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Journal Cover Acta Materialia
  [SJR: 3.683]   [H-I: 202]   [228 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1359-6454
   Published by Elsevier Homepage  [3089 journals]
  • Lattice-alignment mechanism of SiGe on Sapphire
    • Authors: Hyun Jung Kim; Adam Duzik; Sang H. Choi
      Pages: 1 - 7
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Hyun Jung Kim, Adam Duzik, Sang H. Choi
      Heteroepitaxy of silicon germanium (SiGe) prepared on a sapphire substrate (Al2O3) requires scrupulous attention to growth conditions. Previous work was used a substrate temperature of 890°C to grow a SiGe (111) film on the trigonal sapphire (0001) substrate without twin defects. Although the growth conditions were effective for the formation of single crystal film, how the formation of SiGe at the interface of sapphire was not experimentally defined with the order of atomic arrangement. This work presents high resolution transmission electron microscope (TEM) images of the SiGe/Al2O3 interface to show the SiGe/Al2O3 interface bonding for heteroepitaxy mechanism. The first two monolayers of the SiGe are Si-rich as this match with the surface oxygen lattice of the Al2O3 substrate. After the Ge composition increases, the monolayer spacing also increased while maintaining the cubic crystal structure. These results highlight the importance of a cleanliness of sapphire substrate, the Al2O3 termination for SiGe growth, and the cubic structure deformation of SiGe for heteroepitaxy. From the essential understanding of the SiGe/Al2O3 interface and growth mechanism, both low temperature SiGe heteroepitaxy and the III-V or II-VI semiconductor epitaxy are possible.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.031
      Issue No: Vol. 145 (2017)
       
  • Metastable phase transformation and deformation twinning induced
           hardening-stiffening mechanism in compression of silicon nanoparticles
    • Authors: Yu Hong; Ning Zhang; Mohsen Asle Zaeem
      Pages: 8 - 18
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Yu Hong, Ning Zhang, Mohsen Asle Zaeem
      The compressive mechanical responses of silicon nanoparticles with respect to crystallographic orientations are investigated by atomistic simulations. Superelastic and abrupt hardening-stiffening behaviors are revealed in [110]-, [111]- and [112]-oriented nanoparticles. The obtained hardness values of these particles are in good agreement with the experimental results. In particular, [111]-oriented particle is extremely hard since its hardness (∼33.7 GPa) is almost three times greater than that of the bulk silicon (∼12 GPa). To understand the underlying deformation mechanisms, metastable phase transformation is detected in these particles. Deformation twinning of the metastable phase accounts for the early hardening-stiffening behavior observed in [110]-oriented particle. The twin phase then coalescences and undergoes compression to resist further deformation, and leads to the subsequent re-hardening and re-stiffening events. The same metastable phase is also detected to form in [111]- and [112]-oriented particles. The compression of such metastable phase is responsible for their hardening-stiffening behavior. In contrast, the crystal lattice of diamond cubic silicon is merely elastically deformed when compressing along [100] direction. Throughout the simulations, no perfect tetragonal β-tin silicon phase formed due to the deconfinement status of nanoparticle comparing to the bulk silicon. A size effect on hardness of silicon nanoparticles, i.e., “smaller is harder”, is also revealed.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.034
      Issue No: Vol. 145 (2017)
       
  • Sample-size-dependent surface dislocation nucleation in nanoscale crystals
    • Authors: Qing-Jie Li; Bin Xu; Shotaro Hara; Ju Li; Evan Ma
      Pages: 19 - 29
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Qing-Jie Li, Bin Xu, Shotaro Hara, Ju Li, Evan Ma
      The finite-temperature mechanical strength of nanoscale pristine metals at laboratory strain rates may be controlled by surface dislocation nucleation, which was hypothesized to be only weakly dependent on the sample size. Previous studies on surface dislocation nucleation investigated factors such as surface steps, oxidation layers and surface diffusion, while the role of surface stresses and sample size remains unclear. Here we perform systematic atomistic calculations on the activation free energy barriers of surface dislocation nucleation in sub-50 nm nanowires. The results demonstrate that surface stresses significantly influence the activation processes of surface dislocation nucleation. This renders the strength strongly dependent on sample size; whether it is “smaller is stronger” or “smaller is weaker” depends on the combined effects of surface stress and applied axial stress, which can be universally explained in terms of the local maximum resolved shear stress. A linear relation between the activation entropy and activation enthalpy (Meyer-Neldel compensation rule) was found to work well across a range of stresses and sample sizes.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.048
      Issue No: Vol. 145 (2017)
       
  • Atomic and electronic basis for solutes strengthened (010) anti-phase
           boundary of L12 Co3(Al, TM): A comprehensive first-principles study
    • Authors: William Yi Wang; Fei Xue; Ying Zhang; Shun-Li Shang; Yi Wang; Kristopher A. Darling; Laszlo J. Kecskes; Jinshan Li; Xidong Hui; Qiang Feng; Zi-Kui Liu
      Pages: 30 - 40
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): William Yi Wang, Fei Xue, Ying Zhang, Shun-Li Shang, Yi Wang, Kristopher A. Darling, Laszlo J. Kecskes, Jinshan Li, Xidong Hui, Qiang Feng, Zi-Kui Liu
      The crystallographic and electronic structures of (010) APB of L12 Co3Al0.75TM0.25 are studied by high-resolution transmission electron microscopy and first-principles calculations. Effects of solute atoms (TM = Cr, Hf, Mo, Ni, Re, Ru, Ta, Ti, W and Y) on the formation energy, lattice parameters/distortion, magnetism, and bonding strength of the (010) APB in Co3Al0.75TM0.25 are obtained from first-principles calculations. Comparing to the equilibrium volume of Co3Al, it is found that the volume change of the Co3Al0.75TM0.25 with and without the presence of APB increases linearly with the volume of the corresponding FCC elements, indicating the contribution of the solute atoms on lattice distortion of bulk and (010) APB. Particularly, the strong dependence of the APB energy on the composition is comprehensively discussed together with the available experimental and theoretical data in the literature. The negative (010) APB energy indicates that the formation of (010) APB could stabilize the ordered L12 (or the FCC-lattice) Co3Al, and the local L12 → D022 phase transformation can occur. The physical natures of lattice distortions caused by the fault layers of APB and the solute atoms are characterized by bonding charge density. It is found that the solute atoms, occupying Al site of L12 phase and its (010) APB, increase the local bonding strength along (010) through the electron redistribution during forming the chemical bonds with Co, revealing an intrinsic solid-solution strengthening mechanism. This work provides an insight into the atomic and electronic basis for solid-solution strengthening mechanism of L12 Co3Al0.75TM0.25.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.10.041
      Issue No: Vol. 145 (2017)
       
  • Prediction of ceramic fracture with normal distribution pertinent to grain
           size
    • Authors: Chunguo Zhang; Xiaozhi Hu; Tim Sercombe; Qingbin Li; Zhimin Wu; Pengmin Lu
      Pages: 41 - 48
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Chunguo Zhang, Xiaozhi Hu, Tim Sercombe, Qingbin Li, Zhimin Wu, Pengmin Lu
      Fracture of brittle ceramics initiated from shallow surface cracks comparable to their average grain sizes (G) can fluctuate significantly. Such fluctuations can contain crucial information on the inherent relations between the average grain size G and bulk ceramic properties such as the tensile strength f t and fracture toughness K IC. It was proposed in this study that the characteristic crack a*ch = 0.25(K IC/f t)2 = constant × G, inspired by observations of strength distributions with different a*ch/G ratios. It was found that normal distributions with the smallest standard deviation exist around a*ch = (2.5–3.5) × G, based on quasi-brittle fracture results of four different ceramics with G from 2 to 20 μm and shallow surface cracks from 100 nm to 650 μm. Using the average value of the relative characteristic crack a*ch/G ≈ 3, the mean and standard deviation (σ) were determined by normal distributions for both the tensile strength ft and fracture toughness K IC. Quasi-brittle fracture of those fine-grained ceramics based on the mean values and standard deviations was thus predicted. The upper and lower bounds with 96% reliability (±2σ) specified by the normal distributions covered nearly all experimental data ranging from the strength-controlled to toughness-controlled asymptotic limits, and quasi-brittle fracture between the two. With the knowledge of the average grain size G, the tensile strength f t becomes the sole parameter required to describe the entire fracture range.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.041
      Issue No: Vol. 145 (2017)
       
  • The Third Law of Thermodynamics: Phase equilibria and phase diagrams at
           low temperatures
    • Authors: David E. Laughlin; William A. Soffa
      Pages: 49 - 61
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): David E. Laughlin, William A. Soffa
      Great progress has been made over the recent decades in the application of computational thermodynamics (Calphad) and theoretical methodologies (CVM) including so-called first principles approaches to modeling thermodynamic properties and the calculation of phase diagrams of materials. The aim of this paper is to call attention to considerations of the THIRD LAW OF THERMODYNAMICS when evaluating these results when applied to low temperature phase equilibria. In this effort we call attention to the essential content of the modern version of this third principle of thermodynamics using an historical and pedagogical approach. An appreciation of the constraints of the THIRD LAW is shown to be valuable in projecting possible low temperature phase fields and boundaries and predicting thermodynamically consistent phase diagram configurations as T→0 K. The ideas of Simon regarding aspects or subsystems are shown to be of paramount importance in assessing the thermodynamic properties of materials at low temperatures.

