientation deviation (GROD) and GND density matches with the fretting fatigue crack formation position. EBSD analysis shows that the dislocation density distribution on each slip system is closely related to the crack initiation direction. The direction of slip system with the maximum dislocation density agrees with the crack initiation direction obtained by in-situ observation.Graphical abstractImage 1
  • In Situ Characterization of a High Work Hardening Ti-6Al-4V prepared by
           Electron Beam Melting
    • Abstract: Publication date: Available online 21 August 2019Source: Acta MaterialiaAuthor(s): K. Sofinowski, M. Šmíd, I. Kuběna, S. Vives, N. Casati, S. Godet, H. Van Swygenhoven A multi-phase Ti-6Al-4V prepared by electron beam melting and thermal post treatments has been shown to exhibit increased strength and ductility over standard wrought or hot isostatic pressed Ti-6Al-4V. The mechanical improvements are due to a prolonged, continuous work hardening effect not commonly observed in Ti alloys. In situ x-ray diffraction and high resolution digital image correlation are used to examine the strain partitioning between the phases during tensile loading with post-mortem electron microscopy to characterize the deformation behavior in each phase. Specimens heat treated between 850 and 980°C were tested and the effect of annealing temperature on the micromechanical response is discussed. It is shown that the work hardening is the result of composite load-sharing behavior between three mechanically distinct microstructures: large α lamellae and a martensitic region of fine acicular α' and a third phase not previously reported in this alloy.Graphical abstractImage 1
  • Radiation induced solute clustering in high-Ni reactor pressure vessel
    • Abstract: Publication date: Available online 21 August 2019Source: Acta MaterialiaAuthor(s): M.J. Konstantinović, I. Uytdenhouwen, G. Bonny, N. Castin, L. Malerba, P. Efsing The thermal stability and the structure of solute-vacancy clusters formed by neutron irradiation are studied by means of positron annihilation spectroscopy and hardness measurements of post-irradiation annealed reactor pressure vessel steels with high and low Ni contents. Two distinct recovery stages were observed and assigned to (a) the dissolution of vacancy clusters at about 650 K, and (b) the dissolution of solute-vacancy clusters at about 750 K. In steels with high Ni content, hardening mainly recovers during the second stage. Atomistic and coarse grain models suggest that during this stage, the removal of vacancies from vacancy-solute clusters leads to complete cluster dissolution, which indicates that solute clusters are radiation induced.x.Graphical abstractImage 1
  • 60 o +Shuffle+Dislocation+Pileup+against+Different+Grain+Boundaries+in+Silicon+Bicrystal+under+Shear&rft.title=Acta+Materialia&rft.issn=1359-6454&">Amorphization Induced by 60 o Shuffle Dislocation Pileup against
           Different Grain Boundaries in Silicon Bicrystal under Shear
    • Abstract: Publication date: Available online 20 August 2019Source: Acta MaterialiaAuthor(s): Hao Chen, Valery Levitas, Liming Xiong Molecular dynamics (MD) simulations of the amorphous band nucleation and growth ahead of the tip of a shuffle 60o dislocation pileup at different grain boundaries (GBs) in diamond-cubic (dc) silicon (Si) bicrystal under shear are performed. Amorphization initiates when the local resolved shear stress reaches approximately the same value required for amorphization in a perfect single crystal (8.6-9.3GPa) for the same amorphization plane. Since the local stresses at the tip of a dislocation pileup increase when the number of dislocations in the pileup is increased, the critical applied shear stress τap for the formation of an amorphous shear band significantly decreases with the dislocation accumulation at the GBs. In particular, when the number of the dislocations in a pileup increases from 3 to 8, the critical shear stress drops from 4.7GPa to 1.6GPa for both the Σ9 and Σ19 GBs and from 4.6GPa to 2.1GPa for the Σ3 GB, respectively. After the formation of steps and disordered embryos at the GBs, the atomistic mechanisms responsible for the subsequent amorphous shear band formations near different GBs are found to distinct from each other. For a high-angle GB, such as Σ3, an amorphous band propagates through the crystalline phase along the (112) plane. For the Σ9 GB, partial dislocations forming a stacking fault precede the formation of an amorphous band along the (110) plane. For the Σ19 GB, the one-layer stacking fault along the (111) plane transforms into an interesting intermediate phase: a two-layer band with the atomic bonds being aligned along the (111) plane (i.e., rotated by 30o with respect to the atomic bonds outside the band). This intermediate phase transforms to the amorphous band along the (111) plane under a further shearing. The obtained results represent an atomic-level confirmation of the effectiveness of dislocation pileup at the nucleation site for various strain-induced phase transformations (PTs), and exhibit some limitations.Graphical abstractImage 1
  • A Novel Liquid-Mediated Nucleation Mechanism for Explosive Crystallization
