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Journal Cover Acta Materialia
  [SJR: 3.683]   [H-I: 202]   [239 followers]  Follow
    
   Hybrid Journal Hybrid journal (It can contain Open Access articles)
   ISSN (Print) 1359-6454
   Published by Elsevier Homepage  [3120 journals]
  • Core structure and solute strengthening of second-order pyramidal
           〈c+a〉 dislocations in Mg-Y alloys
    • Authors: D. Buey; L.G. Hector; M. Ghazisaeidi
      Pages: 1 - 9
      Abstract: Publication date: 1 April 2018
      Source:Acta Materialia, Volume 147
      Author(s): D. Buey, L.G. Hector, M. Ghazisaeidi
      The ability of Y as an alloying agent to improve the ductility of magnesium is explored using a solid solution strengthening model to determine the relative strengthening effect on the available deformation modes. We use density functional theory calculations to determine the interaction energy between an edge 〈 c + a 〉 dislocation and solute atoms surrounding it. We observe that substituting solute atoms directly into the positions closest to the dislocation significantly changes the structure of the dislocation, making the direct calculation and representation of these interaction energies difficult, and necessitating a modification to the calculation of interaction energies. Next, we apply a solution strengthening model to calculate the relative strengthening effect of the solutes and find that the ratio of the critical resolved shear stress of second-order pyramidal 〈 c + a 〉 slip to that of the basal slip, decreases with increasing Y concentration. The resulting, more isotropic plastic response is beneficial for improving the room temperature ductility of Mg alloys.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.066
      Issue No: Vol. 147 (2018)
       
  • Comparison of the in- and across-plane ionic conductivity of highly
           oriented neodymium doped ceria thin films
    • Authors: George Baure; Hanhan Zhou; Ching-Chang Chung; Marissa N. Buck; Mariia A. Stozhkova; Jacob L. Jones; Juan C. Nino
      Pages: 10 - 15
      Abstract: Publication date: 1 April 2018
      Source:Acta Materialia, Volume 147
      Author(s): George Baure, Hanhan Zhou, Ching-Chang Chung, Marissa N. Buck, Mariia A. Stozhkova, Jacob L. Jones, Juan C. Nino
      To determine the effect of grain boundaries and grain orientation on the electrical properties of solid oxide fuel cell electrolytes, a comparison of the in-plane and across-plane ionic conductivity of both strongly and poorly textured, columnar-grained doped ceria thin films was performed within equivalent temperature ranges (150–300 °C). Additionally, the in-plane conductivity of partially amorphous films, polycrystalline films with randomly oriented grains, and single crystal, epitaxial films with no grain boundaries was determined. Pulsed laser deposition permitted the growth of all these types of films and the ability to grow columnar-grained doped ceria on both conducting and insulating surfaces enabled testing of the films both in-plane and across-plane. Compared to the columnar-grained samples, partially amorphous films exhibited a lower conductivity, while epitaxial doped ceria exhibited an enhancement in conductivity of 2 orders of magnitude. Between 300 and 400 °C, the in-plane conductivity of the strongly textured film was higher than the poorly textured one. The conductivity and activation energy in-plane and across-plane for the strongly textured film was similar (2.75 × 10−5 S/cm, 0.70 eV vs. 5.50 × 10−5 S/cm, 0.68 eV at 250 °C). In contrast, for the poorly textured films, the in-plane and across-plane conductivity values differed by almost an order of magnitude (2.86 × 10−5 S/cm, 0.55 eV vs. 1.99 × 10−4 S/cm, 0.78 eV at 250 °C) suggesting that the boundaries between oriented grains were less resistive. These results further strengthen the argument that grain orientation affects ionic transport through grain boundaries.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.057
      Issue No: Vol. 147 (2018)
       