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.037
      Issue No: Vol. 145 (2017)
       
  • Microstructural effects on effective piezoelectric responses of textured
           PMN-PT ceramics
    • Authors: Chen Ming; Tiannan Yang; Kun Luan; Lei Chen; Liang Wang; Jiangtao Zeng; Yongxiang Li; Wenqing Zhang; Long-Qing Chen
      Pages: 62 - 70
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Chen Ming, Tiannan Yang, Kun Luan, Lei Chen, Liang Wang, Jiangtao Zeng, Yongxiang Li, Wenqing Zhang, Long-Qing Chen
      The effective piezoelectric properties of [001]c fiber textured Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ceramics were studied by phase-field modeling. The effects of microstructures such as texture, grain shape, grain boundaries, residual pores and heterogeneous growth templates were investigated. It was found that the degree of texture plays a dominant role in determining the properties. The pores, heterogeneous templates and grain boundaries reduce the properties significantly at high degrees of texture with the effect diminishing at decreasing degrees of texture. The presence of heterogeneous templates leads to a more significant reduction in the properties than pores although the piezoelectric coefficients of pores are zero. The shape of grains has a weak effect at all degrees of texture. By utilizing the experimentally measured microstructural parameters in the calculations and comparing the computed properties with the corresponding measurements, we showed that the low performance of sintered textured PMN-PT ceramics ( d 33 ∼1000 pC/N) relative to single crystals ( d 33 ∼2800 pC/N) is mainly due to the insufficiently high degree of texture even with Lotgering factors up to 0.9, while the influences of other microstructures are weak.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.043
      Issue No: Vol. 145 (2017)
       
  • Composition and automated crystal orientation mapping of rapid
           solidification products in hypoeutectic Al-4 at.%Cu alloys
    • Authors: K.W. Zweiacker; Can Liu; M.A. Gordillo; J.T. McKeown; G.H. Campbell; J.M.K. Wiezorek
      Pages: 71 - 83
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): K.W. Zweiacker, Can Liu, M.A. Gordillo, J.T. McKeown, G.H. Campbell, J.M.K. Wiezorek
      Rapid solidification can produce metastable phases and unusual microstructure modifications in multi-component alloys during additive manufacturing or laser beam welding. Composition and phase mapping by transmission electron microscopy have been used here to characterize the morphologically distinct zones developing in hypoeutectic Al-4 at.% Cu alloy after pulsed laser melting for different crystal growth rate regimes. Deviations of the compositions of the alloy phases from equilibrium predictions and unique orientation relationships between the solidification transformation products have been determined. Specifically, for the columnar growth zone at solidification rates of 0.8 m s − 1 < v < v a = 1.8 m s − 1 , two distinct orientation relationships were established between the concomitantly forming non-equilibrium phases, supersaturated α-Al solid solution and the discontinuously distributed α-Al2Cu-based θ′-phase, which can be described as {110}θ ∥ {001}α, [001]θ ∥ [110]α and {001}θ ∥ {001}α, [100]θ ∥ [100]α. These orientation relationships permit formation of coherent interphase interfaces with low interfacial free energy. This endows a kinetic advantage to the thermodynamically less stable θ′-Al2Cu phase relative to the more stable equilibrium θ-Al2Cu phase during formation of the morphologically modified eutectic of the columnar growth zone grains, since repeated nucleation is required to establish the discontinuous distribution of θ′-Al2Cu phase.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.040
      Issue No: Vol. 145 (2017)
       
  • Investigating nano-precipitation in a V-containing HSLA steel using small
           angle neutron scattering
    • Authors: Y.Q. Wang; S.J. Clark; V. Janik; R.K. Heenan; D. Alba Venero; K. Yan; D.G. McCartney; S. Sridhar; P.D. Lee
      Pages: 84 - 96
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Y.Q. Wang, S.J. Clark, V. Janik, R.K. Heenan, D. Alba Venero, K. Yan, D.G. McCartney, S. Sridhar, P.D. Lee
      Interphase precipitation (IPP) of nanoscale carbides in a vanadium-containing high-strength low-alloy steel has been investigated. Small angle neutron scattering (SANS) and transmission electron microscopy (TEM) were employed to characterize the precipitates and their size distributions in Fe-0.047C-0.2V-1.6Mn (in wt.%) alloy samples which had been austenitized, isothermally transformed at 700 °C for between 3 and 600 min and water quenched. TEM confirms that, following heat treatment, rows of vanadium-containing nanoscale interphase precipitates were present. Model-independent analysis of the nuclear SANS signal and model fitting calculations, using oblate spheroid and disc-shapes, were performed. The major axis diameter increased from 18 nm after 3 min to 35 nm after 600 min. Precipitate volume percent increased from 0.09 to 0.22 vol% over the same period and number density fell from 2 × 1021 to 5 × 1020 m−3. A limited number of measurements of precipitate maximum diameters from TEM images showed the mean value increased from 8 nm after 5 min to 28 nm after 600 min which is in reasonable agreement with the SANS data.
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      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.032
      Issue No: Vol. 145 (2017)
       
  • First-principles modeling of superlattice intrinsic stacking fault
           energies in Ni3Al based alloys
    • Authors: A. Breidi; J. Allen; A. Mottura
      Pages: 97 - 108
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): A. Breidi, J. Allen, A. Mottura
      High-throughput quantum mechanics based simulations have been carried out to establish the change in lattice parameter and superlattice intrinsic stacking fault (SISF) formation energies in Ni3Al-based alloys using the axial Ising model. We had direct access to the variation in SISF energies due to finite compositional change of the added ternary transition metal (TM) element through constructing large supercells, which was equally necessary to account for chemical disorder. We find that most added TM ternaries induce an important quasi-linear increase in the SISF energy as a function of alloying composition x. The most pronounced increase corresponds to Fe addition, while Co addition decreases the SISF energy monotonically. Our results shed light on the role played by TM elements on strengthening L12 Ni3Al precipitates against stacking fault shear. The data are of high importance for designing new Ni-based superalloys based on computational approaches.
      Graphical abstract image

      PubDate: 2017-12-13T08:05:17Z
      DOI: 10.1016/j.actamat.2017.11.042
      Issue No: Vol. 145 (2017)
       
  • Simultaneous X-ray diffraction, crystallography and fluorescence mapping
           using the Maia detector
    • Authors: Henry J. Kirkwood; Martin D. de Jonge; Ondrej Muránsky; Felix Hofmann; Daryl L. Howard; Chris G. Ryan; Grant van Riessen; Matthew R. Rowles; Anna M. Paradowska; Brian Abbey
      Pages: 1 - 10
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Henry J. Kirkwood, Martin D. de Jonge, Ondrej Muránsky, Felix Hofmann, Daryl L. Howard, Chris G. Ryan, Grant van Riessen, Matthew R. Rowles, Anna M. Paradowska, Brian Abbey
      Interactions between neighboring grains influence the macroscale behavior of polycrystalline materials, particularly their deformation behavior, damage initiation and propagation mechanisms. However, mapping all of the critical material properties normally requires that several independent measurements are performed. Here we report the first grain mapping of a polycrystalline foil using a pixelated energy-dispersive X-ray area detector, simultaneously measuring X-ray fluorescence and diffraction with the Maia detector in order to determine grain orientation and estimate lattice strain. These results demonstrate the potential of the next generation of X-ray area detectors for materials characterization. By scanning the incident X-ray energy we investigate these detectors as a complete solution for simultaneously mapping the crystallographic and chemical properties of the sample. The extension of these techniques to broadband X-ray sources is also discussed.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.025
      Issue No: Vol. 144 (2017)
       
  • Anomalous hardening in magnesium driven by a size-dependent transition in
           deformation modes
    • Authors: Gi-Dong Sim; Gyuseok Kim; Steven Lavenstein; Mohamed H. Hamza; Haidong Fan; Jaafar A. El-Awady
      Pages: 11 - 20
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Gi-Dong Sim, Gyuseok Kim, Steven Lavenstein, Mohamed H. Hamza, Haidong Fan, Jaafar A. El-Awady
      Here, we report a comprehensive study that combines in situ scanning electron microscopy experiments and atomistic simulations to quantify the effect of crystal size on the transformation in deformation modes in a-axis oriented Mg single crystals at room temperature. The experimental results indicate that the deformation is dominated by the nucleation and propagation of tensile twins. The stress required for twin propagation was found to increase with decreasing sample size, showing a typical “smaller is stronger” behavior. Furthermore, an anomalous increase in strain hardening is first reported for microcrystals having diameters larger than ∼18 μm, which is induced by twin-twin and dislocation-twin interactions. The hardening rate gradually decreases toward the bulk response as the microcrystal size increases. Below 18 μm, deformation is dominated by the nucleation and propagation of a single tensile twin followed by basal slip activity in the twinned crystal, leading to no apparent hardening. In addition, molecular dynamics simulations indicate a transition from twinning mediated plasticity to dislocation mediated plasticity for crystal sizes below a few hundred nanometers in size. A deformation mechanism map for twin oriented Mg single crystals, ranging from the nano-scale to bulk scale is proposed based on the current simulations and experiments. The current predicted size-affected deformation mechanism of twin oriented Mg single crystals can lead to better understanding of the competition between dislocations plasticity and twinning plasticity.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.033
      Issue No: Vol. 144 (2017)
       
  • Solubility in Zr-Nb alloys from first-principles
    • Authors: Maeva Cottura; Emmanuel Clouet
      Pages: 21 - 30
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Maeva Cottura, Emmanuel Clouet
      The thermodynamic properties of the Zr-Nb alloy are investigated at temperatures below 890 K with ab initio calculations. The solution energies of the bcc Nb-rich and hcp Zr-rich solid solutions obtained within the framework of density functional theory are in good agreement with experimental data, although insufficient for a quantitative description of the miscibility gap, for which non configurational entropy has to be accounted for. Whereas electronic free energies can be neglected, we show, using the harmonic approximation and the density functional perturbation theory, that both solution free energies are strongly modified by the contribution related to atomic vibrations. Considering this vibrational free energy leads to a good description of the phase diagram.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.035
      Issue No: Vol. 144 (2017)
       