           in Amorphous Germanium
    • Abstract: Publication date: Available online 20 August 2019Source: Acta MaterialiaAuthor(s): Garth C. Egan, Tae Wook Heo, Amit Samanta, Geoffrey H. Campbell We report a novel mechanism for explosive crystallization in amorphous germanium (a-Ge), which operates through liquid-mediated nucleation occurring under extreme thermal gradient conditions. The crystallization kinetics of sputter-deposited films with thicknesses ranging from 30 to 150 nm were characterized using in situ movie-mode dynamic transmission electron microscopy (MM-DTEM). After localized heating from a short laser pulse, explosive liquid phase nucleation (LPN) was observed to occur during the early stage (50 nm) films deposited on silicon nitride substrates. The crystallization front propagated at ∼12-15 m/s and produced nanocrystalline microstructure with ∼50 nm grains. A mechanism involving the existence of a relatively thick (>100 nm) transient liquid layer and a high nucleation rate is proposed to explain the behavior. The key thermodynamic and kinetic features as well as the feasibility of the mechanism are further explored by employing parametric and systematic phase-field modeling and simulations.Graphical abstractImage 1
  • Powder-spreading mechanisms in powder-bed-based additive manufacturing:
           experiments and computational modeling
    • Abstract: Publication date: Available online 20 August 2019Source: Acta MaterialiaAuthor(s): Hui Chen, Qingsong Wei, Yingjie Zhang, Fan Chen, Yusheng Shi, Wentao Yan The packing density of the powder layer plays a key role in the final quality of the parts fabricated via powder-bed-based (PBB) additive manufacturing. This paper presents a combined experimental and computational modeling study on the scraping type of powder-spreading process, in order to understand the fundamental mechanisms of the packing of the powder layer. The deposition mechanisms at the particulate scale, including particle contact stress and particle velocity, are investigated, using the discrete element method, while the macro-scale packing density is validated by experiments. It is found that there is a stress-dip at the bottom of powder pile scraped by the rake. This stress-dip makes the powder particles uniformly deposited. Three kinds of deposition mechanisms dominating the powder-spreading process are identified: cohesion effect, wall effect, and percolation effect. The cohesion effect, which leads to particle agglomerations and thus reduces the packing density, becomes stronger with the decrease of particle size. The wall effect, which leads to more vacancies in the powder layer, becomes stronger with the decrease of layer thickness or the increase of particle size. The percolation effect exists in bimodal powder particles, which leads to particle segregation within the powder layer and thus reduces the packing density. The three kinds of deposition mechanisms compete with each other during the powder-spreading process and make combined effects on the packing density of the powder layer.Graphical abstractImage 1
  • Physical metallurgy-guided machine learning and artificial intelligent
           design of ultrahigh-strength stainless steel
    • Abstract: Publication date: Available online 20 August 2019Source: Acta MaterialiaAuthor(s): Chunguang Shen, Chenchong Wang, Xiaolu Wei, Yong Li, Sybrand van der Zwaag, Wei Xu With the development of the materials genome philosophy and data mining methodologies, machine learning (ML) has been widely applied for discovering new materials in various systems including high-end steels with improved performance. Although recently, some attempts have been made to incorporate physical features in the ML process, its effects have not been demonstrated and systematically analysed nor experimentally validated with prototype alloys. To address this issue, a physical metallurgy (PM) -guided ML model was developed, wherein intermediate parameters were generated based on original inputs and PM principles, e.g., equilibrium volume fraction (Vf) and driving force (Df) for precipitation, and these were added to the original dataset vectors as extra dimensions to participate in and guide the ML process. As a result, the ML process becomes more robust when dealing with small datasets by improving the data quality and enriching data information. Therefore, a new material design method is proposed combining PM-guided ML regression, ML classifier and a genetic algorithm (GA). The model was successfully applied to the design of advanced ultrahigh-strength stainless steels using only a small database extracted from the literature. The proposed prototype alloy with a leaner chemistry but better mechanical properties has been produced experimentally and an excellent agreement was obtained for the predicted optimal parameter settings and the final properties. In addition, the present work also clearly demonstrated that implementation of PM parameters can improve the design accuracy and efficiency by eliminating intermediate solutions not obeying PM principles in the ML process. Furthermore, various important factors influencing the generalizability of the ML model are discussed in detail.Graphical abstractImage 1