  • Fracture toughness, fracture energy and slow crack growth of glass as
           investigated by the Single-Edge Precracked Beam (SEPB) and Chevron-Notched
           Beam (CNB) methods
    • Authors: Theany To; Fabrice Célarié; Clément Roux-Langlois; Arnaud Bazin; Yann Gueguen; Hervé Orain; Mickaël Le Fur; Vincent Burgaud; Tanguy Rouxel
      Pages: 1 - 11
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Theany To, Fabrice Célarié, Clément Roux-Langlois, Arnaud Bazin, Yann Gueguen, Hervé Orain, Mickaël Le Fur, Vincent Burgaud, Tanguy Rouxel
      We show that the Single-Edge Precracked Beam (SEPB) test is not only suitable to the determination of the fracture toughness (K Ic ) of glass, but also offers a unique opportunity to assess the slow crack growth behavior in a single experiment lasting for few minutes. Besides, we found that it is possible to get either a stable or an unstable final fracture regime (pre-cracked specimen) depending on the testing parameters, and that the unstable case is preferable for the estimation of K Ic . The "pop-in" precrack was found mostly to close completely once the load was suppressed on the bridge-flexure device. This led to a reopening event on the loading curves. It is noteworthy that all these original observations were made possible thanks to the design of a very stiff testing apparatus (6.7 MN m−1) allowing for a cross-head speed as small as 0.01 μm s−1. Results obtained on four grades of commercially available glasses are compared to those stemming from Vickers indentation cracking and chevron notched experiments.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.11.056
      Issue No: Vol. 146 (2018)
       
  • Reversed compressive yield anisotropy in magnesium with microlaminated
           structure
    • Authors: Xin Wang; Lin Jiang; Dalong Zhang; Irene J. Beyerlein; Subhash Mahajan; Timothy J. Rupert; Enrique J. Lavernia; Julie M. Schoenung
      Pages: 12 - 24
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Xin Wang, Lin Jiang, Dalong Zhang, Irene J. Beyerlein, Subhash Mahajan, Timothy J. Rupert, Enrique J. Lavernia, Julie M. Schoenung
      To investigate the effect of grain morphology on the mechanical properties of polycrystalline Mg, two types of bulk Mg samples with equiaxed and microlaminated grain structures were fabricated by spark plasma sintering (SPS) of as-received Mg powder and cryomilled disc-shaped Mg powder particles, respectively. Based on a detailed microstructural investigation, the mechanisms by which microstructure evolves and texture development occurs were identified and are discussed. The basal fiber textures in the SPS consolidated samples allow the plastic anisotropy in such textured Mg to be investigated. Compression tests at room temperature parallel and perpendicular to the SPS compaction axis determined that, in comparison to the conventional anisotropy observed in the equiaxed sample, the anisotropy of yield strength is reversed in the microlaminated sample, with the yield strength for c-axis extension being higher than that for c-axis contraction. The reversed compressive yield strength anisotropy observed in the sample that was cryomilled is related to the low twinning activity, limited twinning growth and the anisotropy induced by the microlaminated grain structure, which offers an opportunity to reduce or even reverse the intrinsic plastic anisotropy of hexagonal close packed Mg.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.025
      Issue No: Vol. 146 (2018)
       
  • Variant selection mechanism by elastic anisotropy and double K-S relation
           for transformation texture in steel; difference between martensite and
           ferrite
    • Authors: Toshiro Tomida
      Pages: 25 - 41
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Toshiro Tomida
      Variant selection mechanisms in the phase transformation in steels have been investigated experimentally as well as theoretically. The characteristics of transformation textures from the deformed austenite in 0.1%C-1.1%Mn hot-rolled steel are the development of components lying in the range from {211}<011> to {332}<113>, and the {332}<113> component further evolves by displacive martensite transformation than by diffusive ferrite transformation. Via transformation texture calculation based on the observed textures, various mechanisms of variant selection have been explored particularly for martensite transformation. Whereas all the conventional models examined have failed to quantitatively reproduce the experimental textures of martensite, the mechanism based on the elastic anisotropy in neighboring matrices and the dual orientation relation to two parent grains (Double K-S relation) have successfully reproduced the experimental texture of martensite and the characteristic texture difference between martensite and ferrite at a hitherto unattainable level. The phenomenon of so-called variant pairing in low carbon lath martensite can also be explained by the elastic anisotropy mechanism.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.033
      Issue No: Vol. 146 (2018)
       
