Journal Cover Acta Materialia
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   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1359-6454
   Published by Elsevier Homepage  [3177 journals]
  • Impact of compositional gradients on selectivity of dissolvable support
           structures for directed energy deposited metals
    • Authors: Christopher S. Lefky; Brian Zucker; Abdalla R. Nassar; Timothy W. Simpson; Owen J. Hildreth
      Pages: 1 - 7
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Christopher S. Lefky, Brian Zucker, Abdalla R. Nassar, Timothy W. Simpson, Owen J. Hildreth
      Functionally gradient materials (FGM) have many important applications due to their ability to possess vastly different material properties across the gradient. Recent work on dissolvable supports in stainless steel components fabricated using directed energy deposition (DED) show the utility of exploiting differences in the corrosion suseptibility in FGM metals. In order to better control the feature resolution of DED dissolvable supports, it is first necessary to understand how dilution and mixing within the gradient impact the local corrosion susceptibility and etch rates. In this work, FGMs with varying numbers of tracks and layers were fabricated onto a 304 stainless steel build plate; first one to three layers of 91 carbon steel followed by one to ten layers of 431 stainless steel. Metallography, potentiodynamic polarization plots, energy dispersive x-ray spectroscopy, and 3D contact profilometry data were collected to show that mixing within the gradient is very inhomogeneous. Incomplete mixing is observed throughout individual tracks along with widely varying composition and material properties from track-to-track, even within a single track. This paper demonstrates that the impact of incomplete mixing and composition gradients within a layer must be considered for DED-fabricated FGM dissolvable supports.
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      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.009
      Issue No: Vol. 153 (2018)
       
  • Reversible correlation between subnanoscale structure and Cu content in
           co-evaporated Cu(In,Ga)Se2 thin films
    • Authors: Claudia S. Schnohr; Stefanie Eckner; Philipp Schöppe; Erik Haubold; Francesco d’Acapito; Dieter Greiner; Christian A. Kaufmann
      Pages: 8 - 14
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Claudia S. Schnohr, Stefanie Eckner, Philipp Schöppe, Erik Haubold, Francesco d’Acapito, Dieter Greiner, Christian A. Kaufmann
      Thin film solar cells based on Cu-poor Cu(In,Ga)Se2 absorbers grown by a three-stage co-evaporation process have been shown to strongly benefit from a Cu-rich intermediate growth stage and the presence of alkali elements such as Na, K, and Rb, pushing the current record efficiency to 22.6%. However, some details of the mechanisms underlying these improvements are not yet fully understood, particularly on a very local scale. We therefore used extended X-ray absorption fine structure spectroscopy to study element-specific local structural parameters of Cu(In,Ga)Se2 thin films with varying final Cu content, varying history of the Cu content, and varying alkali treatment. We find that the bulk structure on a subnanometer scale is unaffected by the presence of alkali elements, confirming that their beneficial effect mostly stems from improving the electronic properties of the material. In contrast, local structural disorder clearly increases with decreasing Cu content but does not depend on whether or not the material has passed through a Cu-rich intermediate stage. While the latter is known to improve film morphology and microstructure, it obviously has no effect on the subnanoscale structure, which exhibits a fully reversible correlation with the final Cu content of the Cu(In,Ga)Se2 absorber.
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      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.047
      Issue No: Vol. 153 (2018)
       
  • On spinodal decomposition in alnico - A transmission electron microscopy
           and atom probe tomography study
    • Authors: Lin Zhou; Wei Guo; J.D. Poplawsky; Liqin Ke; Wei Tang; I.E. Anderson; M.J. Kramer
      Pages: 15 - 22
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Lin Zhou, Wei Guo, J.D. Poplawsky, Liqin Ke, Wei Tang, I.E. Anderson, M.J. Kramer
      Alnico is a prime example of a finely tuned nanostructure whose magnetic properties are intimately connected to magnetic annealing (MA) during spinodal transformation and subsequent lower temperature annealing (draw) cycles. Using a combination of transmission electron microscopy and atom probe tomography, we show how these critical processing steps affect the local composition and nanostructure evolution with impact on magnetic properties. The nearly 2-fold increase of intrinsic coercivity (Hci) during the draw cycle is not adequately explained by chemical refinement of the spinodal phases. Instead, increased Fe-Co phase (α1) isolation, development of Cu-rich spheres/rods/blades and additional α1 rod precipitation that occurs during the MA and draw, likely play a key role in Hci enhancement. Chemical ordering of the Al-Ni-phase (α2) and formation of Ni-rich (α3) may also contribute. Unraveling of the subtle effect of these nano-scaled features is crucial to understanding on how to improve shape anisotropy in alnico magnets.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.042
      Issue No: Vol. 153 (2018)
       
  • Recrystallisation-assisted creep of an austenitic Fe-Ni alloy under low
           stresses after hot deformation
    • Authors: Minghao Zhang; Anne-Françoise Gourgues-Lorenzon; Esteban P. Busso; Haiwen Luo; Mingxin Huang
      Pages: 23 - 34
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Minghao Zhang, Anne-Françoise Gourgues-Lorenzon, Esteban P. Busso, Haiwen Luo, Mingxin Huang
      Static and meta-dynamic recrystallisation are known to take place in some steels under low tensile stresses during thermo-mechanical processes, e.g. between rolling stands in a hot strip finishing mill. The main objective of the present work is to understand the mechanisms responsible for the recrystallisation-assisted visco-plastic deformation in a hot worked iron-nickel austenitic alloy subjected to low stress levels at high temperatures. To that purpose, the recrystallisation kinetics of the alloy was first quantified from stress relaxation, interrupted creep and double-hit compression tests, followed by a microstructural study. The evolutions of strain both in the recrystallising and in the fully recrystallised microstructures were quantified. One of the most original aspects of this work is the experimental verification that the acceleration of the viscoplastic deformation of a recrystallising material is unequivocally and intrinsically associated with the on-going recrystallisation phenomenon itself. For the austenitic alloy of interest, the most important contribution to this deformation enhancement was found to arise from the deformation of newly recrystallised grains in the primary creep regime. Finally, it is also shown that this recrystallisation-assisted deformation can be qualitatively predicted with acceptable accuracy using a relatively simple, physics-inspired constitutive model.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.050
      Issue No: Vol. 153 (2018)
       
  • Microstructure and mechanical properties of Al-Mg-Zr alloys processed by
           selective laser melting
    • Authors: Joseph R. Croteau; Seth Griffiths; Marta D. Rossell; Christian Leinenbach; Christoph Kenel; Vincent Jansen; David N. Seidman; David C. Dunand; Nhon Q. Vo
      Pages: 35 - 44
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Joseph R. Croteau, Seth Griffiths, Marta D. Rossell, Christian Leinenbach, Christoph Kenel, Vincent Jansen, David N. Seidman, David C. Dunand, Nhon Q. Vo
      Gas-atomized powders of two ternary alloys, Al-3.60Mg-1.18Zr and Al-3.66Mg-1.57Zr (wt.%), were densified via laser powder bed fusion. At energy densities ranging from 123 to 247 J/mm3, as-fabricated components are near-fully densified (relative density 99.2–99.9%) as verified by X-ray tomography. While Mg acts a solid-solution strengthener, Zr creates two types of metastable L12 Al3Zr precipitates, each playing dual roles: (a) sub-micrometer Al3Zr particles form in the melt upon solidification and act as grain refining agents, nucleating fine aluminum grains, which (i) prevent hot-tearing during the rapid solidification inherent to laser melting and (ii) enhance tensile strength (Hall-Petch strengthening) and ductility (influence a heterogenous grain structure) after fabrication; (b) Al3Zr nano-precipitates form in the solid alloy during subsequent aging, which (i) precipitation-strengthen the alloy leading to an increase of >40% in strength over the as-fabricated value, and (ii) promote thermal stability of the fine grain size (and the associated Hall-Petch strengthening) after exposure to high temperature due to the slow kinetics of Al3Zr coarsening (from the sluggish diffusivity of Zr in solid Al-Mg). While the Zr-richer alloy shows higher yield and ultimate tensile strength in the as-fabricated state, both alloys have identical mechanical properties after peak aging. Interconnected bands of fine (∼0.8 μm), equiaxed, isotropic grains and coarser (∼1 × 10 μm), columnar, textured grains – both containing oxide particles and Al3Zr precipitates - provide a combination of high yield strength and high ductility (e.g., ∼354 MPa, and ∼20%, respectively) with isotropic values in both as-fabricated and peak-aged samples, unlike Al-Si alloys processed via laser fusion of commercial Al-Si-based powders. The pre-alloyed, gas-atomized Al-Mg-Zr powders do not contain expensive alloying elements such as Sc, nor do they require blending with a second powder to nucleate fine grains, making them excellent candidates for economical, large-scale additive manufacturing applications.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.053
      Issue No: Vol. 153 (2018)
       
  • Antiphase domains or dispersed clusters' Neutron diffraction study of
           coherent atomic ordering in Fe3Al-type alloys
    • Authors: Anatoly M. Balagurov; Ivan A. Bobrikov; Sergey V. Sumnikov; Igor S. Golovin
      Pages: 45 - 52
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Anatoly M. Balagurov, Ivan A. Bobrikov, Sergey V. Sumnikov, Igor S. Golovin
      A combination of high-resolution and in situ real-time neutron diffraction was used to collect the experimental data concerning coherent atomic ordering in Fe3Al-type alloys in a wide temperature range. The analysis of the obtained data was carried out within the framework of two models: antiphase domains and dispersed clusters embedded in matrix. The second model, for which the atomic ordering is organized in the form of clusters of mesoscopic sizes (from ∼200 to ∼400 Å) randomly distributed inside a less ordered matrix, provides a better fit of the data. For two chemically identical Fe3Al samples - the first was in the as-cast polycrystalline bulk state, and the second was grown as a single crystal - the initial state can be described as a partially ordered B2 structure (matrix) with dispersed clusters of the D03 ordered phase. The initial (quenched) state of the (Fe0.88Cr0.12)3Al polycrystalline sample is the disordered A2 phase with clusters of the partially ordered B2 phase. After heating and subsequent slow cooling, the structure of both binary and ternary alloys is B2 matrix with dispersed D03 clusters, whose dimensions are increased up to ∼900 Å.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.015
      Issue No: Vol. 153 (2018)
       