  • Short-term creep behavior of an additive manufactured non-weldable
           Nickel-base superalloy evaluated by slow strain rate testing
    • Abstract: Publication date: Available online 20 August 2019Source: Acta MaterialiaAuthor(s): Jinghao Xu, Hans Gruber, Dunyong Deng, Ru Lin Peng, Johan J. Moverare Additive manufacturing (AM) of high γ′ strengthened Nickel-base superalloys, such as IN738LC, is of high interest for applications in hot section components for gas turbines. The creep property acts as the critical indicator of component performance under load at elevated temperature. However, it has been widely suggested that the suitable service condition of AM processed IN738LC is not yet fully clear. In order to evaluate the short-term creep behavior, slow strain rate tensile (SSRT) tests were performed. IN738LC bars were built by laser powder-bed-fusion (L-PBF) and then subjected to hot isostatic pressing (HIP) followed by the standard two-step heat treatment. The samples were subjected to SSRT testing at 850 °C under strain rates of 1×10-5/s, 1×10-6/s, and 1×10-7/s. In this research, the underlying creep deformation mechanism of AM processed IN738LC is investigated using the serial sectioning technique, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM). On the creep mechanism of AM polycrystalline IN738LC, grain boundary sliding is predominant. However, due to the interlock feature of grain boundaries in AM processed IN738LC, the grain structure retains its integrity after deformation. The dislocation motion acts as the major accommodation process of grain boundary sliding. Dislocations bypass the γ′ precipitates by Orowan looping and wavy slip. The rearrangement of screw dislocations is responsible for the formation of subgrains within the grain interior. This research elucidates the short-creep behavior of AM processed IN738LC. It also shed new light on the creep deformation mechanism of additive manufactured γ′ strengthened polycrystalline Nickel-base superalloys.Graphical abstractImage 1
  • Thick amorphous complexion formation and extreme thermal stability in
           ternary nanocrystalline Cu-Zr-Hf alloys
    • Abstract: Publication date: Available online 20 August 2019Source: Acta MaterialiaAuthor(s): Charlette M. Grigorian, Timothy J. Rupert Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with amorphous intergranular films, this paper investigates ternary nanocrystalline Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the formation of disordered complexions. Binary Cu-Zr and Cu-Hf alloys with similar initial grain sizes were also fabricated for comparison. The thermal stability of the nanocrystalline alloys was evaluated by annealing at 950 °C (>95% of the solidus temperatures), followed by detailed characterization of the grain boundary structure. All of the ternary alloys exhibited exceptional thermal stability comparable to that of the binary Cu-Zr alloy, and remained nanocrystalline even after two weeks of annealing at this extremely high temperature. Despite carbide formation and growth in these alloys during milling and annealing, the thermal stability of the ternary alloys is mainly attributed to the formation of thick amorphous intergranular films at high temperatures. Our results show that ternary alloy compositions have thicker boundary films compared to the binary alloys with similar global dopant concentrations. While it is not required for amorphous complexion formation, this work shows that having at least three elements present at the interface can lead to thicker grain boundary films, which is expected to maximize the previously reported toughening effect.Graphical abstractImage 1
  • Effects of thermal aging and low-fluence neutron irradiation on the
           mechanical property and microstructure of ferrite in cast austenitic
           stainless steels
    • Abstract: Publication date: Available online 19 August 2019Source: Acta MaterialiaAuthor(s): Siwei Chen, Yuichi Miyahara, Akiyoshi Nomoto, Kenji Nishida The effects of low-fluence neutron irradiation on hardening and microstructure evolution in ferrite of solution annealed or thermally aged CF3, CF3M, CF8 and CF8M cast austenitic stainless steels (CASSs) have been investigated by means of nanoindentation tests and atom probe tomography (APT). Thermal aging was performed at 400 ºC for 500 h. Neutron irradiation was carried out to a fluence of 4.84×1018 n/cm2 (E>1 MeV) at the temperature ranging from 289 to 292 ºC in the LVR-15 research reactor. Irradiation hardening in thermally-aged specimens was found to be similar with or smaller than that in the corresponding solution annealed specimens. Phase decomposition and formation of solute clusters acted two major factors for the hardening in ferrite with thermal aging and/or neutron irradiation. The phase decomposition of ferrite increased with either the thermal aging or the neutron irradiation for the solution annealed materials; however, the change in the phase decomposition of ferrite was neither significant nor apparent with the low-fluence neutron irradiation for the thermally-aged materials. Ni-Si-Mn enriched solute clusters were observed in the matrix of ferrite in the aged specimens, and the irradiated specimens with/without thermal aging. Mo in the CASSs appeared to inhibit the formation of solute clusters under the neutron irradiations. In the thermally-aged specimen with low-C and without Mo, neutron irradiation enhanced the formation of solute clusters significantly. For the first time we discussed the relationship between hardening and microstructure evolution in ferrite of CASSs with consideration of both thermal aging and neutron irradiation.Graphical abstractImage 1