  • Synergistic effect of ultrasonic melt treatment and fast cooling on the
           refinement of primary Si in a hypereutectic Al–Si alloy
    • Authors: Jae-Gil Jung; Tae-Young Ahn; Young-Hee Cho; Su-Hyeon Kim; Jung-Moo Lee
      Pages: 31 - 40
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Jae-Gil Jung, Tae-Young Ahn, Young-Hee Cho, Su-Hyeon Kim, Jung-Moo Lee
      A mechanism describing the synergistic effect of ultrasonic melt treatment (UST) and subsequent fast cooling on the refinement of primary Si particles in a hypereutectic Al−Si alloy was investigated by examining inoculant particles via high-resolution transmission electron microscopy, serial sectioning, and melt filtration. The application of UST activated non-wetting MgAl2O4 particles with diameters of ∼0.5 μm to nucleate the primary Si phase. The cavitation-enhanced wetting of MgAl2O4 particles caused the formation of the AlP phase at the MgAl2O4 interface, further improving the nucleation potential. The cavitation-enhanced wetting and dispersion of inclusions (such as MgAl2O4) also resulted in the refinement and de-agglomeration of AlP particles. The UST-induced changes to the inoculant particles ultimately increased their number density, and the observed effects became more pronounced after increasing the degree of undercooling up to 20 K, leading to enhanced refinement of primary Si particles at higher cooling rates (up to 102 K s−1).
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.039
      Issue No: Vol. 144 (2017)
       
  • Twinning mechanism at three-grain tri-junction during directional
           solidification of multi-crystalline silicon
    • Authors: T. Jain; H.K. Lin; C.W. Lan
      Pages: 41 - 50
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): T. Jain, H.K. Lin, C.W. Lan
      We propose a model to explain the formation mechanism of twin grains at the three-grain tri-junction (3GTJ) on the growth interface during directional solidification of multi-crystalline silicon. We also attempt to confirm its validity by comparing with the experimental results. This model is an extension of the previous model for the two-dimensional (2D) nucleation at the grain boundaries (GBs). It is found that the energy barriers for faceting and twinning nucleus at the 3GTJ are much smaller than that at GBs. As a result, a higher twinning probability can be obtained at a much lower undercooling. Two types of tri-junctions are considered according to the experiments and the dominant factors which decide the twinning probability on each facet at the 3GTJ are further discussed.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.042
      Issue No: Vol. 144 (2017)
       
  • Strain accumulation during microstructurally small fatigue crack
           propagation in bcc Fe-Cr ferritic stainless steel
    • Authors: E. Malitckii; H. Remes; P. Lehto; Y. Yagodzinskyy; S. Bossuyt; H. Hänninen
      Pages: 51 - 59
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): E. Malitckii, H. Remes, P. Lehto, Y. Yagodzinskyy, S. Bossuyt, H. Hänninen
      Strain accumulation was studied by digital image correlation technique (DIC) during microstructurally small fatigue crack propagation in polycrystalline 18%Cr ferritic stainless steel. Load-controlled fatigue testing was performed with R-ratio of 0.1 and frequency 10 Hz. The maximum applied stress was well below the yield stress of the studied material. The effect of the observed strain field on crack growth rate variation is discussed. Fracture surfaces were studied by scanning electron microscopy (SEM) evidencing the connection between the mechanism of the fatigue crack growth, accumulated strain and crack growth rate. Detailed study of fracture surface morphology was carried out by atomic force microscopy (AFM). Results indicate two processes of material damage accumulation and failure during cyclic loading: 1) local shear strain zones form successively ahead of the crack tip, and 2) fatigue crack growth occurs by both single- and multiple-slip mechanisms. The place and intensity of shear strain localization zones vary during the crack growth that is related closely to the local variation of crack growth rate.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.038
      Issue No: Vol. 144 (2017)
       
  • Similar local order in disordered fluorite and aperiodic pyrochlore
           structures
    • Authors: Jacob Shamblin; Cameron L. Tracy; Raul I. Palomares; Eric C. O'Quinn; Rodney C. Ewing; Joerg Neuefeind; Mikhail Feygenson; Jason Behrens; Christina Trautmann; Maik Lang
      Pages: 60 - 67
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Jacob Shamblin, Cameron L. Tracy, Raul I. Palomares, Eric C. O'Quinn, Rodney C. Ewing, Joerg Neuefeind, Mikhail Feygenson, Jason Behrens, Christina Trautmann, Maik Lang
      A major challenge to understanding the response of materials to extreme environments (e.g., nuclear fuels/waste forms and fusion materials) is to unravel the processes by which a material can incorporate atomic-scale disorder, and at the same time, remain crystalline. While it has long been known that all condensed matter, even liquids and glasses, possess short-range order, the relation between fully-ordered, disordered, and aperiodic structures over multiple length scales is not well understood. For example, when defects are introduced (via pressure or irradiation) into materials adopting the pyrochlore structure, these complex oxides either disorder over specific crystallographic sites, remaining crystalline, or become aperiodic. Here we present neutron total scattering results characterizing the irradiation response of two pyrochlores, one that is known to disorder (Er2Sn2O7) and the other to amorphize (Dy2Sn2O7) under ion irradiation. The results demonstrate that in both cases, the local pyrochlore structure is transformed into similar short range configurations that are best fit by the orthorhombic weberite structure, even though the two compositions have distinctly different structures, aperiodic vs. disordered-crystalline, at longer length scales. Thus, a material's resistance to amorphization may not depend primarily on local defect formation energies, but rather on the structure's compatibility with meso-scale modulations of the local order in a way that maintains long-range periodicity.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.044
      Issue No: Vol. 144 (2017)
       
  • Dislocation interactions at reduced strain rates in atomistic simulations
           of nanocrystalline Al
    • Authors: Maxime Dupraz; Zhen Sun; C. Brandl; Helena Van Swygenhoven
      Pages: 68 - 79
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Maxime Dupraz, Zhen Sun, C. Brandl, Helena Van Swygenhoven
      Molecular dynamics simulations of transient stress drops have been carried out in different regimes on a nanocrystalline Aluminum sample with average grain size of 12 nm. Besides confirming the interpretation of experimental results obtained during in situ X-ray diffraction, the creep simulations performed at 2 or 3 orders of magnitude lower strain rates than usual reveal deformation mechanisms that have not been observed previously. First of all, it is evidenced that the misfit dislocations available at the GB assist the propagation of a lattice dislocation on a plane with low resolved shear stress. Furthermore, it is shown that the interaction of two dislocations gliding on parallel slip planes can result in the emission of a vacancy in the grain interior. Finally, the importance of the Schmid factor in the activation of slip in nanocrystalline structures is discussed.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.043
      Issue No: Vol. 144 (2017)
       
  • Microstructure and mechanical properties of a precipitation-strengthened
           Al-Zr-Sc-Er-Si alloy with a very small Sc content
    • Authors: Anthony De Luca; David C. Dunand; David N. Seidman
      Pages: 80 - 91
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Anthony De Luca, David C. Dunand, David N. Seidman
      The precipitation hardening behavior of an Al-0.08Zr-0.014Sc-0.008Er-0.10Si (at.%) alloy was investigated utilizing microhardness, electrical conductivity, atom-probe tomography (APT), and compressive creep-measurements. This new composition, with a Sc:Zr atomic ratio of less than 1:5 represents a significant reduction of the alloy's cost when compared to the more usual Al-0.06Sc-0.02Zr based alloys with typical Sc:Zr atomic ratios of 3:1. To study the precipitation behavior of this low-Sc alloy, isothermal aging experiments between 350 and 425 °C for a duration of up to 6 months were performed. The low concentration of Sc, compensated by the high Zr concentration, permits the alloy to achieve a higher peak microhardness than the corresponding Sc-richer, Zr-leaner alloys. The low-Sc alloy also shows better over aging resistance, as anticipated from the smaller diffusivity of Zr when compared to Sc, leading to slower coarsening kinetics. Atom-probe tomography demonstrates that the high microhardness is due to the formation of a high number density of nano-precipitates, ∼1023 m−3 for peak aging conditions, with a mean radius of 1.9 nm, thus yielding a high volume fraction (0.35%) of nano-precipitates. Like alloys with much higher Sc and Er concentrations, the (Al,Si)3(Sc,Zr,Er) nano-precipitates still exhibit a core-shell structure with a concentration of Zr in the shell of up to 25 at.%, and a Sc- and Er-enriched core. Compressive creep experiments at 300 °C demonstrate that the new alloy, with only 0.014 at% Sc, is as creep resistant as a binary Al-0.08Sc at.% alloy, displaying a threshold stress of 17.5 ± 0.6 MPa at peak aged condition.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.040
      Issue No: Vol. 144 (2017)
       