  • Compositional optimization of hard-magnetic phases with machine-learning
           models
    • Authors: Johannes J. Möller; Wolfgang Körner; Georg Krugel; Daniel F. Urban; Christian Elsässer
      Pages: 53 - 61
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Johannes J. Möller, Wolfgang Körner, Georg Krugel, Daniel F. Urban, Christian Elsässer
      Machine Learning (ML) plays an increasingly important role in the discovery and design of new materials. In this paper, we demonstrate the potential of ML for materials research using hard-magnetic phases as an illustrative case. We build kernel-based ML models to predict optimal chemical compositions for new permanent magnets, which are key components in many green-energy technologies. The magnetic-property data used for training and testing the ML models are obtained from a combinatorial high-throughput screening based on density-functional theory calculations. Our straightforward choice of describing the different configurations enables the subsequent use of the ML models for compositional optimization and thereby the prediction of promising substitutes of state-of-the-art magnetic materials like Nd2Fe14B with similar intrinsic hard-magnetic properties but a lower amount of critical rare-earth elements.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.03.051
      Issue No: Vol. 153 (2018)
       
  • The effect of zirconium on the omega phase in Ti-24Nb-[0–8]Zr (at.%)
           alloys
    • Authors: E.L. Pang; E.J. Pickering; S.I. Baik; D.N. Seidman; N.G. Jones
      Pages: 62 - 70
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): E.L. Pang, E.J. Pickering, S.I. Baik, D.N. Seidman, N.G. Jones
      Ti-Nb based β-Ti alloys are a promising new class of superelastic, shape-memory, and low-modulus materials for a wide range of applications. A critical phase in β-Ti alloys is the ω phase, which greatly affects the mechanical properties and superelastic/shape-memory behaviour of these materials. Zirconium, an important alloying constituent in many β-Ti alloys, is generally regarded as an ω suppressant, but the body of evidence supporting this view is unconvincing and includes a number of conflicting reports. In this article, the role of Zr on ω phase formation in Ti-Nb alloys is clarified using X-ray diffraction, transmission electron microscopy, and atom-probe tomography. Zirconium additions were found to suppress the formation of athermal ω phase upon quenching from high temperature. However, up to 8 at.% Zr additions to a Ti-24 at.% Nb alloy had little effect on the formation of isothermal ω phase following aging at 300 °C after 100 h. Furthermore, the isothermal ω precipitates were found to be strongly depleted in Nb but only weakly depleted in Zr. These results challenge the belief that Zr suppresses isothermal ω formation in β-Ti alloys, a result that is likely to be applicable beyond the Ti-Nb system considered here and information that can be used to assist in the design of future β-Ti alloys.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.016
      Issue No: Vol. 153 (2018)
       
  • Influences of interstitial and extrusion temperature on grain boundary
           segregation, Y−Ti−O nanofeatures, and mechanical properties of
           ferritic steels
    • Authors: Jae Bok Seol; Daniel Haley; David T. Hoelzer; Jeoung Han Kim
      Pages: 71 - 85
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Jae Bok Seol, Daniel Haley, David T. Hoelzer, Jeoung Han Kim
      Doping with interstitials influences the grain boundary (GB) composition of metallic alloys, enabling changes in elemental GB enrichment, grain size, and mechanical properties or even promoting nanoparticle formation. Yet, little efforts on these doping effects have been made in oxide dispersion-strengthened (ODS) steels. Here, by combining advanced microscopy techniques, we studied the impact of interstitial concentration and extrusion temperature on the GB structure-dependent solute segregation, Y−Ti−O nanofeatures, and mechanical properties of ferritic Fe–14Cr (wt%) ODS steels fabricated by ball milling. We found that doping with high carbon and oxygen contents causes the GB to be decorated with the interstitials and promotes nanoparticle formation along the GBs, thereby retarding capillary-driven grain coarsening. This effect performs twofold, through grain size refinement and particle hardening. For samples with low interstitial contents, altering the extrusion temperature does not significantly change the material's mechanical properties and microstructure or the nonstoichiometric chemistry of nanoparticles, which are highly stable at high temperatures. Further, for all the samples, Y–Al oxides in the initial precipitation stages rapidly become coarsened at high temperatures, as Al weakens the thermal stability of nanoparticles, thereby transforming them to core-shell structures with Y−Al-rich cores and Ti−O-rich shells in the later precipitation stages.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.046
      Issue No: Vol. 153 (2018)
       
  • Formation and autocatalytic nucleation of co-zone {101¯2} deformation
           twins in polycrystalline Mg: A phase field simulation study
    • Authors: H. Liu; F.X. Lin; P. Zhao; N. Moelans; Y. Wang; J.F. Nie
      Pages: 86 - 107
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): H. Liu, F.X. Lin, P. Zhao, N. Moelans, Y. Wang, J.F. Nie
      A phase-field model is developed to study the formation and autocatalytic nucleation of { 10 1 ¯ 2 } twins in polycrystalline Mg. The twins are found to nucleate most favourably in grains with the most negative interaction energy. Within such grains, the energetically most favoured nucleation site is determined by stresses concentrated near the grain boundaries that are related to the elastic anisotropy of the material. Furthermore, in a structure consisting of three lamellar grains with an incoming twin in the central grain, the simulation results show that before autocatalytic nucleation, the incoming twin often has a lenticular shape. The stress field around the tip of the incoming twin plays the major role in the autocatalytic nucleation. After a twin has nucleated in the neighbouring grain, the incoming and the outgoing twins evolve simultaneously, and the shape of the incoming twin gradually changes from lenticular to parallel-sided plate. Under the condition that the crystallographic orientation of the central grain and the applied strain remains unchanged, the driving force for twin nucleation decreases with increasing misorientation (up to 90°) across the grain boundary. It is further derived that the interaction energy values between the pre-existing stress field of the polycrystalline structure and the eigenstrain of the to-be-nucleated twin is mathematically related to the resolved shear stress of twins.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.043
      Issue No: Vol. 153 (2018)
       
  • Grain growth kinetics in submicrometer-scale molecular dynamics simulation
    • Authors: Shin Okita; Eisuke Miyoshi; Shinji Sakane; Tomohiro Takaki; Munekazu Ohno; Yasushi Shibuta
      Pages: 108 - 116
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Shin Okita, Eisuke Miyoshi, Shinji Sakane, Tomohiro Takaki, Munekazu Ohno, Yasushi Shibuta
      Grain growth kinetics under the anisotropic grain boundary properties is investigated by large-scale and long-time molecular dynamics (MD) simulations of contentious processes of nucleation, solidification and grain growth in a submicrometer-scale system. Microstructures obtained via homogeneous nucleation from undercooled melt iron consists of approximately 1500 grains and the number of grains decreases to one tenth of the number via the grain growth process. The grain growth exponent obtained from the MD simulation deviates from the ideal value since anisotropic effects in the grain boundary properties are inherently included in MD simulations. It is confirmed that the decrease of the reduced mobility (i.e., the product of the intrinsic grain boundary mobility and the grain boundary energy) is a dominant factor for the deviation from the ideal grain growth. The deviation from the Mackenzie function for the distribution of the disorientation angle between neighboring grains implies that the preferential selection of grain boundaries with small grain boundary energies occurs during the grain growth. This enhances the anisotropy in grain boundary properties and therefore decreases the reduced mobility of the grain boundary. Moreover, a multi-phase-field simulation starting from a MD configuration results in an ideal grain growth when a constant value of the reduced mobility is employed, which validates the discussion on the reduced mobility. The new insight in this study is achieved for the first time owing to a multi-graphics processing unit (GPU) parallel computation over 50 days for one case using 128 GPUs on the GPU-rich supercomputer.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.060
      Issue No: Vol. 153 (2018)
       
  • Two-phase dielectric polar structures in 0.1NBT-0.6ST-0.3PT solid
           solutions
    • Authors: Š. Svirskas; V.V. Shvartsman; M. Dunce; R. Ignatans; E. Birks; T. Ostapchuk; S. Kamba; D.C. Lupascu; J. Banys
      Pages: 117 - 125
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Š. Svirskas, V.V. Shvartsman, M. Dunce, R. Ignatans, E. Birks, T. Ostapchuk, S. Kamba, D.C. Lupascu, J. Banys
      In this work we address the peculiarities of the macroscopic responses in ternary 0.1Na0·5Bi0·5TiO3-0.6SrTiO3-0.3PbTiO3 (0.1NBT-0.6ST-0.3PT) solid solutions. These solid solutions exhibit a spontaneous first order relaxor to normal ferroelectric phase transition. The phase transition is accompanied by a broad dielectric relaxation which expands over 10 orders of magnitude in frequency just above the phase transition temperature. The temperature dependence of polarization shows that non-zero net polarization persists above the phase transition temperature. Below the phase transition temperature, it is not possible to describe the temperature dependence of polarization with a power law function which is valid in normal ferroelectrics. The piezoresponse force microscopy studies reveal that 0.1NBT-0.6ST-0.3PT solids solutions display several local polarization patterns which arise due to the bimodal distribution of grains in the ceramics. We associate the peculiar macroscopic responses to these complex polar structures and their different temperature behaviours.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.052
      Issue No: Vol. 153 (2018)
       