  • Asymmetric flux-closure domains in compositionally graded nanoscale
           ferroelectrics and unusual switching of toroidal ordering by an
           irrotational electric field
    • Abstract: Publication date: Available online 19 August 2019Source: Acta MaterialiaAuthor(s): Le Van Lich, Minh-Tien Le, Tinh Quoc Bui, Thanh-Tung Nguyen, Takahiro Shimada, Takayuki Kitamura, Trong-Giang Nguyen, Van-Hai Dinh A reversal of polarization vortexlike domains in ferroelectric nanostructures plays important roles for next generations of electronic nanodevices. However, a direct switching of the polarization vortexlike domains in ferroelectrics is a nontrivial task since the toroidal moment is conjugated to a curled electric field rather than a homogeneous one. This work is dedicated to developing an approach to directly switch the toroidal ordering under an irrotational (homogeneous) electric field with the use of compositionally graded ferroelectric (cgFE) nanodot. The variation in material compositions induces an additionally broken spatial inversion symmetry at a scale beyond unit-cell level, giving rise to a formation of asymmetric flux-closure domain (FCD) in cgFE nanodot. More interestingly, such an asymmetric character facilitates to a switch of FCD by an irrotational electric field. In particular, the rotation of polarization can be directly switched from counter-clockwise to clockwise rotations and vice versa without a formation of intermediate domain structures during the switching process. This switching behavior is distinguished from that in homogeneous counterparts. We further demonstrate that the variation in material compositions tailors the distribution of electrostatic and total free energies in the cgFE nanodot that can control the annihilation/initiation process of FCD under irrotational electric field, providing fundamental reason for the direct switching of the toroidal moment. Another interesting issue is found that both the amplitude and frequency of applied electric field strongly affect the switching behavior of FCD in cgFE nanodot.Graphical abstractImage 1
  • Precipitation and Hardening in Irradiated Low Alloy Steels with a Wide
           Range of Ni and Mn Compositions
    • Abstract: Publication date: Available online 18 August 2019Source: Acta MaterialiaAuthor(s): N. Almirall, P.B. Wells, T. Yamamoto, K. Wilford, T. Williams, N. Riddle, G.R. Odette Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPV-type steels, with an unusually wide range of dissolved Mn (0.06-1.34 at.%) and Ni (0.19-3.50 at.%) contents, were irradiated at ≈ 290°C to ≈ 1.4x1020 n/cm2 at an accelerated test reactor flux of ≈ 3.6x1012 n/cm2-s (E> 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with ≈ Ni1.6Mn0.8. Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Ni-silicide phase type compositions are observed.Graphical abstractImage 1
  • Designing solid solution hardening to retain uniform ductility while
           quadrupling yield strength
    • Abstract: Publication date: Available online 17 August 2019Source: Acta MaterialiaAuthor(s): Ping-Jiong Yang, Qing-Jie Li, Wei-Zhong Han, Ju Li, Evan Ma Single-phase metals can be strengthened via cold work, grain refinement, or solid solution hardening. But the yield strength elevation normally comes at the expense of ductility, i.e., a conspicuous decrease of the uniform elongation in uniaxial tension. This strength-ductility trade-off is often a result of inadequate strain hardening rate that can no longer keep up with the elevated flow stress to prevent plastic instability. Here we alleviate this dilemma by designing oxygen interstitial solution hardening in body-centered-cubic niobium: the strain hardening rate is exceptionally high, such that most of the uniform tensile ductility of Nb can be retained despite of quadrupled yield strength. The oxygen solutes impose random force field on moving dislocation line, promoting the formation of cross-kinks that dynamically accumulate vacancy-oxygen complexes. These obstacles enhance the trapping/multiplication of screw dislocations as well as cross-slip, all promoting strain hardening and strain de-localization. This approach utilizes only a low concentration of interstitial solutes to achieve effective strengthening and strain hardening simultaneously, and is an inexpensive and scalable route amenable to the processing of bulk samples.Graphical abstractImage 1
  • Effect of electrochemical charging on the hydrogen embrittlement
           susceptibility of Alloy 718