  • Determining the strengths of HCP slip systems using harmonic analyses of
           lattice strain distributions
    • Authors: Paul R. Dawson; Donald E. Boyce; Jun-Sang Park; Euan Wielewski; Matthew P. Miller
      Pages: 92 - 106
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Paul R. Dawson, Donald E. Boyce, Jun-Sang Park, Euan Wielewski, Matthew P. Miller
      A robust methodology is presented to extract slip system strengths from lattice strain distributions for polycrystalline samples obtained from high-energy x-ray diffraction (HEXD) experiments with in situ loading. The methodology consists of matching the evolution of coefficients of a harmonic expansion of the distributions from simulation to the coefficients derived from measurements. Simulation results are generated via finite element simulations of virtual polycrystals that are subjected to the loading history applied in the HEXD experiments. Advantages of the methodology include: (1) its ability to utilize extensive data sets generated by HEXD experiments; (2) its ability to capture trends in distributions that may be noisy (both measured and simulated); and (3) its sensitivity to the ratios of the family strengths. The approach is used to evaluate the slip system strengths of Ti-6Al-4V using samples having relatively equiaxed grains. These strength estimates are compared to values in the literature.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.032
      Issue No: Vol. 144 (2017)
       
  • Transmission electron microscopy characterization of dislocation structure
           in a face-centered cubic high-entropy alloy Al0.1CoCrFeNi
    • Authors: X.D. Xu; P. Liu; Z. Tang; A. Hirata; S.X. Song; T.G. Nieh; P.K. Liaw; C.T. Liu; M.W. Chen
      Pages: 107 - 115
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): X.D. Xu, P. Liu, Z. Tang, A. Hirata, S.X. Song, T.G. Nieh, P.K. Liaw, C.T. Liu, M.W. Chen
      Structural characterization of dislocations and dislocation reactions in a face-centered cubic high entropy alloy was conducted using the state-of-the-art spherical aberration corrected transmission electron microscopy. We experimentally measured the stacking fault energy of the high entropy alloy from the atomic images and diffraction contrast of dislocation cores. The low stacking fault energy results in widely dissociated dislocations and extensive dislocation reactions, which leads to the formation of immobile Lomer and Lomer-Cottrell dislocation locks. These dislocation locks act as both dislocation barriers and sources and are responsible for the significant work hardening with a large hardening rate in the alloy. Based on the atomic-scale characterization and classical dislocation theory, a simple equation was derived to describe the work hardening behavior of the high entropy alloy in the early-stage of plastic deformation.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.050
      Issue No: Vol. 144 (2017)
       
  • Unveiling the mechanisms of cold sintering of ZnO at 250 °C by varying
           applied stress and characterizing grain boundaries by Kelvin Probe Force
           Microscopy
    • Authors: J. Gonzalez-Julian; K. Neuhaus; M. Bernemann; J. Pereira da Silva; A. Laptev; M. Bram; O. Guillon
      Pages: 116 - 128
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): J. Gonzalez-Julian, K. Neuhaus, M. Bernemann, J. Pereira da Silva, A. Laptev, M. Bram, O. Guillon
      The sintering behavior of nanocrystalline ZnO was investigated at only 250 °C. Densification was achieved by the combined effect of uniaxial pressure and the addition of water both in a Field Assisted Sintering Technology/Spark Plasma Sintering apparatus and a hand press with a heater holder. The final pure ZnO materials present high densities (>90% theoretical density) with nano-grain sizes. By measuring the shrinkage rate as a function of applied stress it was possible to identify the stress exponent related to the densification process. A value larger than one points to non-linear relationship going beyond single solid-state diffusion or liquid phase sintering. Only a low amount of water (1.7 wt%) was needed since the process is dictated by the adsorption on the surface of the ZnO particles. Part of the adsorbed water dissociates into H+ and OH− ions, which diffuse into the ZnO crystal structure, generating grain boundaries/interfaces with high defect chemistry. As characterized by Kelvin Probe Force Microscopy, and supported by impedance spectroscopy, this highly defective grain boundary area presents much higher surface energy than the bulk. This highly defective grain boundary area with high potential reduces the activation energy of the atomic diffusion, leading to sinter the compound at low temperature.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.055
      Issue No: Vol. 144 (2017)
       
  • Composition design of high entropy alloys using the valence electron
           concentration to balance strength and ductility
    • Authors: Ruirun Chen; Gang Qin; Huiting Zheng; Liang Wang; Yanqing Su; YuLung Chiu; Hongsheng Ding; Jingjie Guo; Hengzhi Fu
      Pages: 129 - 137
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Ruirun Chen, Gang Qin, Huiting Zheng, Liang Wang, Yanqing Su, YuLung Chiu, Hongsheng Ding, Jingjie Guo, Hengzhi Fu
      The valence electron concentration (VEC) is an important physical factor for phase formation. A high VEC is conducive to forming an FCC phase and improving an alloy's ductility, while a low VEC is beneficial in forming a BCC phase that improves an alloy's strength. This is demonstrated for two HEAs, CoCrCuFeNi (FCC) and AlCoCrFeNi (BCC), that were designed as matrix alloys, where Ni and Mo are alloyed. The microstructure, phase evolution, and the mechanical properties for (AlCoCrFeNi)100-x Ni x and (CoCrCuFeNi)100-x Mo x were systematically investigated. As the phase structure for the (AlCoCrFeNi)100-x Ni x high entropy alloy (HEA) transformed from a BCC to an FCC crystal structure as the Ni content increased from 0 at.% to 16 at.%, the FCC volume fraction increased from 0% to 85%, its compressive fracture strain increased from 25% to 40%, its VEC increased from 7.2 to 7.6. As the phase structure for the (CoCrCuFeNi)100-x Mo x HEA transformed from FCC to BCC as the Mo content increased from 0 at.% to 16 at.%, the BCC volume fraction increased from 0% to 65%, its compressive yield strength increased from 260 MPa to 928 MPa, its VEC decreased from 8.8 to 8.3. Selecting an element based upon an alloy's VEC is a practical method for designing compositions for HEAs that balance strength and ductility. According to the needs of practical applications, balancing both strength and plasticity requires the following criteria for selecting an element for incorporation into an HEA system: matrix strength is improved by selecting an element with a VEC lower than the average VEC of the matrix, while ductility is improved by selecting another element with a VEC higher than the average VEC for the matrix.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.058
      Issue No: Vol. 144 (2017)
       
  • Microstructural deformation in fatigued nanotwinned copper alloys
    • Authors: Nathan M. Heckman; Matthew F. Berwind; Christoph Eberl; Andrea M. Hodge
      Pages: 138 - 144
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Nathan M. Heckman, Matthew F. Berwind, Christoph Eberl, Andrea M. Hodge
      In this study, the uniaxial tension-tension fatigue behavior of fully nanotwinned magnetron sputtered Cu-6wt%Al, Cu-2wt%Al, and Cu-10 wt%Ni is presented. These alloys have average twin thicknesses ranging from 4 to 8 nm, average grain widths from 90 to 180 nm, and tensile strengths from 1 to 1.5 GPa. In the high cycle regime (103 to 107 cycles), the nanotwinned alloys exhibit fatigue strengths ranging from 210 to 370 MPa, which is higher than previously observed in nanotwinned Cu (fatigue strengths between 80 and 200 MPa). Fatigue strengths are normalized by tensile strength for Cu alloys with different microstructures to study the correlation between tensile and fatigue properties. Post-mortem analysis of the materials reveals a newly observed deformation mechanism, where localized detwinning leads to intergranular fracture between columnar grains. Overall, materials displaying detwinning as a deformation mechanism show lower normalized fatigue strengths in comparison to materials that deform with slip band like behavior.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.057
      Issue No: Vol. 144 (2017)
       
  • Strain-engineered allotrope-like bismuth nanowires for enhanced
           thermoelectric performance
    • Authors: Jeongmin Kim; Min-Wook Oh; Gwansik Kim; Je-Hyeong Bahk; Jae Yong Song; Seong Gi Jeon; Dong Won Chun; Jee-Hwan Bae; Wooyoung Shim; Wooyoung Lee
      Pages: 145 - 153
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Jeongmin Kim, Min-Wook Oh, Gwansik Kim, Je-Hyeong Bahk, Jae Yong Song, Seong Gi Jeon, Dong Won Chun, Jee-Hwan Bae, Wooyoung Shim, Wooyoung Lee
      Allotropy is a fundamental concept that has been frequently studied since the mid-1800s. Although the bulk allotropy of elemental solids is fairly well understood, it remains challenging to reliably produce an allotrope at the nanoscale that has a different crystal structure and accompanies a change in physical properties for specific applications. Here, we demonstrate a "heterostructure" approach to produce allotrope-like bismuth nanowires, where it utilizes the lattice constant difference between bismuth and tellurium in core/shell structure. We find that the resultant strain of [100]-grown Bi nanowires increases the atomic linear density along the c-axis that has been predicted from theoretical considerations, enabling us to establish a design rule for strain-induced allotropic transformation. With our >400-nm-diameter nanowires, we measure a thermoelectric figure of merit ZT of 0.5 at room temperature with reduced thermal conductivity and enhanced Seebeck coefficient, which are primarily a result of the rough interface and the reduced band overlap according to our density-functional calculations.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.062
      Issue No: Vol. 144 (2017)
       