  • Effect of hafnium micro-addition on precipitate microstructure and creep
           properties of a Fe-Ni-Al-Cr-Ti ferritic superalloy
    • Authors: Sung-Il Baik; Michael J.S. Rawlings; David C. Dunand
      Pages: 126 - 135
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Sung-Il Baik, Michael J.S. Rawlings, David C. Dunand
      The Fe-Ni-Al-Cr (FBB8) ferritic alloy contains B2-ordered NiAl precipitates which, upon minor addition of Ti to the alloy, display L21-ordered Ni2TiAl sub-precipitates; this improves creep resistance, consistent with an increase in coherency strains from the hierarchical NiAl/Ni2TiAl precipitates. Here, we study the effect of a small addition of Hf (0.5 wt%) to FBB8-1.5wt.%Ti alloy on precipitate structure and creep properties. The main microstructural changes with Hf addition are relatively minor: (i) decrease of mean radius of Ni2TiAl/NiAl hierarchical precipitates from 84 ± 14 to 78 ± 13 nm, (ii) increase of volume fraction of these precipitates from 18 ± 2 to 19 ± 2%, (iii) increase of their number density from 9 ± 0.3 × 10−19 to 11 ± 0.3 × 10−19 m−3, and (iv) decrease of precipitate edge-to-edge distance from 187 ± 57 to 160 ± 48 nm. The larger volume fraction, higher number density and smaller edge-to-edge distance of B2/L21 precipitates in the FBB8-1.5Ti-0.5Hf alloys, are all favorable to higher creep resistance for a strengthening mechanism based on precipitate climb bypass. However, the threshold stress for creep at 700 °C decreases upon Hf addition to FBB8-1.5Ti, from 156 to 122 MPa. The lower creep resistance is explained by a decrease of the lattice misfit between the Hf-enriched B2-precipitates and the bcc-matrix, and by a decrease of the volume fraction of L21-Ni2TiAl sub-precipitates within B2-NiAl precipitates from 16 ± 4% to 10 ± 3% without and with Hf, respectively.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.044
      Issue No: Vol. 153 (2018)
       
  • Environmentally enhanced creep crack growth by grain boundary cavitation
           under cyclic loading
    • Authors: Jian-Feng Wen; Yu Liu; Ankit Srivastava; Ahmed Amine Benzerga; Shan-Tung Tu; Alan Needleman
      Pages: 136 - 146
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Jian-Feng Wen, Yu Liu, Ankit Srivastava, Ahmed Amine Benzerga, Shan-Tung Tu, Alan Needleman
      Plane strain finite element calculations of mode I crack growth are carried out under small scale creep conditions with an imposed stress intensity factor that is a prescribed cyclic function of time. The finite deformation analyses are based on a constitutive relation that couples creep deformation and damage due to grain boundary cavitation including a simple model of the embrittling effect of solute (oxygen) diffusion along grain boundaries. Isothermal analyses are carried out for two sets of material and grain boundary parameters: (i) parameter values representative of HASTELLOY® X; and (ii) parameter values representative of P91. A stronger detrimental effect of environmentally assisted grain boundary embrittlement is found for HASTELLOY® X than for P91. For a fixed imposed stress intensity factor range the detrimental effect is found to increase with increased hold time for HASTALLOY® X. The variation of the predicted cyclic crack growth rate with imposed stress intensity factor range is found to be in good quantitative agreement with experimental results in the literature for both HASTELLOY® X and P91. Paris law behavior, i.e. the cyclic crack growth rate depending on the imposed stress intensity factor range raised to a power, emerges naturally in the calculations. Parametric studies show that the cyclic crack growth rate and the Paris law exponent are more sensitive to variations in the grain boundary diffusivity, the solute diffusivity and a parameter characterizing the environmental embrittling effect than to parameters characterizing the creep response of the undamaged material. Also, an explicit analytical expression is found that gives a very good fit to the computed dependence of the cyclic crack growth rate on the solute diffusivity.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.034
      Issue No: Vol. 153 (2018)
       
  • Nanochannel structures in W enhance radiation tolerance
    • Authors: Wenjing Qin; Feng Ren; Russell P. Doerner; Guo Wei; Yawei Lv; Sheng Chang; Ming Tang; Huiqiu Deng; Changzhong Jiang; Yongqiang Wang
      Pages: 147 - 155
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Wenjing Qin, Feng Ren, Russell P. Doerner, Guo Wei, Yawei Lv, Sheng Chang, Ming Tang, Huiqiu Deng, Changzhong Jiang, Yongqiang Wang
      Developing high performance plasma facing materials (PFMs) is one of the greatest challenges for fusion reactors, because PFMs face unprecedented harsh environments including high flux plasma exposure, fast neutron irradiation and large transmutation gas. Tungsten (W) is considered as one of the most promising PFMs. Rapid accumulation of helium (He) atoms in such environments can lead to the He bubbles nucleation and even the formation of nano- to micro-scale “fuzz” on W surface, which greatly degrade the properties of W itself. The possible ejection of large W particulates into the core plasma can cause plasma instabilities. Here, we present a new strategy to address the root causes of bubble nucleation and “fuzz” formation by concurrently releasing He outside of W matrix through the nano-engineered channel structure (nanochannels). Comparing to ordinary bulk W, nanochannel W films with high surface-to-volume ratios are found to not only delay the growth of He bubbles, but also suppress the formation of “fuzz” (less than a half of the “fuzz” thickness formation in bulk W). Molecular dynamic (MD) simulation results elucidate that low vacancy formation energy and high He binding energy in the nanochannel surface effectively help He release and affect He clusters distribution in W during He ion irradiation.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.048
      Issue No: Vol. 153 (2018)
       
  • Role of applied stress level on the actuation fatigue behavior of NiTiHf
           high temperature shape memory alloys
    • Authors: O. Karakoc; C. Hayrettin; D. Canadinc; I. Karaman
      Pages: 156 - 168
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): O. Karakoc, C. Hayrettin, D. Canadinc, I. Karaman
      This study presents the actuation fatigue response of nano-precipitation hardened Ni50.3Ti29.7Hf20 high temperature shape memory alloy (HTSMA) undergoing thermal cycling between martensite and austenite under various tensile stress levels up to 500 MPa. Changes in fatigue life, and actuation and irrecoverable strains were monitored as a function of the number of cycles to failure. The experimental results revealed a consistent increase in actuation strain concomitant with the applied load at the expense of fatigue life. Significantly high number of cycles to failure were observed for this class of materials: specimens tested under 200 MPa achieved ∼21,000 cycles with the average actuation strain of ∼2.15% while those tested under 500 MPa experienced ∼2,100 cycles to failure with the average actuation strain of 3.22%. Fracture surface and crack density analyses revealed notable crack formation prior to failure at all stress levels. However, the rate of crack formation during repeated transformation increased with the applied stress. It was also demonstrated that the actuation fatigue lives of the present HTSMAs exhibit an almost perfect power law correlation with average actuation work output. The same work-based power law was shown to successfully capture actuation fatigue lives of several low temperature SMAs. Remarkably, the power law exponents for many SMAs were shown here to be either ∼ −0.5 or ∼ −0.8, which points out the likelihood of the existence of a universal empirical rule for actuation fatigue response of SMAs. The current findings constitute the first report on the applied stress - actuation fatigue interrelationship in NiTiHf HTSMAs.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.021
      Issue No: Vol. 153 (2018)
       
  • Modifying transformation pathways in high entropy alloys or complex
           concentrated alloys via thermo-mechanical processing
    • Authors: Bharat Gwalani; Stephane Gorsse; Deep Choudhuri; Mark Styles; Yufeng Zheng; Rajiv S. Mishra; Rajarshi Banerjee
      Pages: 169 - 185
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Bharat Gwalani, Stephane Gorsse, Deep Choudhuri, Mark Styles, Yufeng Zheng, Rajiv S. Mishra, Rajarshi Banerjee
      Often the experimentally-observed, single-phase high entropy alloy (HEA) is the result of second-phase precipitation constrained by thermodynamic and kinetic factors. Using Al0.3CoCrFeNi as a candidate HEA, this paper demonstrates the strong influence of thermo-mechanical processing on the transformation pathway adopted for isothermal second-phase precipitation. A traditional thermo-mechanical processing route comprised of homogenization cold-rolling solution treatment in the single fcc phase region, followed by a precipitation anneal at a lower temperature, results in a homogeneous distribution of nanometer scale-ordered L12 (gamma prime-like) precipitates within the fcc matrix. In contrast, if cold-rolling is followed directly by annealing at the precipitation temperature, then the resulting microstructural evolution pathway changes completely, with concurrent recrystallization of the matrix fcc grains and precipitation of B2 and sigma phases, largely at the grain boundaries. These experimentally observed variations in transformation pathway have been rationalized via the competition between the thermodynamic driving force and activation barrier for second-phase nucleation in this alloy, coupled with the kinetics of the process. The microstructural variations that result from these dramatically different phase transformation pathways can lead to some rather exceptional mechanical properties that can be varied over a large range even for a single Al0.3CoCrFeNi HEA composition.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.05.009
      Issue No: Vol. 153 (2018)
       
  • In situ observation of grain-boundary development from a facet-facet
           groove during solidification of silicon
    • Authors: Kuan-Kan Hu; Kensaku Maeda; Haruhiko Morito; Keiji Shiga; Kozo Fujiwara
      Pages: 186 - 192
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Kuan-Kan Hu, Kensaku Maeda, Haruhiko Morito, Keiji Shiga, Kozo Fujiwara
      The evolution of the facet-facet groove at grain boundaries in multi-crystalline Si during solidification was investigated by in situ observation of the melt/crystal interface. The grain boundaries changed their propagation direction without any new grain formation or grain boundaries interaction during crystal growth. We monitored the melt/crystal interface over time and carefully estimated the growth velocities on two facets. We found that the facet velocities are different in some of our experimental observations and the development of random grain boundary is strongly dependent on the facet velocities. On the basis of our experimental observations, we discussed the direction of random grain boundary during solidification by considering the growth velocities on two facets.
      Graphical abstract image

      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.062
      Issue No: Vol. 153 (2018)
       