    • Abstract: Publication date: Available online 17 August 2019Source: Acta MaterialiaAuthor(s): X. Lu, D. Wang, D. Wan, Z.B. Zhang, N. Kheradmand, A. Barnoush The susceptibility of age-hardened nickel-based 718 superalloy to hydrogen embrittlement was studied by the controlled electrochemical charging combined with slow strain-rate tensile tests (SSRT) and advanced characterization techniques. We proposed some novel ideas of explaining hydrogen embrittlement mechanisms of the studied material in regard to two cracking morphologies: transgranular and intergranular cracking. It is for the first time to report that electrochemical charging alone could cause slip lines, surface and subsurface cracks on nickel-based superalloys. The formation of pre-damages was discussed by calculating the hydrogen concentration gradient generated during cathodic charging. Pre-damages were proved to result in transgranular cracks and lead to the evident reduction of mechanical properties. In addition, the STRONG (Slip Transfer Resistance of Neighbouring Grains) model was used to analyze the dependence of hydrogen-assisted intergranular cracking on the microscopic incompatibility of the grain boundaries. The results show that in the presence of hydrogen, grain boundaries with a lower dislocation slip transmission are more prone to cracking during loading and vice versa.Graphical abstractImage 1
  • A coupled microstructural-structural mechanism governing thermal
           depolarization delay in Na0.5Bi0.5TiO3-based piezoelectrics
    • Abstract: Publication date: Available online 15 August 2019Source: Acta MaterialiaAuthor(s): Dipak Kumar Khatua, Anupam Mishra, Naveen Kumar, Gobinda Das Adhikary, Uma Shankar, Bhaskar Majumdar, Rajeev Ranjan Driven by environmental concerns and governmental directives, a sustained research effort in the last decade and half has led to the development of lead-free alternatives which can potentially replace the commercial lead-based piezoceramics in niche applications. Na0.5Bi0.5TiO3 (NBT)-based lead-free piezoceramics have found acceptance as promising lead-free transducers in high power ultrasonic devices. An issue of concern however is the low depolarization temperature which limits the device’s tolerance for temperature rise during operation. While several strategies have been reported to improve thermal depolarization in NBT-based piezoceramics, there is a lack of consensus regarding the most fundamental factor/mechanism which enhances the depolarization temperature. In this paper we unravel a coupled microstructural-structural mechanism which controls the thermal depolarization in NBT-based piezoceramics. First, we demonstrate the phenomenon of a considerable increase in the depolarization temperature, without significantly losing the piezoelectric property in unmodified NBT by increasing the grain size. We then establish a grain size controlled structural mechanism and demonstrate that the rise in depolarization temperature is primarily associated with the bigger grains allowing relatively large lattice distortion to develop in the poling stabilized long range ferroelectric phase. We reconfirmed the validity of this mechanism in the model morphotropic phase boundary (MPB) composition 0.94Na0.5Bi0.5TiO3-0.06BaTiO3. For the sake of generalization, we demonstrate that the same mechanism is operative in another lead-based relaxor-ferroelectric system 0.62PbTiO3-0.38Bi(Ni0.5Hf0.5)O3. Our study provides the fundamental structural basis for understanding thermal depolarization delay in relaxor ferroelectric based piezoceramics.Graphical abstractImage 1
  • Interfacial Ponderomotive Force in Solids Leads to Field Induced
           Dissolution of Materials and Formation of Non-equilibrium Nanocomposites
    • Abstract: Publication date: Available online 14 August 2019Source: Acta MaterialiaAuthor(s): Amin Nozariasbmarz, Mahshid Hosseini, Daryoosh Vashaee We report that microwave radiation can decompose continuous solid-solution materials into their constituent phases – a process that is thermodynamically unfavorable at equilibrium. A detailed analysis of the interaction of the electromagnetic wave with the material showed that a strong ponderomotive force preferentially separates the constituent phases via an enhanced mass transport process amplified particularly near the interfaces. The proof of concept experiments showed that the material, whether it is a solid-solution of two elements, e.g. (Si1-xGex), or two compounds, e.g. (Bi2Te3)1-x(Sb2Te3)x, decomposes into the constituent phases when radiated by a polarized microwave field. The dissolution happens in the bulk of the material and even below the melting point. The degree of decomposition can be controlled by radiation parameters to produce structures composed of gradient phases of the solid-solution. This offers a novel and facile method for synthesizing gradient composite and complex structures for application in thermoelectricity as well as fabrication of core-shell structures for catalysts and biomedical applications.Graphical abstractImage 1
  • Relations between Material Properties and Barriers for Twin Boundary
           Motion in Ferroic Materials