  • Spectral graph theory for characterization and homogenization of grain
           boundary networks
    • Authors: Oliver K. Johnson; Jarrod M. Lund; Tyler R. Critchfield
      Pages: 42 - 54
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Oliver K. Johnson, Jarrod M. Lund, Tyler R. Critchfield
      Grain boundary networks (GBNs) have a profound influence on the properties of both structural and functional materials. However, existing methods to characterize their complex structure have almost universally relied upon a binary classification of GBs as either “special” or “general”, which ignores the rich and continuous spectrum of GB types and properties. Furthermore, characterizing the aggregate network structure of GBs has proven complicated, with traditional methods focusing on local structure and also relying on a binary GB taxonomy, e.g. by evaluating how many “special” or “general” boundaries meet at triple junctions or quadruple nodes. Here we develop new structural metrics for GBNs, based on spectral graph theory, that encode both global network topology and the full spectrum of constituent GB properties, enabling high-fidelity characterization of arbitrary GBNs. Using these metrics, we derive an new structure-property relation for GBN diffusivity. The dominant term in this expression provides an efficient and accurate approximation, whose corresponding spectral metrics reveal the dominant microstructural features influencing the property of interest. The spectral index of this term serves as a type of global order parameter that reveals a fundamental structural transition in GBNs. This work provides a new framework to characterize the structure of GBNs in greater generality than previously possible and facilitates the development of new defect-sensitive structure-property models.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.11.054
      Issue No: Vol. 146 (2018)
       
  • Slip mode dependency of dislocation shearing and looping of precipitates
           in Mg alloy WE43
    • Authors: J.J. Bhattacharyya; F. Wang; N. Stanford; S.R. Agnew
      Pages: 55 - 62
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): J.J. Bhattacharyya, F. Wang, N. Stanford, S.R. Agnew
      The effect of precipitates on the strength of Mg alloy, WE43, has been investigated. Transmission electron microscopy (TEM) provides unequivocal evidence that the ordered prismatic plate-shaped β′ precipitates are sheared by basal slip of <a> dislocations. However, non-basal <a> and pyramidal <c+a> dislocations are unable to shear the precipitates, and instead bow around them during plastic deformation. In the latter case the Orowan looping model accurately predicts precipitate hardening. For the case of the shearable particles, it is proposed that order strengthening is the dominant mechanism by which the β′ phase provides strengthening to the basal slip system, and a new model quantifying this effect is developed. Using this model in conjunction with TEM observations and elastoplastic self-consistent polycrystal modeling, the anti-phase boundary energy (APB) of the precipitates is estimated to be 210 mJ/m2. The present work has significant implications for Mg alloy design strategies, and emphasizes the importance of considering both precipitate shearing and Orowan looping of precipitates. Finally, it suggests the value of making first-principles predictions of APB energies (γ-surfaces) of the candidate strengthening phases.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.043
      Issue No: Vol. 146 (2018)
       
  • Peritectic transformation with non-linear solute distribution in all three
           phases: Analytical solution, phase-field modeling and experiment
           comparison
    • Authors: Shiyan Pan; Mingfang Zhu
      Pages: 63 - 75
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Shiyan Pan, Mingfang Zhu
      An analytical model is derived to predict the kinetics of a peritectic transformation involving solute diffusion in the L-, δ- and γ-phases with non-zero constant diffusivities. A similarity method is adopted to solve the diffusion equations in the bulk L-, δ-, and γ-phases coupled with the solute conservation equations at the γ/L and γ/δ interfaces. The proposed model is applied to predict the kinetics of the isothermal peritectic transformation of Fe-C alloys for two conditions, zero and non-zero supersaturation, in the δ- and L-phases. An excellent match between the analytical solutions and phase-field simulations is achieved. In the case of zero supersaturation, the time evolution of γ-phase thickness and non-linear concentration distribution in the γ-phase predicted by the present analytical model agree well with the experimental data reported in the literature. When the holding temperature decreases, the parabolic rate constant at the γ/δ interface increases non-linearly, while it remains nearly unchanged at the γ/L interface. Additionally, the non-zero supersaturation in the parent phases increases the diffusion flux jump significantly at the γ/L interface but only slightly at the γ/δ interface. As a result, the growth velocity increases noticeably at the γ/L interface, but it is nearly unchanged at the γ/δ interface.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.034
      Issue No: Vol. 146 (2018)
       