  • Origin of hydrogen trapping site in vanadium carbide precipitation
           strengthening steel
    • Authors: Jun Takahashi; Kazuto Kawakami; Yukiko Kobayashi
      Pages: 193 - 204
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Jun Takahashi, Kazuto Kawakami, Yukiko Kobayashi
      We successfully achieved atomic scale visualization of hydrogen atoms at trapping sites associated with the nano-sized precipitates of titanium carbide and vanadium carbide in ferritic steels using a modified three-dimensional atom probe with a deuterium charge cell. We proposed that the hydrogen trapping sites of fine carbide precipitates were at the (001) broad interface between the precipitate and the ferrite matrix. In this study, the precipitate size dependence in the trapping site and its energy was systematically investigated in vanadium carbide precipitation strengthening steels with various aging times. Hydrogen thermal desorption spectrometry analysis showed that the hydrogen trapping energy in the peak- and over-aged steels was larger than that in the under-aged steel. Atom probe tomography analysis showed the {001} platelets of vanadium carbide were covered by charged deuterium atoms in the peak-aging steel with large trapping energy, whereas no deuterium atoms around the {001} platelets were observed in the under-aged steel with small trapping energy. High-resolution transmission electron microscopy observation showed that misfit dislocations hardly appeared on the (001) surface of the precipitates in the two steels with large and small trapping energies. In contrast, the vanadium–carbon atomic ratios of vanadium carbide precipitates were definitely different between the two steels with large and small trapping energies. The precipitates in the under-aged steel with small trapping energy showed a chemical composition similar to VC, whereas the precipitates in the peak- and over-added steels with large trapping energy showed a chemical composition similar to V4C3. These results suggested that the origin of the hydrogen trapping site with large trapping energy is not the misfit dislocation core but the carbon vacancy on the (001) broad surface of V4C3 precipitates.
      Graphical abstract image

      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.05.003
      Issue No: Vol. 153 (2018)
       
  • Effect of oxygen partial pressure on grain-boundary transport in alumina
    • Authors: Yan Wang; Helen M. Chan; Jeffrey M. Rickman; Martin P. Harmer
      Pages: 205 - 213
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Yan Wang, Helen M. Chan, Jeffrey M. Rickman, Martin P. Harmer
      The oxidation of the bond coat in a thermal barrier coating results in the formation of a protective alumina scale. Such scales experience large gradients in oxygen partial pressure ( P O 2 ) during formation, but it is unclear how P O 2 affects grain-boundary (GB) transport in alumina. To study the effect of P O 2 on GB kinetics in alumina, the rate of oxygen penetration was measured at 1400 °C over a wide range of P O 2 . This was accomplished using a novel reduction experiment using nickel aluminate spinel particles. The results indicated that the dependence of the boundary transport on P O 2 is mainly due to the driving force, namely the gradient in P O 2 across the alumina. Point-defect models were then invoked to interpret the measurements, and it was found that transport in an alumina scale is controlled by extrinsic defects.
      Graphical abstract image

      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.028
      Issue No: Vol. 153 (2018)
       
  • Phase prediction in high entropy alloys – A kinetic approach
    • Authors: C. Chattopadhyay; Anil Prasad; B.S. Murty
      Pages: 214 - 225
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): C. Chattopadhyay, Anil Prasad, B.S. Murty
      A simple and completely predictive model has been developed to predict whether a multicomponent equiatomic alloy will form a single phase BCC, FCC, HCP or a combination of two or more solid solution phases or intermetallic compounds (IM) or an amorphous phase. This approach is based on the viscosity of alloys as a function of temperature, utilising the viscosities of its constituting elements, and suitably incorporating the crystal structure information. Some other parameters affecting viscosity of an alloy like atomic size of constituting elements, packing density of the unit cell, etc., are suitably incorporated into the model. The temperature-time-transformation (TTT) diagrams were generated with the help of the viscosity data of five widely experimentally examined alloys, CoCrCuFeNi, CoCrFeMnNi, AlCoCrFeNi, AlCuMgMnZn and ZrTiCuNiBe. The chance of formation of preferable lower order alloys has also been considered. In this regard, all the possible binary to quinary alloys that can form from the constituting elements have been studied. The formation of the single phase BCC, FCC, or formation of multi phases, IMs or an amorphous phase in these alloys has been excellently predicted by the model. It has also been revealed that AlCuMgMnZn alloy prefers to form a number of IMs with a rare HCP phase, which matches excellently with the experimental evidence. The most important part of the present work is that it acts as an efficient guide about the processing route that should be used to form an intended phase in a particular alloy via the critical cooling rate R c obtained through the predicted TTT diagrams. Further, two alloys (AlCoCrFeNi and CoCrFeMnNi) could not be vitrified even via melt spinning route as predicted by the model. Almost all the equiatomic alloys found so far ranging from ternary to octanary according to literature have been studied by the present model. The phase formation in most of the alloys has been predicted correctly by the model.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.05.002
      Issue No: Vol. 153 (2018)
       
  • On the mechanism of sodium migration in transition aluminas with
           calcination
    • Authors: Grant J. McIntosh; Hasini Wijayaratne; Andrew Chan; Linus Perander; Margaret Hyland
      Pages: 226 - 234
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Grant J. McIntosh, Hasini Wijayaratne, Andrew Chan, Linus Perander, Margaret Hyland
      We have characterized unwashed agglomerate Bayer gibbsite, a major precursor for industrial-grade and specialty aluminas, and the transition aluminas/corundum formed on calcination to follow the migration of sodium with phase evolution. Sodium is found initially to be intercalated, likely in the OH-rich planes in hydroxide precursors, with Na 1s XPS spectra demonstrating much becomes incorporated into the alumina structure with calcination. 23Na-NanoSIMS, XRF, and XPS measurements reveal sodium then migrates from within the crystallites to accumulate in cracks, interfaces or defects and, eventually, at the grain exterior during calcination without significant losses. Changes in pore structure with acid leaching and subsequent re-calcination suggests sodium occupies surface Al-O-Na environments analogous to surface hydroxide, and that near-surface bulk sites act as a reservoir to repopulate lost surface sodium. These bulk sites are argued to be the alumina cation vacancies, the distribution and occupancy of which is still debated despite implications for bulk and surface chemistry properties, a contention strongly supported by XPS and Na K-edge NEXAFS spectra. This indicates that, while surface segregation is thermodynamically favoured, bulk incorporation of sodium occurs for kinetic reasons. We tentatively suggest that bulk hydrogen may similarly be present in activated transition aluminas due to analogous kinetic considerations.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.049
      Issue No: Vol. 153 (2018)
       
  • Role of layered structure in ductility improvement of layered Ti-Al metal
           composite
    • Authors: Meng Huang; Chao Xu; Guohua Fan; Emad Maawad; Weimin Gan; Lin Geng; Fengxiang Lin; Guangze Tang; Hao Wu; Yan Du; Danyang Li; Kesong Miao; Tongtong Zhang; Xuesong Yang; Yiping Xia; Guojian Cao; Huijun Kang; Tongmin Wang; Tiqiao Xiao; Honglan Xie
      Pages: 235 - 249
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Meng Huang, Chao Xu, Guohua Fan, Emad Maawad, Weimin Gan, Lin Geng, Fengxiang Lin, Guangze Tang, Hao Wu, Yan Du, Danyang Li, Kesong Miao, Tongtong Zhang, Xuesong Yang, Yiping Xia, Guojian Cao, Huijun Kang, Tongmin Wang, Tiqiao Xiao, Honglan Xie
      Layered Ti-Al metal composite (LMC) was designed and fabricated by hot-rolling and annealing of pure Ti and Al sheets. The as-prepared composite exhibits high tensile ductility, being superior to any individual Ti or Al sheets. The stress/strain evolution and fracture behavior of the LMC were analyzed by in-situ observations during the tensile deformation. Three deformation stages of LMC were clearly observed by neutron diffraction: elastic stage, elastic-plastic stage and plastic stage. It is found that stress partitioning at the elastic-plastic deformation stage improves the strain balance of LMC, but leads to an internal stress accumulated at the interface. Additionally, a strain-transfer from Ti to adjacent Al layers relieves the strain localization of Ti layers in LMC, which improves the ductility of Ti. Both stress partitioning and strain localization of Ti layers facilitate the nucleation of cracks at a low macro strain. However, the crack propagation is constrained by layered structure. In terms of the Al layers, the constrained micro-cracks relieve the stress concentration in Al layer and improve the ductility of Al layers, so that cracking indirectly affects the plastic deformation behavior of LMC, then improving its entire ductility. This work provides a new structural strategy towards simultaneously improving strength and ductility to develop high performance LMC by structural design.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.05.005
      Issue No: Vol. 153 (2018)
       
  • Hydrogen diffusion in titanium dihydrides from first principles
    • Authors: I.I. Novoselov; A.V. Yanilkin
      Pages: 250 - 256
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): I.I. Novoselov, A.V. Yanilkin
      Transition metal hydrides are of great practical interest due to high volume concentration of hydrogen. Titanium dihydride is a typical representative of such materials. Safe and reliable application of the dihydride requires established values of hydrogen diffusion rates. Experimental studies of the matter provide somewhat controversial results, therefore we investigate the process from first principles. We calculate hydrogen diffusion rates in representative compounds ( T i H 1.75 and T i H 2.0 ) and demonstrate that the dominant diffusion mechanism depends on temperature and composition.
      Graphical abstract image

      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.059
      Issue No: Vol. 153 (2018)
       
  • Multiresolution mechanical characterization of hierarchical materials:
           Spherical nanoindentation on martensitic Fe-Ni-C steels
    • Authors: Ali Khosravani; Lutz Morsdorf; Cemal Cem Tasan; Surya R. Kalidindi
      Pages: 257 - 269
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Ali Khosravani, Lutz Morsdorf, Cemal Cem Tasan, Surya R. Kalidindi
      Systematic length scale studies are required for understanding effects of microstructural features that determine the mechanical properties of hierarchical materials. Recent advances in spherical indentation stress-strain protocols have made it possible to characterize the local mechanical responses at different length scales, from hundreds of nanometers to hundreds of microns. In this paper, two model martensitic steels Fe-5.1Ni-0.13C (wt.%) and Fe-5.0Ni-0.30C (wt.%) with different carbon contents were investigated using spherical nanoindentation stress-strain curves to quantify the mechanical behavior of lath martensite at multiple length scales using different spherical indenter tip radii. The indentation yield strength is dominated by the nanoscale defect structure for all indenter radii (1 μm, 16 μm and 100 μm) and does not exhibit any discernible size effect in the measured yield strengths at different length scales. The work hardening rates measured in the indentation tests at the different length scales coincide until the indentation zones grow large enough, so that a significant increase of work hardening occurs which is attributed to the presence of high-angle block boundaries in the indentation probed volumes. Characteristic pop-ins were observed in the indentations performed with the 1 μm and 16 μm indenter tip sizes and have been attributed to the interaction of dislocations with lath boundaries and their eventual transmission. In addition, the correlations between the properties measured from these indentation protocols and those measured in uniaxial tensile tests are critically examined.
      Graphical abstract image