    • Abstract: Publication date: Available online 14 August 2019Source: Acta MaterialiaAuthor(s): Bar Danino, Gil Gur-Arieh, Doron Shilo, Dan Mordehai Ferroic materials typically exhibit a microstructure that contains twins or domains separated by twin boundaries (walls). The deformation of these materials is governed by twin boundary motion under mechanical/electrical/magnetic driving force. The Landau-Ginzburg model is a widely accepted phenomenological model used to describe twin boundary properties. However, it is incapable of describing energy barriers for motion due to the lack of atomistic description. In this work, we present a model interatomic potential for studying the relations between the lattice barrier for twin boundary motion and measurable material properties. The interatomic potential emulates the continuum Landau-Ginzburg model and reproduces known results of twin boundary thickness and energy as a function of the model parameters. An atomic model system is constructed, with a single twin boundary separating crystals of different orientations and we employ the Nudged Elastic Band method to calculate the energy barriers for the motion of twin boundaries with different thicknesses under different externally-applied shear stresses. The results are summarized in a closed-form expression relating the energy barriers with material properties and the external loading. The energy barrier function extends the Landau-Ginzburg model and allows treating the motion of twin boundary as a thermally activated process.Graphical abstractImage 1
  • Strong metal–metal interaction and bonding nature in metal/oxide
           interfaces with large mismatches
    • Abstract: Publication date: Available online 14 August 2019Source: Acta MaterialiaAuthor(s): Hongping Li, Mitsuhiro Saito, Chunlin Chen, Kazutoshi Inoue, Kazuto Akagi, Yuichi Ikuhara Metal/oxide heterointerfaces are ubiquitous in functional materials, and their microstructures frequently govern the macroscopic properties. It has been believed that the interfacial interactions are very weak at incoherent interfaces with large mismatches. Combining atomic-resolution scanning transmission electron microscopy with density functional theory calculations, we investigated the interaction and bonding reconstruction at Pd/ZnO{0001} interfaces, which have large mismatches. Molecular beam epitaxy was employed to grow Pd films on clean Zn-terminated ZnO(0001) and O-terminated ZnO(0001¯) polarized surfaces. Atomically sharp Zn-terminated interfaces formed on both substrates, and the large lattice misfit between the films and substrate was not strongly accommodated, suggesting the formation of incoherent regions. The interfacial atoms were located almost at bulk lattice points in the stoichiometric Zn-terminated Pd(111)/ZnO(0001) structure, whereas the interfacial Pd and Zn atoms underwent relatively large relaxations on the interfacial plane in the nonstoichiometric Zn-terminated Pd(111)/ZnO(0001¯) interface. Effective Pd-Zn chemical bonds were formed across both interfaces, but the bonding mechanisms were quite different, depending on the local atomic geometry. The Pd-Zn bonds exhibited site-dependent characteristics and gradually transitioned from covalent to ionic at the Pd(111)/ZnO(0001) interface, whereas most of Pd-Zn bonds exhibited strong covalent behavior at the Pd/ZnO(0001¯) interface. The adhesive energies indicated that the Zn-terminated Pd/ZnO(0001¯) interface is energetically preferable to the Zn-terminated Pd/ZnO(0001) interface. Thus, the interfacial interaction can be strong and direct metal–metal interactions can play a critical role in metal/oxide heterointerfaces with large mismatches, opening up a new avenue for understanding the origins of interface-related issues.Graphical abstractImage 1
  • Effective cluster interactions and pre–precipitate morphology in
           binary Al-based alloys
    • Abstract: Publication date: Available online 10 August 2019Source: Acta MaterialiaAuthor(s): O.I. Gorbatov, A.Yu. Stroev, Yu.N. Gornostyrev, P.A. Korzhavyi The strengthening by coherent, nano-sized particles of metastable phases (pre-precipitates) continues to be the main design principle for new high-performance aluminium alloys. To describe the formation of such pre-precipitates in Al–Cu, Al–Mg, Al–Zn, and Al–Si alloys, we carry out cluster expansions of ab initio calculated energies for supercell models of the dilute binary Al-rich solid solutions. Effective cluster interactions, including many-body terms and strain-induced contributions due to the lattice relaxations around solute atoms, are thus systematically derived. Monte Carlo and statistical kinetic theory simulations, parameterized with the obtained effective cluster interactions, are then performed to study the early stages of decomposition in the binary Al-based solid solutions. We show that this systematic approach to multi-scale modelling is capable of incorporating the essential physical contributions (usually referred to as atomic size and electronic structure factors) to the free energy, and is therefore able to correctly describe the ordering temperatures, atomic structures, and morphologies of pre-precipitates in the four studied alloy systems.Graphical abstractImage 1
  • Assessment of ductile character in superhard Ta-C-N thin films