  • High frequency in situ fatigue response of Ni-base superalloy René-N5
           microcrystals
    • Authors: Steven Lavenstein; Bryan Crawford; Gi-Dong Sim; Paul A. Shade; Christopher Woodward; Jaafar A. El-Awady
      Pages: 154 - 163
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Steven Lavenstein, Bryan Crawford, Gi-Dong Sim, Paul A. Shade, Christopher Woodward, Jaafar A. El-Awady
      A novel in situ scanning electron microscope (SEM), high frequency fatigue testing methodology is developed using a combination of laser milling, focused ion beam fabrication and nanoindentation. This methodology is used to investigate crack initiation, propagation, fracture, fatigue life, and the mechanical response of microcantilever samples of a Ni-based superalloy (René-N5) under different cyclic strain amplitudes. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam's dynamic stiffness and continuous SEM imaging. The dynamic stiffness response of the micro-beams exhibits a transition from softening to hardening at a critical strain amplitude of 7 × 10 − 3 . Theoretical analysis indicates that this transition corresponds to the stress required to shear γ ′ precipitates. SEM imaging reveals the evolution of significant extrusions, intrusions, and slip traces during cyclic loading above this critical strain amplitude. Below this strain amplitude, very little surface roughening is observed. In addition, the measured dynamic stiffness is observed to exhibit two regimes of decrease after crack initiation. These two regimes correspond to short and large crack propagation. Finally, an overall increase in fatigue life is observed when comparing to bulk scale experiments on nickel-base superalloys. It is proposed that this is an inherent size effect in the small-volume, single crystal specimens tested.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.049
      Issue No: Vol. 144 (2017)
       
  • Hydrogen-modified dislocation structures in a cyclically deformed
           ferritic-pearlitic low carbon steel
    • Authors: Shuai Wang; Akihide Nagao; Petros Sofronis; Ian M. Robertson
      Pages: 164 - 176
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Shuai Wang, Akihide Nagao, Petros Sofronis, Ian M. Robertson
      The fatigue-crack growth rate of a ferritic-pearlitic low carbon steel was faster when the tests were conducted in high-pressure H2 gas environments than in air. The predominant fracture feature changed from ductile fatigue striations with some “quasi-cleavage-like” regions when the test was conducted in air to mixed “quasi-cleavage” and “flat” facets when tested in a H2 gas environment. The microstructure beneath the fracture surfaces produced in air was sub-grains, and over a distance of 15 μm from the fracture surface, the dimensions of the sub-grains increased. With hydrogen, dense dislocation bands and refined dislocation cells existed beneath the “quasi-cleavage” and “flat” fracture surfaces. The cell size increased with distance from the fracture surface. The decrease in the dimensions of the key microstructural features as the fracture surface is approached is attributed to the propagation of the crack through an already deformed matrix. The differences in evolved dislocation structure are explained in terms of the hydrogen-enhanced localized plasticity mechanism, and the hydrogen-modified dislocation structure establishes the local conditions that promote the fracture mode transition from ductile fatigue striations to a mixture of “quasi-cleavage” and “flat” features, which directly leads to enhanced fatigue-crack growth.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.034
      Issue No: Vol. 144 (2017)
       
  • Alternative misfit dislocations pattern in semi-coherent FCC {100}
           interfaces
    • Authors: Shuai Shao; Firas Akasheh; Jian Wang; Yue Liu
      Pages: 177 - 186
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Shuai Shao, Firas Akasheh, Jian Wang, Yue Liu
      The character of interface misfit dislocations is determined according to interface crystallography and minimization of interface energy, which includes coherent interface energy and dislocation line energy. The core energy of dislocations is generally ignored in such analysis. In this work, we demonstrate that the core energy of misfit dislocations is dependent on the mechanical and thermal loading condition, and ultimately determines the nature of interface misfit dislocation patterns (MDP). Employing atomistic simulations with empirical interatomic potentials, we show the transformation of conventional MDP consisting of a/2<110> dislocation into an alternative MDP consisting of mixed a<100> and a/2<110> dislocations under elevated temperatures and/or normal-to-interface tensile stresses. Although a<100> type dislocations typically have greater line energy in bulk, molecular statics/dynamics calculations show that a<100> type misfit dislocations are preferred over a/2<110> type under elevated temperatures and/or normal-to-interface tensile stresses due to their reduced core energy. In addition, we found that the a<100> dislocations possess significantly reduced vacancy formation energies compared to the a/2<110> dislocations. The potential application of this unique property of the alternative dislocation pattern for nanoscale multilayered composite as a functional material is discussed.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.052
      Issue No: Vol. 144 (2017)
       
  • Surface termination analysis of stoichiometric metal hexaborides: Insights
           from first-principles and XPS measurements
    • Authors: K.M. Schmidt; O. Jaime; J.T. Cahill; D. Edwards; S.T. Misture; O.A. Graeve; V.R. Vasquez
      Pages: 187 - 201
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): K.M. Schmidt, O. Jaime, J.T. Cahill, D. Edwards, S.T. Misture, O.A. Graeve, V.R. Vasquez
      We present a modeling framework and discuss the energetics and structural features of the surface terminations of Ca, Ba, Sr and La hexaborides using density functional theory analysis in combination with X-ray photoelectron spectroscopy. There is significant uncertainty in the literature about the nature of the surface terminations in metal hexaborides in terms of metal versus boron terminations. We show from electronic structure calculations that segregated regions of metal and boron-terminations produce the lowest energies for di-cations of CaB6, SrB6 and BaB6, while trivalent LaB6 minimizes the surface energy by arranging the metal ions in parallel rows on the surface. XPS measurements show that CaB6 and SrB6 have surfaces that are close to stoichiometric for the compound, while BaB6 has surfaces that are Ba-rich. Energetic barriers are calculated for transitions between each of the surface geometries considered. There is a substantial increase in the activation energy for the lanthanum migrations compared to the divalent cations. We also find that the boron octahedra units in these materials tend to contract or expand from their bulk values depending on the proximity to regions of high metal concentrations. These materials have many attractive features, such as low work functions, high hardness, low thermal expansion coefficients, and high melting points, among many other properties of interest for industrial applications. Promising uses of these materials also include catalytic applications for chemical dissociation reactions of various molecules such as hydrogen, water and carbon monoxide, for example, thus, the interest in determining relevant surface properties.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.045
      Issue No: Vol. 144 (2017)
       
  • Strong and tough metal/ceramic micro-laminates
    • Authors: Claudio Ferraro; Sylvain Meille; Julien Réthoré; Na Ni; Jerome Chevalier; Eduardo Saiz
      Pages: 202 - 215
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Claudio Ferraro, Sylvain Meille, Julien Réthoré, Na Ni, Jerome Chevalier, Eduardo Saiz
      There is a growing interest in the development of composites with complex structures designed to generate enhanced mechanical properties. The challenge is how to implement these structures in practical materials with the required degree of control. Here we show how freeze casting of ceramic preforms combined with metal infiltration can be used to fabricate Al2O3/Al-4wt% Mg micro-laminated composites. By manipulating the solid content of the suspension and the morphology of the ceramic particles (from platelets to round particles) it is possible to access a range of structures with layer thickness varying between 1 and 30 μm and metallic contents between 66 and 86 vol%. The mechanical response of the materials is characterized by combining bending tests with observation of crack propagation in two and three dimensions using different imaging techniques. These composites are able to combine high strength and toughness. They exhibit a rising R-curve behaviour although different structures generate different toughening mechanisms. Composites fabricated with Al2O3 particles exhibit the highest fracture resistance approaching 60 MPa m1/2, while laminates prepared from Al2O3 platelets exhibit higher strengths (above 700 MPa) while retaining fracture resistance up to ∼40 MPa m1/2. The results provide new insights on the effect of structure on the mechanical properties in metal-ceramic composites as well as on the design of appropriate testing procedures.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.059
      Issue No: Vol. 144 (2017)
       
  • Tailoring the mechanical properties of sputter deposited nanotwinned
           nickel-molybdenum-tungsten films
    • Authors: Gi-Dong Sim; Jessica A. Krogstad; Kelvin Y. Xie; Suman Dasgupta; Gianna M. Valentino; Timothy P. Weihs; Kevin J. Hemker
      Pages: 216 - 225
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Gi-Dong Sim, Jessica A. Krogstad, Kelvin Y. Xie, Suman Dasgupta, Gianna M. Valentino, Timothy P. Weihs, Kevin J. Hemker
      Advanced metallic alloys are attractive in microelectromechanical systems (MEMS) applications that require high density, electrical and thermal conductivity, strength, and dimensional stability. Here we report the mechanical behavior of direct current (DC) magnetron sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) films annealed at various temperatures. The films deposit as single-phase nanotwinned solid solutions and possess ultra-high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and nucleation of Mo-rich precipitates. While films annealed at 600 °C or 800 °C for 1 h still showed brittle behavior, films annealed at 1,000 °C for 1 h were found to exhibit strength greater than 1.2 GPa and near 10% tensile ductility. In addition to the excellent mechanical properties, alloy films further exhibit remarkably improved dimensional stability – a lower coefficient of thermal expansion and greater microstructural stability. An excellent balance between mechanical properties and dimensional stability make sputter deposited Ni-Mo-W alloys promising structural materials for MEMS applications.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.065
      Issue No: Vol. 144 (2017)
       
  • Texture-directed twin formation propensity in Al with high stacking fault
           energy
    • Authors: S. Xue; W. Kuo; Q. Li; Z. Fan; J. Ding; R. Su; H. Wang; X. Zhang
      Pages: 226 - 234
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): S. Xue, W. Kuo, Q. Li, Z. Fan, J. Ding, R. Su, H. Wang, X. Zhang
      Twin boundaries can enhance the strength and preserve the ductility of a variety of metallic materials with face centered cubic structures. However, twin boundaries are rare in aluminum due to its high stacking fault energy. There are limited successes for the formation of nanotwins in Al. Here, we show that twin morphology and twin density in Al can be altered by tailoring the textures of films. Transmission Kikuchi diffraction and transmission electron microscopy studies on (111), (110) and (112) textured Al films indicate that Al (112) film has the highest twin density. Two design criteria are identified for the introduction of a high density of growth twins into Al.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.053
      Issue No: Vol. 144 (2017)
       