      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.063
      Issue No: Vol. 153 (2018)
       
  • In situ observation on formation process of nanoscale cracking during
           tension-compression fatigue of single crystal copper micron-scale specimen
           
    • Authors: Takashi Sumigawa; Kim Byungwoon; Yuki Mizuno; Takuma Morimura; Takayuki Kitamura
      Pages: 270 - 278
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Takashi Sumigawa, Kim Byungwoon, Yuki Mizuno, Takuma Morimura, Takayuki Kitamura
      This study aims to develop an in situ observation technique of micron-scale metal specimens under fully-reversed tension-compression cyclic loading and to investigate the unique cracking process in low-cycle fatigue. The fatigue experiment was conducted on a single crystal copper micron-scale specimen under a constant displacement amplitude. While crystallographic slip spread over the entire test section during the tensile first half-cycle, a locally concentrated slip band appeared during reverse loading (compression). In situ observations using scanning electron microscopy revealed that crystallographic slip occurred only near the localized band in consecutive cycles, and strain localization due to slip bands led to nanoscale extrusion/intrusion at the surface. This indicates that, unlike during the fatigue of a typical bulk pure metal, no characteristic dislocation substructure was formed during the extrusion/intrusion process. The extrusion/intrusion was the cause of failure of the micron-scale metal specimen in fatigue. Moreover, the process required a much higher stress amplitude than in the case of bulk copper specimen. Thus, the fatigue cracking process for copper micro-scale specimen differs significantly from that for bulk copper specimen.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.061
      Issue No: Vol. 153 (2018)
       
  • Mechanical properties and optimal grain size distribution profile of
           gradient grained nickel
    • Authors: Y. Lin; J. Pan; H.F. Zhou; H.J. Gao; Y. Li
      Pages: 279 - 289
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Y. Lin, J. Pan, H.F. Zhou, H.J. Gao, Y. Li
      Gradient structured (GS) materials are ubiquitous in biological systems and now increasingly adopted in engineering systems to achieve desirable combinations of mechanical properties. However, how to control and characterize the gradient structure still remains challenging. In the present work, pure Ni samples possessing a gradient structure with a change in the grain size up to three orders of magnitude from 29 nm to 4 μm are prepared by electrodeposition, where the degree of grain size gradient is accurately controlled. The GS Ni samples exhibit a favorable combination of high strength and high ductility. An optimal grain size distribution profile is discovered which gives rise to a yield strength of 460 MPa and a uniform elongation of 8.9%, the latter even better than that of the coarse-grained Ni. Experimental observations and molecular dynamics (MD) simulations reveal that the surface roughening of coarse grains and strain localization of nano-grains can be effectively suppressed by the mutual constraint between nano-grains and coarse grains, leading to the observed superior uniform elongation. This work not only reports a promising methodology of producing materials possessing both high strength and high ductility, but also provides a model for investigating the deformation mechanisms in GS materials.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.065
      Issue No: Vol. 153 (2018)
       
  • The effect of phase chemistry on the extent of strengthening mechanisms in
           model Ni-Cr-Al-Ti-Mo based superalloys
    • Authors: A.J. Goodfellow; E.I. Galindo-Nava; K.A. Christofidou; N.G. Jones; C.D. Boyer; T.L. Martin; P.A.J. Bagot; M.C. Hardy; H.J. Stone
      Pages: 290 - 302
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): A.J. Goodfellow, E.I. Galindo-Nava, K.A. Christofidou, N.G. Jones, C.D. Boyer, T.L. Martin, P.A.J. Bagot, M.C. Hardy, H.J. Stone
      The exceptional mechanical properties of polycrystalline nickel-based superalloys arise through various concurrent strengthening mechanisms. Whilst these mechanisms are generally understood, consensus has yet to be established on the precise contribution of each to the overall alloy strength. Furthermore, changes in alloy chemistry influence several different mechanisms, making the assessment of individual alloying elements complex. In this study, a series of model quinary Ni-based superalloys has been investigated to systematically study the effect of varying Mo content on the contributing strengthening mechanisms. Using microstructural data, the yield strength was modelled by summing the individual effects of solid solution in both the γ and γʹ phases, coherency, grain boundary and precipitation strengthening. The total predicted yield stress increased with Mo content despite the diminishing contribution of precipitation strengthening. It is shown that solid solution strengthening of the ordered γʹ precipitate phase is a key contributor to the overall strength, and that variations in composition between the tertiary and secondary γʹ lead to significant changes in mechanical properties that should be accounted for in models of alloy strength.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.064
      Issue No: Vol. 153 (2018)
       
  • Fundamental aspects about the first steps of irradiation-induced phase
           transformations in fluorite-related oxides
    • Authors: Bertrand Lacroix; Rolly J. Gaboriaud; Fabien Paumier
      Pages: 303 - 313
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Bertrand Lacroix, Rolly J. Gaboriaud, Fabien Paumier
      Fundamental aspects of ion-irradiation-induced phase transformations in fluorite-related oxides, from cubic to monoclinic structures, are investigated through xenon implantation at medium energy in yttrium sesquioxide (Y2O3). For that purpose, an original approach considering this material fabricated as a thin film is proposed to control the pristine structure before irradiation and to allow fine measurements of internal stresses. It is evidenced that this oxide can be strongly disordered up to an unexpected amorphization, depending on the initial damage state. We emphasize that point defects created during the irradiation process generate strong internal stresses which turn out to be a major parameter in the mechanism involved during such phase transition. When the oxide structure is initially weakly disordered, the evolutions of the biaxial and triaxial residual stresses, together with the lattice parameter, presents two different steps. Those steps are interpreted at the atomic scale by the nucleation of oxygen vacancy aggregates which collapse in extended dislocation loops that are considered as monoclinic nuclei. The geometry of these extended defects has been considered to explain why the monoclinic phase appears with clearly defined crystallographic orientation relationship (4 0 2 ¯ ) B //(2 2 2) C relative to the initial cubic phase. The link between the energy deposited by irradiation and the critical size of these defects, analyzed from a thermodynamical approach by the evaluation of the Gibbs free energy, also elucidates the presence of sharp thresholds observed between the different phases (amorphous/cubic and monoclinic/amorphous) along the depth of the film.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.04.058
      Issue No: Vol. 153 (2018)
       
  • Design of crystalline-amorphous nanolaminates using deformation mechanism
           maps
    • Authors: Bin Cheng; Jason R. Trelewicz
      Pages: 314 - 326
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Bin Cheng, Jason R. Trelewicz
      The deformation behavior of crystalline-amorphous nanolaminates is controlled by a confluence of mechanisms involving dislocation and grain boundary (GB) plasticity in the crystalline phase, shear transformation zone (STZ) plasticity and its localization into shear bands in the amorphous phase, and their coupling across the amorphous-crystalline interface. Leveraging molecular dynamics simulations, the influence of microstructural length scales on the mechanical behavior is quantified using deformation mechanism maps, which provide mechanistic insights into the scaling of mechanical properties with layer thickness ratio and grain size. We find that flow stress primarily scales with the relative phase fraction as deformation shifts toward STZ dominated plasticity with increasing amorphous layer thickness while the onset of plasticity is also influenced by grain size due to the role of GB plasticity in yielding. Toughness limiting mechanisms involve void formation at GBs and shear localization in the amorphous layer, where the former is attributed to dislocation-GB interactions during mixed mode deformation and the latter to dislocation slip bands biasing the process of shear localization. An Ashby plot representation combining flow stress with void and shear band localization factors demonstrates that configurations with microstructural length scales promoting cooperative strain accommodation through the coupling of dislocation, GB, and STZ plasticity exhibited limited strain localization while retaining a high flow stress, thus providing a mechanistic basis for microstructural design of crystalline-amorphous nanolaminates.
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      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.05.006
      Issue No: Vol. 153 (2018)
       
  • Effect of solutes on ideal shear resistance and electronic properties of
           magnesium: A first-principles study
    • Authors: P. Garg; I. Adlakha; K.N. Solanki
      Pages: 327 - 335
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): P. Garg, I. Adlakha, K.N. Solanki
      Solution strengthening or softening is an effective way to enhance mechanical properties, especially in magnesium based alloys due to their inability to activate adequate non-basal deformation mechanisms at the room temperature. Hence, using first-principles calculations, the effects of several different alloying elements on the ideal shear resistance across various slip systems of Mg were investigated. The results reveal that the addition of a Ce or Zr solute atom decreases the ideal shear resistance (softening); whereas, the substitution of a Sn, Li or Zn atom increases the ideal shear resistance of Mg (strengthening). The dominant slip system in Mg was found to change from the basal partial (0001) [ 10 1 ¯ 0 ] to prismatic (10 1 ¯ 0)[11 2 ¯ 0] with the addition of a Ce or Zr solute atom; whereas, the addition of a Sn, Li or Zn solute atom had negligible effect on the plastic anisotropy. Furthermore, the electronic density of states and valence charge transfer, which provides a quantum mechanical insight into the underlying factors influencing the observed softening/strengthening behavior, was probed. For instance, the electronic density of states calculations show that the contribution from d states of Ce and Zr solute atoms decreases the electronic structure stability of their respective solid solution, thereby enhancing slip activities. In the end, theoretical analyses were performed and a shearability parameter was introduced to understand the implications of the observed variation in ideal shear resistance on the macroscopic behavior of Mg alloys.
      Graphical abstract image

      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.05.014
      Issue No: Vol. 153 (2018)
       