    • Abstract: Publication date: Available online 10 August 2019Source: Acta MaterialiaAuthor(s): T. Glechner, R. Hahn, T. Wojcik, D. Holec, S. Kolozsvári, H. Zaid, S. Kodambaka, P.H. Mayrhofer, H. Riedl Using a combination of density functional theory calculations and nanomechanical testing of sputter-deposited, 110-oriented Ta0.47C0.34N0.19 thin films, we show that non-metal alloying – substituting C with N atoms – in TaC results in a super-hard material with enhanced ductility. Based on the calculated elastic constants, with Pugh and Pettifor criteria for ductile character, we predict that stoichiometric and sub-stoichiometric Ta-C-N alloys are more ductile than Ta-C compounds. From nanoindentation of the as-deposited coating, we measure hardness of 43 ± 1.4 GPa. In situ scanning electron microscopy (SEM) based micro-compression of cylindrical pillars, prepared via focused ion beam milling of the coating, revealed that Ta-C-N alloys are ductile and undergo plastic deformation with a yield strength of 17 ± 1.4 GPa. The post-compression SEM images of the pillars show {111} as the active slip system operating during compression. Additional in situ SEM based cantilever tests suggest that the Ta-C-N films exhibit superior fracture toughness compared to Ta-C coatings. Our results provide a new perspective on the role of alloying on the mechanical behavior of ultra-high temperature compounds such as transition-metal carbides.Graphical abstractImage 1
  • X-ray characterization of the micromechanical response ahead of a
           propagating small fatigue crack in a Ni-based superalloy
    • Abstract: Publication date: Available online 8 August 2019Source: Acta MaterialiaAuthor(s): Diwakar P. Naragani, Paul A. Shade, Peter Kenesei, Hemant Sharma, Michael D. Sangid The small fatigue crack (SFC) growth regime in polycrystalline alloys is complex due to the heterogeneity in the local micromechanical fields, which result in high variability in crack propagation directions and growth rates. In this study, we employ a suite of techniques, based on high-energy synchrotron-based X-ray experiments that allow us to track a nucleated crack, propagating through the bulk of a Ni-based superalloy specimen during cyclic loading. Absorption contrast tomography is used to resolve the intricate 3D crack morphology and spatial position of the crack front. Initial near-field high-energy X-ray diffraction microscopy (HEDM) is used for high-resolution characterization of the grain structure, elucidating grain orientations, shapes, and boundaries. Cyclic loading is periodically interrupted to conduct far-field HEDM to determine the centroid position, average orientation, and average lattice strain tensor for each grain within the volume of interest. Reciprocal space analysis is used to further examine the deformation state of grains that plasticize in the vicinity of the crack. Analysis of the local micromechanical state in the grains ahead of the crack front is used to rationalize the advancing small crack path and growth rate. Specifically, the most active slip system in a grain, determined by the maximum resolved shear stress, aligns with the crack growth direction; and the degree of microplasticity ahead of the crack tip helps to identify directions for potential occurrences of crack arrest or propagation. The findings suggest that both the slip system level stresses and microplasticity events within grains are necessary to get a complete description of the SFC progression. Further, this detailed dataset, produced by a suite of X-ray characterization techniques, can provide the necessary validation, at the appropriate length-scale, for SFC models.Graphical abstractImage 1
  • Phase transformation mechanisms during Quenching and Partitioning of a
           ductile cast iron
    • Abstract: Publication date: Available online 7 August 2019Source: Acta MaterialiaAuthor(s): Arthur S. Nishikawa, Goro Miyamoto, Tadashi Furuhara, André P. Tschiptschin, Hélio Goldenstein The modification of the matrix of ductile cast irons by heat treatments has been of interest of researchers for many years. Among these treatments, in the last years the Quenching & Partitioning (Q&P) process has emerged as a viable way to produce microstructures containing controlled amounts of martensite and retained austenite, providing a good combination of strength and ductility. In this work, the different mechanisms of phase transformations occurring during the Q&P heat treatment applied to a ductile cast iron alloy is investigated. Microsegregation, inherent to cast irons, was analyzed by means of Electron Probe Microanalysis (EPMA). Microstructural characterization was performed with Scanning Electron Microscopy (SEM) and Electron Backscattered Diffraction (EBSD), while kinetics of carbon redistribution and competitive reactions were studied using dilatometry and in situ synchrotron X-ray diffraction. It was found that either transition carbides or cementite precipitate in martensite depending on the partitioning temperature. Despite of carbides precipitation, evidence of carbon partitioning from martensite to austenite was obtained. Formation of bainitic ferrite occurs