  • Material structure-property linkages using three-dimensional convolutional
           neural networks
    • Authors: Ahmet Cecen; Hanjun Dai; Yuksel C. Yabansu; Surya R. Kalidindi; Le Song
      Pages: 76 - 84
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Ahmet Cecen, Hanjun Dai, Yuksel C. Yabansu, Surya R. Kalidindi, Le Song
      The core materials knowledge needed in the accelerated design, development, and deployment of new and improved materials is most accessible when cast in the form of computationally low cost (reduced-order) and reliable process-structure-property (PSP) linkages. Quantification of the material structure (also referred as microstructure) is the core challenge in this task. Conventionally, microstructure quantification has been addressed using highly simplified measures suggested by the governing physics, with the list of measures often suitably augmented by the intuition of the materials expert. In this paper, we develop an objective (data-driven) approach to efficiently and accurately link a three-dimensional (3-D) microstructure to its effective (homogenized) properties. Our method employs a 3-D convolutional neural network (CNN) to learn the salient features of the material microstructures that lead to good predictive performance for the effective property of interest. We then utilize 3-D CNN learned features as estimators of higher-order spatial correlations, and formulate an integrated framework combining 3-D CNN features with 2-point spatial correlations. In this work, we created an extremely large microstructure-property benchmark dataset of 5900 microstructures, and demonstrated that our CNN based approach not only learns interpretable microstructure features, but also leads to improved accuracy in property predictions for new microstructures, while achieving a dramatic reduction in the computation time.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.11.053
      Issue No: Vol. 146 (2018)
       
  • In-situ magnetic force microscopy analysis of magnetization and
           demagnetization behavior in Al3+ substituted Sr-hexaferrite
    • Authors: F. Rhein; T. Helbig; V. Neu; M. Krispin; O. Gutfleisch
      Pages: 85 - 96
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): F. Rhein, T. Helbig, V. Neu, M. Krispin, O. Gutfleisch
      The sintering temperature of an Al3+ substituted Sr-hexaferrite composite was systematically varied from 1180 °C to 1280 °C resulting in different microstructures. The grain size was found to range from a few hundred nanometers to several hundred micrometers depending on Al content and sintering temperature. Adding an Al substituted powder to a commercial powder increased the coercivity from 360 mT to 470 mT, at the same time, decreasing remanence from 350 mT to 305 mT. Magnetization and demagnetization processes from the thermally demagnetized state (TDS) and DC-demagnetized state (DCD) have been investigated systematically by in-situ magnetic force microscopy (MFM) under magnetic field. From the surface domain contrast a polarization was derived which quantitatively matches the global i.e. bulk polarization obtained by superconducting quantum interface device (SQUID) magnetometry. The shape of the initial polarization curve and the polarization from the DCD state were correlated with the in-situ MFM data revealing a distinctly different magnetization behavior depending on grain size. The presented results enable a better understanding of local nucleation mechanisms, global influences of pinning centers and further opportunities to improve rare earth (RE) free permanent magnets based on ferrites.

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.010
      Issue No: Vol. 146 (2018)
       
  • Post-sinter annealing influences on coercivity of multi-main-phase
           Nd-Ce-Fe-B magnets
    • Authors: Yujing Zhang; Tianyu Ma; Mi Yan; Jiaying Jin; Bo Wu; Baixing Peng; Yongsheng Liu; Ming Yue; Chuyang Liu
      Pages: 97 - 105
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Yujing Zhang, Tianyu Ma, Mi Yan, Jiaying Jin, Bo Wu, Baixing Peng, Yongsheng Liu, Ming Yue, Chuyang Liu
      Post-sinter annealing (PSA) plays an essential role on enhancing coercivity of the single-main-phase (SMP) Nd2Fe14B-based magnets due to the formation of continuous Nd-rich grain boundary (GB) layers to decouple the adjacent grains. Here we investigated the PSA effects on coercivity of the multi-main-phase (MMP) [(Nd,Pr)0.55Ce0.45]30.5FebalM1.0B1.0 (wt.%) magnet prepared by sintering the mixture of Ce-free and Ce-containing 2:14:1 powders, which has shown superior magnetic properties to the SMP one at the same average composition. The annealing temperature dependent investigation showed that the coercivity can be enhanced when annealing at low temperatures, but becomes even lower than that of the as-sintered state when annealing at higher temperatures, despite that continuous RE-rich GB layers are formed after annealing. When raising the annealing temperature, the RE gradients within the 2:14:1 phase grains are reduced to reach the ‘‘close to equilibrium (or SMP)’’ state due to the further elemental inter-diffusions among the Ce-lean and Ce-rich 2:14:1 grains, which weakens the magnetic impedance effect among them. These findings suggest a unique clue to enhance coercivity of the MMP RE-Fe-B magnets by PSA, i.e. the competitive contributions between the formation of continuous GB layers and the gradual chemical homogenization should be balanced.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.027
      Issue No: Vol. 146 (2018)
       