  • Grain boundary mediated plasticity: On the evaluation of grain boundary
           migration - shear coupling
    • Authors: Konstantin D. Molodov; Dmitri A. Molodov
      Pages: 336 - 353
      Abstract: Publication date: July 2018
      Source:Acta Materialia, Volume 153
      Author(s): Konstantin D. Molodov, Dmitri A. Molodov
      The solutions of the Frank-Bilby equation for the dislocation content of symmetric tilt grain boundaries were analyzed for a number of grain and twin boundaries. Based on this analysis, the geometrical model of grain boundary migration - shear coupling was extended to evaluate the coupling factor (amount of shear) for twin and grain boundaries in various crystal structures. The coupling factors calculated according to the proposed general formula are in excellent agreement with respective experimental and simulation data known from literature. For a given grain/twin boundary the coupling factor was found to result from a combination of two elementary coupling modes, the balance between which is constituted by an introduced weighting factor. Hence, the coupling factor (or the twinning shear) depends not only on the tilt angle of the grain/twin boundary, but also on the weighting factor, which may vary for crystallographically equivalent boundaries. With respect to deformation twinning, the weighting factor is intimately linked to the amount and direction of twinning shear produced by the boundary. A variation of the weighting factor for the same boundary can be interpreted as a result of a structural change of the grain/twin boundary, i.e. grain boundary phase transformation.
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      PubDate: 2018-05-18T14:01:59Z
      DOI: 10.1016/j.actamat.2018.04.057
      Issue No: Vol. 153 (2018)
       
  • Efficient exploration of the High Entropy Alloy composition-phase space
    • Authors: A. Abu-Odeh; E. Galvan; T. Kirk; H. Mao; Q. Chen; P. Mason; R. Malak; R. Arróyave
      Pages: 41 - 57
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): A. Abu-Odeh, E. Galvan, T. Kirk, H. Mao, Q. Chen, P. Mason, R. Malak, R. Arróyave
      High Entropy Alloys (HEAs), Multi-principal Component Alloys (MCA), or Compositionally Complex Alloys (CCAs) are alloys that contain multiple principal alloying elements. While many HEAs have been shown to have unique properties, their discovery has been largely done through costly and time-consuming trial-and-error approaches, with only an infinitesimally small fraction of the entire possible composition space having been explored. In this work, the exploration of the HEA composition space is framed as a Continuous Constraint Satisfaction Problem (CCSP) and solved using a novel Constraint Satisfaction Algorithm (CSA) for the rapid and robust exploration of alloy thermodynamic spaces. The algorithm is used to discover regions in the HEA Composition-Temperature space that satisfy desired phase constitution requirements. The algorithm is demonstrated against a new (TCHEA1) CALPHAD HEA thermodynamic database. The database is first validated by comparing phase stability predictions against experiments and then the CSA is deployed and tested against design tasks consisting of identifying not only single phase solid solution regions in ternary, quaternary and quinary composition spaces but also the identification of regions that are likely to yield precipitation-strengthened HEAs.

      PubDate: 2018-04-23T08:53:02Z
      DOI: 10.1016/j.actamat.2018.04.012
      Issue No: Vol. 152 (2018)
       
  • Revisiting building block ordering of long-period stacking ordered
           structures in Mg–Y–Al alloys
    • Authors: H. Zhang; C.Q. Liu; Y.M. Zhu; H.W. Chen; L. Bourgeois; J.F. Nie
      Pages: 96 - 106
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): H. Zhang, C.Q. Liu, Y.M. Zhu, H.W. Chen, L. Bourgeois, J.F. Nie
      Long-period stacking-ordered (LPSO) structures in Mg alloys have received considerable attention in the past decade due to their unique crystallographic features, but their precise structures are still not fully established. In this work, the 18R LPSO structure in two Mg–Y–Al alloys, in both cast and homogenized conditions, is systematically investigated using atomic-scale high-angle annular dark-field scanning transmission electron microscopy and density functional theory. Our observations reveal that there exist at least four types of building clusters—three of them being metastable and hitherto unreported—in the 18R structure in the as-cast condition. Each of the four types of building clusters comprises three Al–Y cubes that are connected by one Mg–Y cube. After annealing at 530–550 °C for 47–48 h, the three metastable building clusters transform to a stable configuration that is characterized by the Al6Y8 L12-type building cluster. These transformations are validated by first-principles calculations. Based on the experimental observations and computational results, the evolution of the building clusters in the 18R structure is elaborated.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
      DOI: 10.1016/j.actamat.2018.04.010
      Issue No: Vol. 152 (2018)
       
  • 3D multi-layer grain structure simulation of powder bed fusion additive
           manufacturing
    • Authors: Johannes A. Koepf; Martin R. Gotterbarm; Matthias Markl; Carolin Körner
      Pages: 119 - 126
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Johannes A. Koepf, Martin R. Gotterbarm, Matthias Markl, Carolin Körner
      In powder bed fusion (PBF) additive manufacturing, powder layers are locally melted with a laser or an electron beam to build a component. Hatching strategies and beam parameters as beam power, scan velocity and line offset significantly affect the grain structure of the manufactured part. While experiments reveal the result of specific parameter combinations, the precise impact of distinct parameters on the resulting grain structure is widely unknown. This knowledge is necessary for a reliable prediction of the microstructure and consequently the mechanical properties of the manufactured part. We introduce the adaption of a three-dimensional model for the prediction of dendritic growth for use with PBF. The heat input is calculated using an analytical solution of the transient heat conduction equation. Massively parallel processing on a high-performance cluster computer allows the computation of the grain structure on the scale of small parts within reasonable times. The model is validated by accurately reproducing experimental grain structures of Inconel 718 test specimens manufactured by selective electron beam melting. The grain selection zone within the first layers as well as the subsequent microstructure in several millimeters build height is modeled in unprecedented level of detail. This model represents the cutting-edge of grain structure simulation in PBF and enables a reliable numerical prediction of appropriate beam parameters for arbitrary applications.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
      DOI: 10.1016/j.actamat.2018.04.030
      Issue No: Vol. 152 (2018)
       
  • The influence of nanoparticles on dendritic grain growth in Mg alloys
    • Authors: Enyu Guo; Sansan Shuai; Daniil Kazantsev; Shyamprasad Karagadde; A.B. Phillion; Tao Jing; Wenzhen Li; Peter D. Lee
      Pages: 127 - 137
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Enyu Guo, Sansan Shuai, Daniil Kazantsev, Shyamprasad Karagadde, A.B. Phillion, Tao Jing, Wenzhen Li, Peter D. Lee
      Melt processing offers a cost effective method for producing metal matrix nanocomposite (MMNC) components; however, the influence of nanoparticles on the evolving microstructure during solidification is still not well understood. In this study, the effect of SiC nanoparticles on α-Mg dendrite evolution in a Mg-25Zn-7Al (wt.%) alloy was investigated through 4D (three dimensions plus time) synchrotron tomographic quantification of solidification experiments conducted at different cooling rates with and without nanoparticles. Key features of the solidifying primary α-Mg dendritic grains were quantified, including grain morphology, size distribution, and dendrite tip velocity. To obtain the high-contrast tomography dataset necessary for structure quantification, a new image reconstruction and processing methodology was implemented. The results reveal that the addition of nanoparticles increases grain nucleation whilst restricting dendritic growth and altering the dendritic grain growth morphology. Using LGK model calculations, it is shown that these changes in solidification microstructure occur as a result of nanoparticle-induced restriction in Zn's effective diffusivity ahead of the dendrite tips, reducing tip velocity. The results both suggest the key phenomena required to be simulated when numerically modelling solidifying Mg-based MMNC and provide the data required to validate those models.
      Graphical abstract image

      PubDate: 2018-04-23T08:53:02Z
      DOI: 10.1016/j.actamat.2018.04.023
      Issue No: Vol. 152 (2018)
       
  • Sacrificing trap density to achieve short-delay and high-contrast
           mechanoluminescence for stress imaging
    • Authors: Jun-Cheng Zhang; Xin-Hua Fan; Xu Yan; Feng Xia; Weijin Kong; Yun-Ze Long; Xusheng Wang
      Pages: 148 - 154
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Jun-Cheng Zhang, Xin-Hua Fan, Xu Yan, Feng Xia, Weijin Kong, Yun-Ze Long, Xusheng Wang
      Trap-controlled mechanoluminescence (ML) enables the direct observation of stress concentration of load-bearing objects through imaging the ML distribution, showing numerous prospects in stress detection, bio-imaging and optical displays. However, the applications of trap-controlled ML materials universally require long-time delay to fade the noise of symbiotic persistent luminescence (PersL) in order to achieve high-contrast ML images. In view of the difficulty to solve the PersL problem through individually eliminating the PersL traps, herein we propose a novel strategy of sacrificing trap density which decreases PersL and ML traps as a whole. By employing Sr2+ substitution to decrease the trap density of Ca2Nb2O7:Pr3+, we identify a novel composition of (Ca0.5Sr0.5)2Nb2O7:Pr3+ displaying short-delay and high-contrast ML images, and evaluate its practicability through a 2-dimensional in-situ imaging experiment of dynamic stress distribution. The underlying mechanism is ascribed to the greater decrease ratio of PersL intensity than ML intensity as a result of the larger detrapping rate of traps due to stress (leading to ML) than that due to thermal energy (PersL). Furthermore, multi-spectral investigations of (Ca,Sr)2Nb2O7:Pr3+ system reveal a distinctive electron transition process co-regulated by trap levels, charge transfer state and crystal field. The proposed strategy and the associated phosphors are expected to initiate the reconstruction of PersL-type ML materials and bring important implications for real-world stress imaging.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.011
      Issue No: Vol. 152 (2018)
       
  • Fabrication of the porous MnCo2O4 nanorod arrays on Ni foam as an advanced
           electrode for asymmetric supercapacitors
    • Authors: Jiasheng Xu; Yudong Sun; Mingjun Lu; Lin Wang; Jie Zhang; E. Tao; Jianhua Qian; Xiaoyang Liu
      Pages: 162 - 174
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Jiasheng Xu, Yudong Sun, Mingjun Lu, Lin Wang, Jie Zhang, E. Tao, Jianhua Qian, Xiaoyang Liu
      In this work, the porous MnCo2O4 nanorod arrays on three-dimensional Ni foam (PMCN@NF) as electrode material have been fabricated through a mild co-precipitating reaction at room temperature followed by a subsequent thermal treatment. The PMCN@NF electrode material have been characterized by XRD, SEM, TEM, EDS-mapping, BET and XPS technologies. The capacitive performances of the PMCN@NF electrode have been investigated by CV, GCD and EIS. This porous structure possesses an average diameter of ∼6.7 nm. BET interface area is measured to be 105.6 m2 g−1 for this PMCN@NF electrode. This PMCN@NF electrode exhibits the good capacitance of 845.6 F g−1 (tested condition: 1 ampere per gram, 1 A g−1). After 2000 cycles test, it shows a 90.2% retention for specific capacitance of the first test. The MnCo2O4//rGO asymmetric supercapacitor with the stable opening voltage of 1.6 V delivers a maximum energy density of 53.7 Wh kg−1 (when the power density is 1600 W kg−1). When this power density up to 8000 W kg−1, it still achieves an energy density of 31.6 Wh kg−1. The cyclic stability of this device after 5000 cycles can achieve an initial capacitance retention of 82.0%. These results indicate that the porous MnCo2O4 nanorod arrays electrode material is a promising functional material for advanced energy conversion and storage equipment.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.025
      Issue No: Vol. 152 (2018)
       