during the partitioning step, further contributing to carbon enrichment of austenite. The experimental results are compared with a local field model that computes the local kinetics of carbon redistribution by simultaneously considering carbides precipitation and growth of bainitic ferrite. Results showed that kinetics of carbon partitioning from martensite to austenite depends on the carbides free energy. More stable carbides do not dissolve and prevent the escape of carbon from martensite. Fast carbon partitioning occurs by dissolution of less stable carbides, but it is slowed down as growth of bainitic ferrite proceeds. This result is explained by the overlapping of the diffusion fields (soft impingement) of the carbon partitioned from martensite and the carbon rejected from growth of bainitic ferrite.Graphical abstractImage 1
  • A new scenario for ‹c› vacancy loop formation in zirconium based on
           atomic-scale modeling
    • Abstract: Publication date: Available online 3 August 2019Source: Acta MaterialiaAuthor(s): B. Christiaen, C. Domain, L. Thuinet, A. Ambard, A. Legris The growth of zirconium alloys under irradiation is a phenomenon experimentally identified and associated with the development beyond a threshold dose of dislocation loops with vacancy character having a Burgers vector with a component parallel to the c axis. In this work, by combining atomic simulations (DFT and empirical potential) and continuous modeling, we show that prismatic stacking fault pyramids or bipyramids whose base rests on the basal plane of the hcp structure are likely precursors to the formation of c vacancy loops. In other words, these would not be formed by progressive accretion of vacancies but rather by collapse of the pyramids or bipyramids beyond a certain size. This mechanism could explain the fact that the ‹c› vacancy loops are never observed below a size of the order of 10 nm and their appearance at high fluence.Graphical abstractImage 1
  • Direct observation of the displacement field and microcracking in a glass
           by means of X-ray tomography during in situ Vickers indentation experiment
    • Abstract: Publication date: Available online 30 July 2019Source: Acta MaterialiaAuthor(s): Tanguy Lacondemine, Julien Réthoré, Éric Maire, Fabrice Célarié, Patrick Houizot, Clément Roux-Langlois, Christian M. Schlepütz, Tanguy Rouxel The actual displacement field in a glass during an in-situ Vickers indentation experiment was determined by means of X-ray tomography, thanks to the addition of 4 vol. % of X-ray absorbing particles, which acted as a speckle to further proceed through digital volume correlation. This displacement was found to agree well with the occurrence of densification beneath the contact area. The intensity of the densification contribution (Blister field proposed by Yoffe) was characterized and provides evidence for the significant contribution of densification to the mechanical fields. Densification accounts for 27 % of the volume of the imprint for the studied glass, that is expected to be less sensitive to densification than amorphous silica or window glass. A major consequence is that indentation cracking methods for the evaluation of the fracture toughness, when they are based on volume conservation, as in the case of Hill-Eshelby plastic inclusion theory, are not suitable to glass. The onset for the formation of the subsurface lateral crack was also detected. The corresponding stress is ≈ 14 GPa and is in agreement with the intrinsic glass strength.Graphical abstractImage 1
  • High-temperature X-ray absorption spectroscopy study of thermochromic
           copper molybdate
    • Abstract: Publication date: Available online 21 June 2019Source: Acta MaterialiaAuthor(s): Inga Jonane, Andris Anspoks, Giuliana Aquilanti, Alexei Kuzmin X-ray absorption spectroscopy at the Cu and Mo K-edges was used to study the effect of heating on the local atomic structure and dynamics in copper molybdate (α-CuMoO4) in the temperature range from 296 to 973 K. The reverse Monte-Carlo (RMC) method was successfully employed to perform accurate simulations of EXAFS spectra at both absorption edges simultaneously. The method allowed us to determine structural models of α-CuMoO4 being consistent with the experimental EXAFS data. These models were further used to follow temperature dependencies of the local environment of copper and molybdenum atoms and to obtain the mean-square relative displacements for Cu–O and Mo–O atom pairs. Moreover, the same models were able to interpret strong temperature-dependence of the Cu K-edge XANES spectra. We found that the local environment of copper atoms is more affected by thermal disorder than that of molybdenum atoms. While the MoO4 tetrahedra behave mostly as the rigid units, a reduction of correlation in atomic motion between copper and axial oxygen atoms occurs upon heating. This dynamic effect seems to be the main responsible for the temperature-induced changes in the O2−→Cu2+ charge transfer processes and, thus, is the origin of the thermochromic properties of α-CuMoO4 upon heating above room temperature.Graphical abstractImage 1
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