  • Spatially resolved localization and characterization of trapped hydrogen
           in zero to three dimensional defects inside ferritic steel
    • Authors: Waldemar Krieger; Sergiy V. Merzlikin; Asif Bashir; Agnieszka Szczepaniak; Hauke Springer; Michael Rohwerder
      Pages: 235 - 244
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Waldemar Krieger, Sergiy V. Merzlikin, Asif Bashir, Agnieszka Szczepaniak, Hauke Springer, Michael Rohwerder
      In this work, localized hydrogen (H) detection measurements were conducted on a model ferritic (Fe 5 wt.-% Ni) steel which enables a systematic study of 0- (vacancies), 1- (dislocations), 2- (grain boundaries) and 3- (inclusions) dimensional defects, induced by varying mechanical and thermal treatments, without changing the chemical composition of the material. Spatially resolved detection with Scanning Kelvin Probe Force Microscopy (SKPFM) as an electrochemical technique with a resolution on a nanometric scale in combination with Thermal Desorption Spectroscopy (TDS) and microstructure characterization using electron microscopy indicated a domination of at least two trapping sites. Step by step the dominating H trapping sites were identified as dislocations and vacancies with estimated desorption energies of 29 ± 5 and 38 ± 5 kJ mol−1. Furthermore, voids, inclusions and their interface to the matrix where found to be trapping sites binding low amounts of H, invisible for TDS measurements but detectible with SKPFM. Unexpectedly, no H was detected inside high angle grain boundaries. This study shows that dislocation and vacancies are the main trapping sites whereas vacancies getting increasingly important after cold rolling of steel rich of inclusions. Furthermore, the importance of using several techniques to understand the trapping behaviour of H even in simplified model alloys is underlined.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.066
      Issue No: Vol. 144 (2017)
       
  • Towards the prediction of hydrothermal ageing of 3Y-TZP bioceramics from
           processing parameters
    • Authors: Chong Wei; Laurent Gremillard
      Pages: 245 - 256
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Chong Wei, Laurent Gremillard
      Hydrothermal ageing of yttria-stabilized tetragonal zirconia ceramics can have a strong influence on the lifetime of zirconia devices. Ageing kinetics are often described by the Mehl–Avrami–Johnson equation, most often used as a phenomenological description. This work seeks to relate the parameters of MAJ equations (Vmax, n, b0 and Q) to microstructural characteristics of the zirconia material: grain sizes, Y2O3 partitioning, monoclinic, tetragonal and cubic phases ratio. Samples with identical nominal composition of 3Y-TZP were prepared with grain sizes ranging from 190 nm to 773 nm. From their microstructural parameters, a relationship between microstructural parameters and sintering cycles was first proposed, followed by a relationship between ageing parameters and microstructural parameters. These results provide a convenient framework to better develop the sintering cycle of zirconia biomaterial in order to maximize their resistance to hydrothermal ageing.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.061
      Issue No: Vol. 144 (2017)
       
  • In situ micromechanical testing in environmental scanning electron
           microscope: A new insight into hydrogen-assisted cracking
    • Authors: Bjørn Rune Sørås Rogne; Nousha Kheradmand; Yun Deng; Afrooz Barnoush
      Pages: 257 - 268
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Bjørn Rune Sørås Rogne, Nousha Kheradmand, Yun Deng, Afrooz Barnoush
      The susceptibility of Fe–26Al–0.5Cr (at.%) intermetallic alloy to hydrogen assisted cracking was evaluated by micromechanical fracture mechanics specimens. The notched micro-beams were loaded in situ in an environmental scanning electron microscope under two conditions: one with low pressure to avoid any hydrogen effect and the other with water vapour to promote hydrogen uptake and hydrogen assisted cracking. Fractographic and electron backscatter diffraction analysis carried out on the fracture surfaces of the ruptured beams revealed the mechanisms of the crack propagation. Under both conditions, the failure was cleavage-like, accommodated with plastic deformation. The results show the influence of hydrogen-induced embrittlement on the initiation of the fracture and the plasticity of the crack tip during the propagation of the crack, where the latter becomes localised and uniform over the fracture surface. From the observations, a three stage crack propagation process is proposed. Also, the fracture toughness of the samples was evaluated by linear elastic fracture mechanics and the validity of the results are discussed.
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      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.037
      Issue No: Vol. 144 (2017)
       
  • Microstructure evolution during austenite reversion in Fe-Ni martensitic
           alloys
    • Authors: H. Shirazi; G. Miyamoto; S. Hossein Nedjad; T. Chiba; M. Nili Ahmadabadi; T. Furuhara
      Pages: 269 - 280
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): H. Shirazi, G. Miyamoto, S. Hossein Nedjad, T. Chiba, M. Nili Ahmadabadi, T. Furuhara
      The change of microstructure during reverse transformation by continuous heating and isothermal holding above Af temperature were studied in Fe-11, 18 and 23 Ni (mass %) alloys. In-situ observation by using confocal laser scanning microscopy (CLSM) and in-situ/ex-situ electron backscatter diffraction (EBSD) analysis were used for direct observation of reverse transformation. It was found that the start temperatures (As) for austenite reversion decrease with increasing of Ni content while they are higher than T0 temperatures. Reverse transformation in the Fe-23 Ni alloy is accompanied with a sharp surface relief indicating that reverse transformation occurs martensitically in this alloy. EBSD measurements show that reversed austenite grains in this alloy are formed with nearly identical crystallographic orientations to the prior one, which means orientations and boundaries of prior austenite grains are preserved due to the austenite memory effect. By further holding above Af temperature spontaneous recrystallization of reverted austenite proceeds. The Fe-18 Ni alloy also shows similar microstructure change during reversion. Near Kurdjamov-Sachs (K-S) orientation relationship is found between reversed austenite and initial martensite during reversion of the Fe-18 and 23 Ni alloys. However, when the Ni content is decreased to 11%, no specific orientation relationship is found between reversed austenite and initial martensite, indicating that the reversion mechanism is changed from martensitic to partitionless diffusional (massive) mechanism.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.068
      Issue No: Vol. 144 (2017)
       
  • Unraveling the origin of twin related domains and grain boundary evolution
           during grain boundary engineering
    • Authors: Christopher M. Barr; Asher C. Leff; Ryan W. Demott; Roger D. Doherty; Mitra L. Taheri
      Pages: 281 - 291
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Christopher M. Barr, Asher C. Leff, Ryan W. Demott, Roger D. Doherty, Mitra L. Taheri
      Grain boundary engineering of Fe-based austenitic stainless steels and other materials has been successful in producing a large increase in twin and twin related grain boundaries from a wide range of thermomechanical treatments. However, the exact mechanisms and effective grain boundary network descriptors to create the heavily twinned microstructures are yet to be fully understood. In this study, we provide insight into the grain boundary engineering process by examining sequential progression of the same spatial location of a twin related microstructure through thermomechanical processing. The results show that clusters of twin related grain boundaries called twin related domains form during primary recrystallization. The size of the twin related domains increases as the level of strain falls toward the critical strain for recrystallization. Growth of twin related domains during recrystallization results in the formation of twin boundaries behind the migrating grain boundary front. Formation of higher order twin boundaries occurs when two separate grain boundary fronts of the same twin related domain impinge upon each other. We also present relevant microstructural descriptors with emphasis on twin related domain statistics to recrystallization phenomena in grain boundary engineering materials.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.007
      Issue No: Vol. 144 (2017)
       
  • Defect-mediated multiple-enhancement of phonon scattering and decrement of
           thermal conductivity in (YxYb1-x)2SiO5 solid solution
    • Authors: Zhilin Tian; Chunfu Lin; Liya Zheng; Luchao Sun; Jialin Li; Jingyang Wang
      Pages: 292 - 304
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Zhilin Tian, Chunfu Lin, Liya Zheng, Luchao Sun, Jialin Li, Jingyang Wang
      Rare earth (RE) silicates are promising candidates for environmental and thermal barrier coating (ETBC) materials. Low thermal conductivity is one of the main concerned thermal properties in ETBC design. We herein adopted multiple phonon scattering mechanisms to lower thermal conductivity of (YxYb1-x)2SiO5 solid solutions. Bulk samples were prepared by hot pressing method and RE atomic occupations, Raman spectra, thermal conductivities were measured as well as Debye temperature was obtained from temperature dependent Young's modulus. It is interesting to note that huge mass and size misfits between Yb and Y ions dominate the decrement of thermal conductivity. Furthermore, Yb2+ increases the concentration of oxygen vacancy, and it further decreases heat conduction. This work highlights the possible defect engineering in RE silicates for their advances in ETBC applications.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.064
      Issue No: Vol. 144 (2017)
       
  • Strong converse magnetoelectric effect in (Ba,Ca)(Zr,Ti)O3 - NiFe2O4
           multiferroics: A relationship between phase-connectivity and interface
           coupling
    • Authors: M. Naveed-Ul-Haq; Vladimir V. Shvartsman; Harsh Trivedi; Soma Salamon; Samira Webers; Heiko Wende; Ulrich Hagemann; Jörg Schröder; Doru C. Lupascu
      Pages: 305 - 313
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): M. Naveed-Ul-Haq, Vladimir V. Shvartsman, Harsh Trivedi, Soma Salamon, Samira Webers, Heiko Wende, Ulrich Hagemann, Jörg Schröder, Doru C. Lupascu
      Studying multiferroic magnetoelectrics has been a focus field for the last decade and a half, and the exploration of new materials is one of the several aspects of this quest. Here we report on the synthesis and characterization of NiFe2O4-based multiferroic composites which employ (Ba,Ca)(Zr,Ti)O3 as the ferroelectric/piezoelectric component and NiFe2O4 as the magnetostrictive phase. We find that these composites show excellent magnetoelectric properties. Especially the composite with 30 vol% of NiFe2O4 has a converse ME coefficient approximately two times larger than the previously reported one for BaTiO3-CoFe2O4 composites. A relationship between the phase connectivity within these composites and the ME properties was explored by the time of flight secondary ion mass microscopy. We believe that our investigation will be helpful for the design of magnetoelectric materials as components of sensors and memory devices.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.048
      Issue No: Vol. 144 (2017)
       