  • Effect of heat treatment on the microstructural evolution of a
           nickel-based superalloy additive-manufactured by laser powder bed fusion
    • Authors: Fan Zhang; Lyle E. Levine; Andrew J. Allen; Mark R. Stoudt; Greta Lindwall; Eric A. Lass; Maureen E. Williams; Yaakov Idell; Carelyn E. Campbell
      Pages: 200 - 214
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Fan Zhang, Lyle E. Levine, Andrew J. Allen, Mark R. Stoudt, Greta Lindwall, Eric A. Lass, Maureen E. Williams, Yaakov Idell, Carelyn E. Campbell
      Elemental segregation is a ubiquitous phenomenon in additive-manufactured (AM) parts due to solute rejection and redistribution during the solidification process. Using electron microscopy, in situ synchrotron X-ray scattering and diffraction, and thermodynamic modeling, we reveal that in an AM nickel-based superalloy, Inconel 625, stress-relief heat treatment leads to the growth of unwanted δ-phase precipitates on a time scale much faster than that in wrought alloys (minutes versus tens to hundreds of hours). The root cause for this behavior is the elemental segregation that results in local compositions of AM alloys outside the bounds of the allowable range set for wrought alloys. In situ small angle scattering experiments reveal that platelet-shaped δ phase precipitates grow continuously and preferentially along their lateral dimensions during stress-relief heat treatment, while the thickness dimension reaches a plateau very quickly. In situ XRD experiments reveal that nucleation and growth of δ-phase precipitates occur within 5 min during stress-relief heat treatment, indicating a low nucleation barrier and a short incubation time. An activation energy for the growth of δ phase was found to be (131.04 ± 0.69) kJ mol−1. We further demonstrate that a subsequent homogenization heat treatment can effectively homogenize the AM alloy and remove the deleterious δ phase. The combined experimental and modeling methodology in this work can be extended to elucidate the phase evolution during heat treatments in a broad range of AM materials.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.03.017
      Issue No: Vol. 152 (2018)
       
  • Critical role of ZrO2 on densification and microstructure development in
           spark plasma sintered NbB2
    • Authors: T.N. Maity; Krishanu Biswas; Bikramjit Basu
      Pages: 215 - 228
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): T.N. Maity, Krishanu Biswas, Bikramjit Basu
      The present work reports the densification and microstructure development in niobium diboride (NbB2) containing ZrO2, consolidated using four stage spark plasma sintering with final holding at 1750°C for 2 min. The adopted processing approach enables to obtain fully dense (99.6% ρth) NbB2-ZrO2 ceramic with good mechanical properties. The sintered microstructure is characterized by the uniform dispersion of intragranular ZrO2 particles within the NbB2 grains. Fine scale microstructure analysis using high resolution transmission electron microscopy (HRTEM) together with HAADF (high angle annular dark field) technique reveals the dispersion of t-ZrO2 or o-ZrO2 and the defect structure including edge dislocations and asymmetric twins in o-ZrO2. An attempt has been made to rationalize the enhanced densification in terms of quantitative analysis of ‘constriction resistance’ induced localized heating and defect structure. The evaluation of basic mechanical properties revealed that a combination of high hardness (∼22 GPa) together with moderate indentation toughness (∼5.2 MPa m1/2) can be achieved with solid state sintered NbB2 containing ZrO2. The present study, for the first time, conclusively establishes the role of zirconia in obtaining dense NbB2 with good mechanical property combination at relatively lower sintering temperature of 1750 °C.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.03.049
      Issue No: Vol. 152 (2018)
       
  • Computational design of metal oxides to enhance the wetting and adhesion
           of silver-based brazes on yttria-stabilized-zirconia
    • Authors: Thanaphong Phongpreecha; Jason D. Nicholas; Thomas R. Bieler; Yue Qi
      Pages: 229 - 238
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Thanaphong Phongpreecha, Jason D. Nicholas, Thomas R. Bieler, Yue Qi
      Ag–CuO is a broadly used reactive air brazing (RAB) system for effectively bonding ceramics and metal interfaces, especially for sealing yttria-stabilized zirconia (YSZ) to metals in solid-oxide fuel cells (SOFCs). To understand the superior performance of this braze, density functional theory (DFT) calculations were employed to investigate two mechanisms that can potentially increase the work of adhesion ( W adh ) and hence reduce the wetting angle of Ag on YSZ. It was found while the formation of Ag–dissolved O clusters at the Ag–YSZ interface can promote wetting, a much greater wetting angle reduction comes from the formation of CuO interlayers between Ag and YSZ. Further, the W adh of an Ag/CuO and CuO/YSZ interface was found to be significantly higher than that of an Ag/YSZ interface. Based on simulation-obtained insights into metal to oxide bond formation, a simple descriptor was developed to predict the Ag/oxide interface energies, the Ag/oxide W adh , and to search for potential oxide interlayers capable of promoting the wetting and adhesion of Ag on YSZ. Many simple metal oxides (single cation) were examined, however their W adh with Ag were less than that of an Ag/CuO interface. Expanding the search to multi-cation oxides led to several promising candidates, such as CuAlO2, CuGaO2, and Cu3TiO4; all of which are also stable in the reducing SOFC conditions. Depending upon their solubility in molten Ag, these newly-identified oxides could either be pre-applied as wetting promoting interlayers or directly incorporated into Ag to form new reactive air brazes.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.024
      Issue No: Vol. 152 (2018)
       
  • Plastic yielding in lath martensites – An alternative viewpoint
    • Authors: Bevis Hutchinson; Pete Bate; David Lindell; Amer Malik; Matthew Barnett; Peter Lynch
      Pages: 239 - 247
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Bevis Hutchinson, Pete Bate, David Lindell, Amer Malik, Matthew Barnett, Peter Lynch
      In recent literature the gradual yielding of quenched martensitic steels has been attributed to either heterogeneous microstructures having different strength levels or to the presence of small scale, Type II, residual stresses. Using in-situ tensile testing in synchrotron diffraction experiments in combination with crystal plasticity finite element modelling (CPFEM) we show that the dominant influence on yielding derives from the residual stresses which are a product of the displacive transformation from austenite during quenching. As plastic straining proceeds, the measured diffraction peaks become narrower and asymmetric, as predicted by the CPFEM calculations. The model predictions are generally in good agreement with published results showing large variations in local strains in different microstructural elements.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.039
      Issue No: Vol. 152 (2018)
       
  • A new concept for growth restriction during solidification
    • Authors: Z. Fan; F. Gao; L. Zhou; S.Z. Lu
      Pages: 248 - 257
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Z. Fan, F. Gao, L. Zhou, S.Z. Lu
      Growth restriction refers to the phenomenon of reduced growth velocity due to the solute enrichment/depletion at the solid/liquid interface during alloy solidification. Although significant progress has been made to understand this phenomenon, so far there has been no effective parameter to quantify growth restriction. In this paper, we have derived a new parameter, β, to quantify the growth restriction in multicomponent systems effectively, and which incorporates the nature of solutes, solute concentrations and solidification conditions holistically. Theoretical analysis and phase field simulations have confirmed that growth velocity is a unique function of β regardless of the nature of solutes, solute concentrations and solidification conditions, but it is not a unique function of the widely used growth restriction factor, Q. Our analysis suggests that the overall β for a multicomponent alloy system can be either calculated accurately by the ratio of the liquid fraction to the solid fraction (β = f L /f S ) or approximated with great confidence by a linear addition of the β values of the constituent binary systems. In addition, we have shown theoretically that for a given alloy system solidifying under a given undercooling, there is a critical solute concentration, below which solidification becomes partitionless and therefore there is no growth restriction during solidification. Furthermore, our analysis has shown that the physical origin of growth restriction is the blockage of the supply of the critical elements for crystal growth, i.e., solvent atoms in the case of eutectic-forming.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.045
      Issue No: Vol. 152 (2018)
       
  • Structure-property correlations and origin of relaxor behaviour in
           BaCexTi1-xO3
    • Authors: Giovanna Canu; Giorgia Confalonieri; Marco Deluca; Lavinia Curecheriu; Maria Teresa Buscaglia; Mihai Asandulesa; Nadejda Horchidan; Monica Dapiaggi; Liliana Mitoseriu; Vincenzo Buscaglia
      Pages: 258 - 268
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Giovanna Canu, Giorgia Confalonieri, Marco Deluca, Lavinia Curecheriu, Maria Teresa Buscaglia, Mihai Asandulesa, Nadejda Horchidan, Monica Dapiaggi, Liliana Mitoseriu, Vincenzo Buscaglia
      Macroscopic properties measurements, such as dielectric permittivity and ferroelectric hysteresis, differential scanning calorimetry, and average structure information are combined with complementary techniques sensitive to the local structure, i.e. Pair Distribution Function (PDF) and Raman spectroscopy, to gain comprehensive insight into the structure-property relationships and origin of relaxor behaviour in BaCexTi1-xO3 ceramics over a broad composition (x = 0.02–0.30) and temperature (100–450 K) range. The resulting phase diagram displays sequential phase transitions with a tricritical point (TCP) at x = 0.09 and a ferroelectric to relaxor crossover (FRC) at x ≈ 0.20. In contrast, the local structure is rhombohedral irrespective of x and the PDF reveals the existence of a high level of disorder and significant local strains determined by the ionic size mismatch (Ce4+: 0.87 Å, Ti4+: 0.605 Å). The diffuse character of the phase transitions observed when x ≥ 0.05 is most likely originated by these deformations. Parallel of BaMxTi1-xO3 phase diagrams (M = Sn, Hf, Zr, Ce) shows that the compositions corresponding to TCP and FRC are nearly independent of M. This suggests that, irrespective of the ionic radius of M4+, in homovalent-substituted BaTiO3 a critical number of Ti-O-Ti bonds has to be broken before a new “state” is established, whereas local electric and strain fields seem to have a marginal effect.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.038
      Issue No: Vol. 152 (2018)
       