  • Phase-field modeling of reactive wetting and growth of the intermetallic
           Al2Au phase in the Al-Au system
    • Authors: Fei Wang; Andreas Reiter; Michael Kellner; Jürgen Brillo; Michael Selzer; Britta Nestler
      Pages: 106 - 118
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Fei Wang, Andreas Reiter, Michael Kellner, Jürgen Brillo, Michael Selzer, Britta Nestler
      When an Al-droplet is in contact with an Au-solid substrate, the liquid phase reacts with the substrate and an intermetallic layer is formed at the solid-liquid interface due to diffusion and reaction. This phenomenon has been commonly observed in the soldering process and the wetting is termed as reactive wetting, in contrast to the inert wetting where the droplet does not react with the base materials and the substrate is flat. Young's law can be used to interpret the contact angle in the static state, but is not able to predict the dynamics in the process of reactive wetting. In this work, we present a multi-phase model including phase transition and fluid dynamics to investigate the effect of the formation of the intermetallic Al2Au phase and capillary flow on the reactive wetting in the Al-Au system. In order to capture phase boundaries of solid-, liquid- and intermetallic-vapor, phase-field simulations are performed based on a ternary (Al-Au-X) phase diagram concept and using experimental data. It has been found that the increase of both the liquid-intermetallic interfacial tension as well as the capillary flow lead to an inhibition effect for the growth of the intermetallic phase.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.015
      Issue No: Vol. 146 (2018)
       
  • Phase stability and distortion in high-entropy oxides
    • Authors: G. Anand; Alex P. Wynn; Christopher M. Handley; Colin L. Freeman
      Pages: 119 - 125
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): G. Anand, Alex P. Wynn, Christopher M. Handley, Colin L. Freeman
      The present investigation demonstrates how configurational entropy stabilises rock-salt type single phase multi-component solid solution oxides. Classical simulations have been used to extensively sample the configurational landscape of such oxides using both random and genetic algorithm sampling strategies. The thermodynamic properties including the enthalpy and free energy of various oxide mixes have been calculated to show the influence of the chemical identity of the oxides on the phase stability. Additionally, a distance analysis between all the cation-cation and cation-anion pairs has been carried out in order to quantify the distortion in the lattice. The correlation between the multiplicity of cations in such systems with consequent enthalpy and configurational entropy has been enumerated and its relation with emergent distortion has been analysed.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.037
      Issue No: Vol. 146 (2018)
       