  • Twin-interface interactions in nanostructured Cu/Ag: Molecular dynamics
           study
    • Authors: R. Béjaud; J. Durinck; S. Brochard
      Pages: 314 - 324
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): R. Béjaud, J. Durinck, S. Brochard
      The interaction of deformation twins with interfaces in nanostructured Cu/Ag is studied using molecular dynamics simulations. The influence of the interface structure on twin nucleation, propagation and thickening is analysed, and the role of the misfit interfacial dislocations mesh is detailed. In particular, we show that the interface can induce, directly or indirectly via Lomer dislocations, the nucleation of twinning dislocations. A thorough description of the involved mechanisms is given. Through this atomic scale approach, our study offers some useful understanding of the mechanical twinning process in nanolamellar composites, where twinning appears to be a common plasticity mechanism.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.036
      Issue No: Vol. 144 (2017)
       
  • Temperature-dependence of mode I fracture toughness of a bulk metallic
           glass
    • Authors: Devaraj Raut; R.L. Narayan; Parag Tandaiya; Upadrasta Ramamurty
      Pages: 325 - 336
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Devaraj Raut, R.L. Narayan, Parag Tandaiya, Upadrasta Ramamurty
      Within the temperature range over which the shear band (SB)-mediated plastic deformation is dominant, metallic glasses exhibit an intermediate temperature ductility minimum (ITDM), which occurs at about 65% of the glass transition temperature, T g . This ITDM is associated with a small number of SBs, with each band carrying large amount of plastic strain, which in turn leads to their easy transition to shear cracks, eventually leading to fracture. Some MGs are known to exhibit high room temperature (RT) fracture toughness, which has been associated with SB-mediated crack-tip plasticity. Hence, it is expected that ITDM would also correspond to a minimum in toughness. In order to ascertain this, temperature-dependence of mode I fracture toughness, J c , of a bulk metallic glass (BMG), Vitreloy 105, was investigated by recourse to 4-point bend testing of single edge notched specimens within 298–475 K range, which corresponds to ∼0.44 and 0.7T g of the tested BMG. Complementary finite element analyses were utilized to convert the critical load for fracture into J c . Results confirm a minimum in J c at ∼0.67T g , which is in agreement with the results of unnotched 3-point bend experiments on unnotched bars that show ITDM at 0.65T g . These observations are rationalized with the aid of notch plastic deformation and post mortem fractographic characterizations and in terms of the influence of temperature on factors such as the number of shear bands, the barrier for their conversion into shear cracks, and hydrostatic stress gradients ahead of the notch tip. This study highlights the sensitive nature of BMGs' fracture toughness, even when they are nominally ductile, to temperature.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.063
      Issue No: Vol. 144 (2017)
       
  • The α→ω and β→ω phase transformations in Ti–Fe alloys under
           high-pressure torsion
    • Authors: A.R. Kilmametov; Yu. Ivanisenko; A.A. Mazilkin; B.B. Straumal; A.S. Gornakova; O.B. Fabrichnaya; M.J. Kriegel; D. Rafaja; H. Hahn
      Pages: 337 - 351
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): A.R. Kilmametov, Yu. Ivanisenko, A.A. Mazilkin, B.B. Straumal, A.S. Gornakova, O.B. Fabrichnaya, M.J. Kriegel, D. Rafaja, H. Hahn
      The formation of ω-phase under high-pressure torsion (HPT) has been studied in Ti–Fe alloys. Seven alloys with Fe concentration from 0 to 10 wt % have been annealed between 600 and 950 °C, quenched and HPT-treated at 7 GPa, 1 rpm, 5 and 0.1 anvil rotations (equivalent strain e eq = 156 and = 3.1, respectively). The strain after 0.1 rot. corresponds to the transient state of HPT, and that after 5 rot. corresponds to the HPT steady-state and to the dynamic equilibrium between formation and annihilation of microstructure defects. A defect-rich high-pressure ω-phase forms after HPT and persists in the samples also after the pressure release. The amount of retained ω-phase after HPT depends on the iron concentration. It increases from 40% in pure titanium, reaches maximum of 95% at 4 wt % Fe and then decreases again to 10% at 10 wt % Fe. It is because the addition of iron influences the lattice parameters in β and ω-phases in a different manner. The minimal lattice mismatch between β- and ω-phases is reached at 4 wt % Fe. A good conformity between the lattices of the β- and ω-phases enhances the probability of the martensitic (diffusionless) β→ω transformation. Based on the XRD and TEM observations, the crystallography and mechanisms of α→ω and β→ω phase transformations (which can be diffusionless as well as controlled by mass transfer) under the influence of pure shear by HPT are discussed.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.051
      Issue No: Vol. 144 (2017)
       
  • Phase differentiation by electron backscatter diffraction using the
           dictionary indexing approach
    • Authors: Farangis Ram; Marc De Graef
      Pages: 352 - 364
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Farangis Ram, Marc De Graef
      Using the dictionary approach to Electron Backscatter Diffraction (EBSD) analysis, we address: (1) phase differentiation in highly-deformed or fine-grained materials; and (2) phase differentiation between phases with the same Bravais lattice and different lattice parameters. We introduce a phase differentiation confidence index based on the dictionary indexing approach and apply the index to scenarios where the classical EBSD indexing approach has major difficulties in delivering reliable results. We show that dictionary indexing successfully differentiates the phases in these scenarios and report on the confidence of such differentiations. We also show that dictionary indexing is sensitive to hydrostatic strain and can identify the correct amount of hydrostatic strain present in a material.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.069
      Issue No: Vol. 144 (2017)
       
  • On the role of twinning and stacking faults on the crystal plasticity and
           grain refinement in magnesium alloys
    • Authors: S.Q. Zhu; Simon P. Ringer
      Pages: 365 - 375
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): S.Q. Zhu, Simon P. Ringer
      We have investigated the detailed microstructural mechanisms associated with the excellent crystal plasticity and ultra-fine grain refinement observed under high strain-rate deformation of Mg alloys, focusing on ZK60. Firstly, we have identified the clear formation of stacking faults in deformation-induced twinned crystal segments. Specifically, we have found that intrinsic I 1 and I 2 stacking faults bounded by 1 6 < 2 ¯ 023> and 1 3 <10 1 ¯ 0> partial dislocations, respectively, were found to occur in very high number densities within the twins. This was due to the high Schmid factor for stacking fault shearing in twins and the critical role that twin boundaries played in emitting partial dislocations. Secondly, we have clarified the interplay between twinning and stacking faults on the enhanced crystal plasticity. Apart from the strain accommodated by the extensive twinning itself, we propose that the improved plasticity during high strain-rate deformation is mainly due to the nucleation of 1 3 < 11 ¯ 23>{11 2 ¯ 2} dislocation within twins, which provides enough independent slip systems to achieve a homogeneous deformation in the material. Finally, we have demonstrated the interplay between twinning and stacking fault formation on the nucleation of new grains via dynamic recrystallisation. The twin boundaries and stacking faults, especially those of the I 1 type, facilitate the formation of low-angle grain boundaries that can subsequently transition into high-angle grain boundaries, and form ultra-fine dynamically recrystallised grains.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.11.004
      Issue No: Vol. 144 (2017)
       
  • Elastic properties and plastic deformation of TiC- and VC-based
           pseudobinary alloys
    • Authors: D. Edström; D.G. Sangiovanni; L. Hultman; Ivan Petrov; J.E. Greene; V. Chirita
      Pages: 376 - 385
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): D. Edström, D.G. Sangiovanni, L. Hultman, Ivan Petrov, J.E. Greene, V. Chirita
      Transition-metal (TM) carbides are an important class of hard, protective coating materials; however, their brittleness often limits potential applications. We use density functional theory to investigate the possibility of improving ductility by forming pseudobinary cubic M1M2 C alloys, for which M1  = Ti or V and M2  = W or Mo. The alloying elements are chosen based on previous results showing improved ductility of the corresponding pseudobinary nitride alloys with respect to their parent compounds. While commonly-used empirical criteria do not indicate enhanced ductility in the carbide alloys, calculated stress/strain curves along known slip systems, supported by electronic structure analyses, indicate ductile behavior for VMoC. As VMoC layers are sheared along the 1 1 ¯ 0 direction on {111} planes, the stress initially increases linearly up to a yield point where the accumulated stress is partially dissipated. With further increase in strain, the stress increases again until fracture occurs. A similar mechanical behavior is observed for the corresponding TM nitride VMoN, known to be a ductile ceramic material [1]. Thus, our results show that VMoC is a TM carbide alloy which may be both hard and ductile, i.e. tough.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.047
      Issue No: Vol. 144 (2017)
       