  • High speed dynamic deformation of polysynthetic twinned titanium aluminide
           intermetallic compound
    • Authors: Guang Yang; Kui Du; Dongsheng Xu; Hui Xie; Wenqing Li; Dingming Liu; Yang Qi; Hengqiang Ye
      Pages: 269 - 277
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Guang Yang, Kui Du, Dongsheng Xu, Hui Xie, Wenqing Li, Dingming Liu, Yang Qi, Hengqiang Ye
      Impact deformed TiAl polysynthetic twinned crystal was investigated by scanning and transmission electron microscopy including aberration-corrected scanning transmission electron microscopy. Shear bands, deformation twins and dislocations were observed in γ-TiAl phase. Deformation twins were identified on both primary (11 1 ¯ ) and secondary (1 1 ¯ 1 ¯ ) twin planes, and dislocation structures were resolved on the coherent {111} twin boundaries. Among them, 1/6 < 211] pseudo-twin dislocations were observed on the coherent twin boundaries co-existing with 1/2 < 101] super-partial dislocations. Two types of shear bands were identified, one was composed of (11 1 ¯ ) twin lamellae inclined to the propagation direction of shear bands, while the other was composed of curved (1 1 ¯ 1 ¯ ) twin lamellae and its propagation is nearly parallel to the (1 1 ¯ 1 ¯ ) twin plane.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.037
      Issue No: Vol. 152 (2018)
       
  • Investigation of the phase change mechanism of Ge6Sn2Sb2Te11
    • Authors: Christine Koch; Torben Dankwort; Anna-Lena Hansen; Marco Esters; Dietrich Häußler; Hanno Volker; Alexander von Hoegen; Matthias Wuttig; David C. Johnson; Wolfgang Bensch; Lorenz Kienle
      Pages: 278 - 287
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Christine Koch, Torben Dankwort, Anna-Lena Hansen, Marco Esters, Dietrich Häußler, Hanno Volker, Alexander von Hoegen, Matthias Wuttig, David C. Johnson, Wolfgang Bensch, Lorenz Kienle
      Thin films of Ge6Sn2Sb2Te11 were synthesized and compared to the well-known unsubstituted phase change material (PCM) Ge8Sb2Te11. In situ X-ray diffraction (XRD) and temperature dependent sheet resistance measurements evidenced a significant decrease of the phase change temperature from 144 °C for Ge8Sb2Te11 to 112 °C for Ge6Sn2Sb2Te11. The resistance measurements also revealed an intermediate step during the phase change. Detailed in situ transmission electron microscopy (TEM) and (XRD) investigations on structural ordering phenomena suggest that this intermediate step is associated with the disorder of structural vacancies on the cationic sites stable up to 130 °C. Annealing the sample beyond 130 °C leads to a subsequent ordering of vacancies and thus to the formation of a metastable primitive trigonal phase with vacancy layers. At ∼240 °C - ∼300 °C, a transition to the stable phase is observed. For the first time, an in plane movement of bi-layer defects is observed by in situ TEM as a result of a self-ordering mechanism. These findings represent new insights into the transition process on the nanoscale and suggest that tin substituted PCMs may represent promising candidates for multi-level data storage applications.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.029
      Issue No: Vol. 152 (2018)
       
  • Thermo-kinetic design of retained austenite in advanced high strength
           steels
    • Authors: Zongbiao Dai; Ran Ding; Zhigang Yang; Chi Zhang; Hao Chen
      Pages: 288 - 299
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Zongbiao Dai, Ran Ding, Zhigang Yang, Chi Zhang, Hao Chen
      Design of metastable retained austenite has been one of the most key issues in the development of advanced high strength steels (AHSSs) as mechanical properties of AHSSs are directly linked to the amount of retained austenite and its stability. In the past decades, several approaches, e.g. isothermal bainitic transformation, quenching & partitioning, austenite reversion transformation et al. have been successfully proposed to obtain retained austenite in the AHSSs. However, up to now, optimization of alloy composition and processing parameters in the above approaches is primarily by “trial and error” experiments or thermodynamic calculations. In this study, an integrated thermo-kinetic computational model, in which thermodynamics-kinetics of phase transformations and alloying elements partitioning are carefully considered, is used to design multi-phase microstructure of AHSSs with an emphasis on retained austenite. The current model is benchmarked by a comparison with the available experimental data for the conventional transformation-induced plasticity and quenching & partitioning steels, and the effects of alloy composition and processing parameters on the amount of retained austenite and its composition will also be discussed.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.040
      Issue No: Vol. 152 (2018)
       
  • Crystal defect associated selection of phase transformation orientation
           relationships (ORs)
    • Authors: Meishuai Liu; Yudong Zhang; Xinli Wang; Benoit Beausir; Xiang Zhao; Liang Zuo; Claude Esling
      Pages: 315 - 326
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Meishuai Liu, Yudong Zhang, Xinli Wang, Benoit Beausir, Xiang Zhao, Liang Zuo, Claude Esling
      Phase transformation in solids always follows specific orientation relationships (ORs). The OR usually ensures a minimum lattice deformation for the structure change. However, in many cases different ORs are respected by the same transformation. The selection role and the link between the OR and the existing crystal defects needs further investigation. Thus, in this work, the α to β heating phase transformation induced by high density Electric Current Pulse (ECP) treatments in an annealed Cu–40%Zn alloy was investigated. Results show that the β phase obeys the K-S OR when formed along the α grain boundaries or in their vicinities, or the N-W OR when formed in the α grain interiors. In the former sites, the {111}α/< 1 1 ¯ 0 >α dislocation arrays were frequently observed, whereas in the latter, the {111}α/< 11 2 ¯ >α stacking faults were often found. Transformation strain analyses revealed that under the K-S OR the maximum lattice deformation required is a shear on the {111}α plane in the < 1 1 ¯ 0 >α direction, whereas under the N-W OR the maximum deformation is a shear on the {1 11 }α plane in the < 11 2 ¯ >α direction. Thus the existing {111}α/< 1 1 ¯ 0 >α dislocation arrays along the α grain boundaries and in their vicinities provide pre-strain required by the transformation via the K-S path, whereas the {111}α/< 11 2 ¯ >α stacking faults boarded by {111}α/< 11 2 ¯ >α partial dislocations offer pre-strain facilitating the transformation via the N-W path. The present results provide new information on the role of crystal defects on phase transformation strain path and the selection of transformation ORs.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.031
      Issue No: Vol. 152 (2018)
       
  • Ostwald ripening of spheroidal particles in multicomponent alloys
    • Authors: Kyoungdoc Kim; Peter W. Voorhees
      Pages: 327 - 337
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): Kyoungdoc Kim, Peter W. Voorhees
      We propose a general theory of Ostwald ripening for prolate spheroidal particles in a nonideal nondilute multicomponent alloy. The diffusion problem of a growing or shrinking particle is solved using prolate spheroidal coordinates under the assumption that the spheroidal particle has a constant Wulff shape. The result shows that the diffusional growth rate increases with an increasing particle aspect ratio due to the increased surface area per volume. The anisotropic interfacial energy necessary to guarantee that the particles are always prolate spheroids with a given aspect ratio is also determined. We find that the chemical potential decreases with an increasing particle aspect ratio under a constant volume-equivalent radius. Based on the two correction factors, asymptotic analysis reveals that the temporal exponents for the coarsening laws for spheroid particles are identical to that for spherical particles. However, as the aspect ratio increases the amplitudes of the temporal power laws of the average equivalent radius, the matrix supersaturations, and the particle composition decrease, whereas the amplitude of the number of particles per volume increases. It is also shown that the particle shape anisotropy affects the amplitudes, but not the direction of the vector representing the matrix supersaturation and particle composition.
      Graphical abstract image

      PubDate: 2018-05-01T07:25:01Z
      DOI: 10.1016/j.actamat.2018.04.041
      Issue No: Vol. 152 (2018)
       
  • Texture evolution and microcracking mechanisms in as-extruded and
           cross-rolled conditions of a 14YWT nanostructured ferritic alloy
    • Authors: S. Pal; M.E. Alam; S.A. Maloy; D.T. Hoelzer; G.R. Odette
      Pages: 338 - 357
      Abstract: Publication date: 15 June 2018
      Source:Acta Materialia, Volume 152
      Author(s): S. Pal, M.E. Alam, S.A. Maloy, D.T. Hoelzer, G.R. Odette
      Cr-stabilized nanostructured ferritic alloys (NFAs), dispersion strengthened by an ultra-high density of nanooxides, are attractive candidates for many nuclear energy applications due to their high-temperature strength, in-service stability and remarkable irradiation tolerance. However, typical NFA deformation processing paths lead to crystallographic texturing, formation of brittle microstructures and low toughness orientations, making fabricating components very difficult. Here, we characterize the dislocation-mediated deformation mechanisms that lead to the brittle texture component. The as-extruded bar is less brittle than the cross-rolled plate, which contains a large population of pre-existing cleavage microcracks. More generally, deformed ODS/NFAs are most often textured and have anisotropic low toughness orientations, even absent microcracks. However, cross-rolling produces a very high volume fraction of a plate normal {001}<110>-texture component, which constitutes the brittle cleavage system in iron. Microcracks propagate along {001} low angle deformation induced subgrain boundaries in <110> directions after nucleating by the Cottrell mechanism.
      Graphical abstract image

      PubDate: 2018-05-17T03:14:16Z
      DOI: 10.1016/j.actamat.2018.03.045
      Issue No: Vol. 152 (2018)
       
 
 
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