  • The austenite reversion and co-precipitation behavior of an ultra-low
           carbon medium manganese quenching-partitioning-tempering steel
    • Authors: Yu Li; Wei Li; Wenqing Liu; Xiaodong Wang; Xueming Hua; Huibin Liu; Xuejun Jin
      Pages: 126 - 141
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Yu Li, Wei Li, Wenqing Liu, Xiaodong Wang, Xueming Hua, Huibin Liu, Xuejun Jin
      The multiphase microstructure evolution and mechanical properties of an ultra-low carbon medium manganese quenching-partitioning-tempering (QPT) steel have been investigated based on the nanoscale austenite reversion correlated with the co-precipitation behavior. The blocky austenite (aspect ratio < 3) was found to nucleate in the partitioning process, and growth occurred in the tempering stage, but the film austenite (aspect ratio > 3) always formed during the tempering procedure in the alloy-enriched structures after partitioning. The adjacent co-precipitation of Ni-rich particles shells with Cu-rich precipitates was observed in quenching-tempering (QT) steels. The lower annealing temperature of QPT-1 steels resulted in a multiphase constitution of mostly film austenite, nanoscale dispersed Ni-rich precipitates and in the obvious coarsening and anisotropic growth of Cu-rich precipitates. With a high annealing temperature, QPT-2 steels showed a large percentage of blocky austenite, an entire core-shell co-precipitation structure of Ni-rich and Cu-rich precipitates in martensite and elongated Cu-rich particles in the blocky austenite. The introduction of nanoparticles in dual phase has rarely been found in ultra-low carbon steels with a low alloying degree. The co-precipitation mechanisms are closely related to the cooperative austenite reversion process, which is governed by Mn diffusion and segregation: this leads to a different element enrichment degree in dual phase. In addition, the intergranular precipitates in QT-steels result in stress concentration in the grain boundaries and a very low ductility, although with strong modulus and Orowan hardening effects. The difference of yield strength (67 MPa) in the two QPT steels mainly originate from the contribution of dispersion strengthening effect (44 MPa) under the consideration of the constituent phases. The lower mechanical stability of the blocky austenite in QPT-2 steels results in a lower uniform elongation and impact toughness.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.035
      Issue No: Vol. 146 (2018)
       
  • Low-field-actuated giant magnetocaloric effect and excellent mechanical
           properties in a NiMn-based multiferroic alloy
    • Authors: D.Y. Cong; L. Huang; V. Hardy; D. Bourgault; X.M. Sun; Z.H. Nie; M.G. Wang; Y. Ren; P. Entel; Y.D. Wang
      Pages: 142 - 151
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): D.Y. Cong, L. Huang, V. Hardy, D. Bourgault, X.M. Sun, Z.H. Nie, M.G. Wang, Y. Ren, P. Entel, Y.D. Wang
      Multiferroic magnetic shape memory alloys with first-order magntostructural transformation exhibit much enhanced magnetocaloric effect which incorporates the latent heat associated with the phase transformation itself, but they suffer from the drawbacks of large hysteresis and transformation interval and consequently too high critical field to actuate the magnetocaloric effect, greatly impeding their applications. Here, by generating a kind of specific stacking-mediated structure of martensite through minor Al substitution to improve the geometric compatibility between martensite and austenite in the Ni40Co10Mn40Sn9Al1 alloy, we greatly reduced the thermal hysteresis and transformation temperature interval while conserving the large magnetization difference between the two phases. Consequently, a low-field-actuated giant magnetocaloric effect with isothermal entropy change of 23 J kg−1 K−1 for a field change from 0 to 2 T, which is among the highest values reported heretofore for all magnetocaloric materials, was successfully achieved. Meanwhile, with minor Al substitution, the present single-phase multiferroic alloy that is intermetallic in nature exhibits superior mechanical properties, including excellent compressive properties over a wide temperature range and a relatively high fracture toughness, which are quite beneficial for practical applications. Incorporating the advantages of low cost, environment friendliness and easy fabrication, this alloy shows great potential for magnetocaloric applications.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.047
      Issue No: Vol. 146 (2018)
       
  • Thermal decomposition of sol-gel derived Zn0.8Ga0.2O precursor-gel: A
           kinetic, thermodynamic, and DFT studies
    • Authors: Asad Mahmood; Fatih Tezcan; Gulfeza Kardaş
      Pages: 152 - 159
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Asad Mahmood, Fatih Tezcan, Gulfeza Kardaş
      Gallium-doped zinc oxide has been widely accepted as the potential alternative to expensive tin-doped indium oxide (ITO) for transparent conductive layer applications. An extensive study reports the processing of ZnO-based semiconductor materials by utilizing a sol-gel method, however, no study has been reported to investigate both the kinetic and thermodynamic aspects of the gel decomposition for these materials. Here, we studied the kinetic and thermodynamic parameters of the sol-gel derived Zn0.8Ga0.2O precursor gel decomposition utilizing the thermogravimetric (TG) and differential thermal analysis (DTA). A non-isothermal method was used to calculate the activation energy, pre-exponential factors, reaction mechanism function, and order of reaction. In addition, density functional theory (DFT) was used to calculate the optical band gap and density of states. The results suggested that the gel decomposition followed the Jander: 3D model. The study is important for understanding the components of synthesis ranging from the formation of an activated complex to gel decomposition, while this study can be extended to other semiconductor processing methods.
      Graphical abstract image