  • A strategy to predict the fracture toughness of steels with a banded
           ferrite–pearlite structure based on the micromechanics of brittle
           fracture initiation
    • Authors: Kazuki Shibanuma; Yoshiki Nemoto; Takashi Hiraide; Katsuyuki Suzuki; Sunao Sadamatsu; Yoshitaka Adachi; Shuji Aihara
      Pages: 386 - 399
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Kazuki Shibanuma, Yoshiki Nemoto, Takashi Hiraide, Katsuyuki Suzuki, Sunao Sadamatsu, Yoshitaka Adachi, Shuji Aihara
      This paper presents a strategy to predict the fracture toughness of steels with a banded ferrite–pearlite structure using a new model based on the micromechanics of brittle fracture initiation. The model requires only the (1) ferrite grain size and pearlite band thickness distributions, (2) the stress–strain curve, and (3) the specimen geometry and boundary conditions of the fracture toughness test, without the need for any parameter fittings from the experimental results. The model is based on the multiscale model synthesis approach, consisting of three elemental models: (1) the microstructural spatial distribution, (2) a macroscopic finite element analysis, and (3) the microscopic fracture initiation processes, wherein the respective formulations of the fracture criteria of the three stages are proposed, namely, Stage I: micro-crack formation in shear in the pearlite colony; Stage II: the crack entering the adjacent ferrite grain; and Stage III: the propagation of the crack across ferrite grain boundary. The proposed model was validated by comparing it with the experimental results of five kinds of steels with a range of carbon concentrations, ferrite grain sizes, and pearlite band thicknesses. The predicted and experimental results agreed well for all steel samples and temperatures. In addition, the influence of the microstructure on the fracture toughness was discussed using virtual candidate steels containing various carbon concentrations, ferrite grain sizes, and pearlite band thicknesses. The results demonstrate that the proposed model is an effective and powerful tool for quantitatively predicting the fracture toughness of steel with a banded ferrite–pearlite microstructure.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.046
      Issue No: Vol. 144 (2017)
       
  • Hierarchical aging pathways and reversible fragile-to-strong transition
           upon annealing of a metallic glass former
    • Authors: Isabella Gallino; Daniele Cangialosi; Zach Evenson; Lisa Schmitt; Simon Hechler; Moritz Stolpe; Beatrice Ruta
      Pages: 400 - 410
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Isabella Gallino, Daniele Cangialosi, Zach Evenson, Lisa Schmitt, Simon Hechler, Moritz Stolpe, Beatrice Ruta
      The change of physical properties during aging and the associated microscopic dynamics of the Au49Cu26.9Si16.3Ag5.5Pd2.3 bulk metallic glass are investigated using a broad collection of laboratory and synchrotron-based techniques, such as differential- and fast-scanning calorimetry, thermomechanical testing, and x-ray photon correlation spectroscopy. We observe multiple decays in the enthalpy change during aging. This is reflected by a microscopic ordering consisting of distinct stationary dynamical regimes interconnected by abrupt aging processes. The stationary regimes are representative of states of local and transient equilibrium with increasingly higher activation energies. Furthermore, the aging study is conducted with the kinetically fragile frozen-in structure and the underlying fragile-to-strong transition is accessed by the ultra-viscous liquid state during annealing on a long-time scale and corresponds to the last observed enthalpy equilibration decay. The experimental work verifies, for the first time, that in a metallic glass forming system, the fragile-to-strong transition can also occur below the conventional glass transition temperature. Upon reheating, the reverse transformation, i.e. the strong-to-fragile transition, is observed with an entropy change of 0.19 J/(g-atom K), which is 2.4% of the entropy of fusion.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.060
      Issue No: Vol. 144 (2017)
       
  • Relaxation and saturation of electrostriction in 10 mol% Gd-doped
           ceria ceramics
    • Authors: Nimrod Yavo; Ori Yeheskel; Ellen Wachtel; David Ehre; Anatoly I. Frenkel; Igor Lubomirsky
      Pages: 411 - 418
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Nimrod Yavo, Ori Yeheskel, Ellen Wachtel, David Ehre, Anatoly I. Frenkel, Igor Lubomirsky
      10 mol% Gd-doped ceria (10GDC) ceramics, with grain size in the single micron range, display electrostrictive behavior under ambient conditions of temperature and pressure. In weak, quasi-static electric fields, i.e. <1 kV/cm, frequency <1 Hz, the longitudinal strain is measured to be proportional to the square of the applied electric field, albeit with the corresponding electrostrictive strain coefficient (M 33 ) displaying large variability between samples: −(2-20)·10−17 (m/V)2. Nevertheless, M 33 of all samples exceeds the values expected on the basis of the classical (Newnham) electrostriction scaling law by up to two orders of magnitude. A systematic study reveals the functional dependence of M 33 on frequency: above 10 Hz, M33 decreases to ≈10−18 (m/V)2, which may be characterized as non-Debye relaxation with non-ideality factor 0.35–1.13. For frequencies ≤1.5 Hz, increasing the field strength beyond 1 kV/cm results in an exponential decrease in M 33 : the longitudinal strain saturates at 1-4 ppm. Dielectric impedance spectra suggest that partitioning of the applied voltage between grain boundaries and grain cores may be a factor contributing both to the large variability in the electrostriction parameters, and to the strong dependence on electric field amplitude. The frequency dependence may have two sources: the slow electric field-driven reorganization of the Ce-containing active complexes in the electrostrictive medium as well as the influence of the grain boundaries. 10GDC ceramics may therefore be added to the list of non-classical electrostrictors which includes reduced and Gd-doped ceria thin films and (Nb,Y)-doped bismuth oxide ceramics.
      Graphical abstract image

      PubDate: 2017-11-11T16:08:53Z
      DOI: 10.1016/j.actamat.2017.10.056
      Issue No: Vol. 144 (2017)
       
  • Microstructural evolution and deformation behavior of Al-Cu alloys: A
           Transmission X-ray Microscopy (TXM) and micropillar compression study
    • Authors: C. Shashank Kaira; Christopher Kantzos; Jason J. Williams; Vincent De Andrade; Francesco De Carlo; Nikhilesh Chawla
      Pages: 419 - 431
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): C. Shashank Kaira, Christopher Kantzos, Jason J. Williams, Vincent De Andrade, Francesco De Carlo, Nikhilesh Chawla
      A unique approach to correlating an evolving 3D microstructure in an Al-Cu alloy and its micro-scale mechanical properties has been introduced. Using these nanoscale three-dimensional microstructures derived from Transmission X-ray Microscopy (TXM), individual contributions from different strengthening mechanisms were quantified. The spatial distribution and morphology of the individual θ′ and θ phases were seen to play an important role in influencing dislocation storage. Uniaxial micro-compression experiments were used to quantify the stress-strain response of the alloy at different aging times. Transmission electron microscopy (TEM) aided in discerning dislocation activity at these precipitates. A model is proposed to accurately predict the variation in yield stress by using appropriate morphological parameters from the 3D microstructure and its validity has been corroborated using experimental measurements. Distributions of 2D and 3D inter-precipitate spacing were seen to provide crucial insights on influencing deformation in such precipitation-strengthened alloys. Finally, the transition in deformation behavior and origin of numerous strain bursts were investigated using in situ micropillar compression testing.
      Graphical abstract image

      PubDate: 2017-11-24T07:20:18Z
      DOI: 10.1016/j.actamat.2017.11.009
      Issue No: Vol. 144 (2017)
       
  • Role of the X and n factors in ion-irradiation induced phase
           transformations of Mn+1AXn phases
    • Authors: Chenxu Wang; Tengfei Yang; Cameron L. Tracy; Jingren Xiao; Shaoshuai Liu; Yuan Fang; Zhanfeng Yan; Wei Ge; Jianming Xue; Jie Zhang; Jingyang Wang; Qing Huang; Rodney C. Ewing; Yugang Wang
      Pages: 432 - 446
      Abstract: Publication date: 1 February 2018
      Source:Acta Materialia, Volume 144
      Author(s): Chenxu Wang, Tengfei Yang, Cameron L. Tracy, Jingren Xiao, Shaoshuai Liu, Yuan Fang, Zhanfeng Yan, Wei Ge, Jianming Xue, Jie Zhang, Jingyang Wang, Qing Huang, Rodney C. Ewing, Yugang Wang
      Phase transitions induced in hcp M n+1AX n phases (Ti2AlN, Ti2AlC, and Ti4AlN3) by 1 MeV Au+ ion irradiation were investigated, over a series of ion fluences ranging from 1 × 1014 to 2 × 1016 ions cm−2, by transmission electron microscopy (TEM) and synchrotron grazing incidence X-ray diffraction (GIXRD). Irradiation-induced structural evolutions were observed using high-resolution TEM (HRTEM) imaging and selected area electron diffraction (SAED). Based on phase contrast imaging and electron diffraction pattern (EDP) simulations, the atomic-scale mechanisms for the phase transitions were determined. Transformations of the initial hcp phases to the intermediate γ-phases and fcc phases were driven by the formation of Ti/Al antisite defects and extended stacking faults induced by ion irradiation. By comparing the transformation behavior of Ti2AlN with that of Ti2AlC and Ti4AlN3 under the same irradiation conditions, using both the experimental data and first-principles calculations, the role of the X and n parameters in the radiation responses of Mn+1AXn phases were elucidated. The susceptibilities of materials in this Ti-Al-X (X = C, N) system to irradiation-induced phase transitions were determined with respect to the bonding characteristics and compositions of these MAX phases. Ti2AlC is slightly less susceptible to the radiation-induced phase transformation than Ti2AlN, which is attributed to the stronger Ti-Al bond covalency in Ti2AlN. Ti4AlN3 is more resistant to radiation-induced phase transformations than is Ti2AlN, due to the lower Al content and lower anion vacancy ratio in the irradiation-induced solid solution phases.
      Graphical abstract image

      PubDate: 2017-11-24T07:20:18Z
      DOI: 10.1016/j.actamat.2017.11.008
      Issue No: Vol. 144 (2017)
       
 
 
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