      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.065
      Issue No: Vol. 146 (2018)
       
  • Influence of spinodal decomposition structures on the strength of Fe-Cr
           alloys: A dislocation dynamics study
    • Authors: A. Takahashi; T. Suzuki; A. Nomoto; T. Kumagai
      Pages: 160 - 170
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): A. Takahashi, T. Suzuki, A. Nomoto, T. Kumagai
      This paper presents a numerical analysis of the influence of spinodal decomposition on the strength of Fe-Cr alloys using the dislocation dynamics (DD) method. In the DD simulations, the structure of a spinodally decomposed chromium distribution is approximated using a cosine function. Using the equation for internal stress distribution, the interaction between a dislocation and the internal stress distribution is precisely accounted for in the DD simulations. The structure of the spinodally decomposed chromium distribution is parameterized using four variables, including the magnitude of chromium concentration, wave length of chromium distribution, position of the slip plane of dislocation, and the dislocation character (angle between the dislocation line and Burgers vector). Using these variables, the influence of the structure of chromium distribution on the critical resolved shear stress (CRSS) is studied. Furthermore, we focused on two major slip systems of the BCC structure, { 110 } 〈 111 〉 and { 112 } 〈 111 〉 , and discuss the difference in the influence between the different slip systems. In the { 110 } 〈 111 〉 slip system, the Δ CRSS appears only for a mixed dislocation with θ = 54.7°, because of the stripe pattern of the resolved shear stress distribution. On the other hand, in the { 112 } 〈 111 〉 slip system, the dislocations with θ = 39.2° and 90° have a large Δ CRSS. There is a plateau of Δ CRSS in a range of 39.2 ∘ < θ < 90 ∘ . The slip plane position does not change the Δ CRSS. There is a dependence of Δ CRSS on the wave-length of the chromium distribution. The dependence of Δ CRSS on the wave-length can be found not with a straight dislocation but with a curved dislocation using the DD simulations. The information is a new finding, and is meaningful in understanding the relationship between the material strength and the structure of spinodal decomposition.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.051
      Issue No: Vol. 146 (2018)
       
  • High-performance ZnS/GaN heterostructure photoanode for
           photoelectrochemical water splitting applications
    • Authors: Mostafa Afifi Hassan; Jin-Ho Kang; Muhammad Ali Johar; Jun-Seok Ha; Sang-Wan Ryu
      Pages: 171 - 175
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Mostafa Afifi Hassan, Jin-Ho Kang, Muhammad Ali Johar, Jun-Seok Ha, Sang-Wan Ryu
      We present a study of ZnS/GaN heterostructure photoanodes fabricated by using ZnS thin films deposited on GaN by atomic layer deposition and a significantly enhanced photoelectrochemical (PEC) water splitting performance was demonstrated. The PEC performance of the photoanodes was investigated for various ZnS thicknesses and GaN doping concentrations. The photocurrent density of the ZnS/GaN photoanode at zero bias was enhanced by a factor of 1.75 compared to that for the reference GaN structure. Furthermore, significantly enhanced photoanode stability was observed with the optimized ZnS coating. The high performance of the ZnS/GaN photoanode is attributed to the type-II band alignment of the heterojunction, which forms a potential barrier for electron injection to the electrolyte while facilitating hole transfer. Therefore, using a ZnS cocatalyst coated on GaN is a promising technique to fabricate photoanodes for PEC-based solar energy harvesting.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.063
      Issue No: Vol. 146 (2018)
       
  • Coarsening evolution of dendritic sidearms: From synchrotron experiments
           to quantitative modeling
    • Authors: H. Neumann-Heyme; N. Shevchenko; Z. Lei; K. Eckert; O. Keplinger; J. Grenzer; C. Beckermann; S. Eckert
      Pages: 176 - 186
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): H. Neumann-Heyme, N. Shevchenko, Z. Lei, K. Eckert, O. Keplinger, J. Grenzer, C. Beckermann, S. Eckert
      The local dynamics of dendritic sidearms during coarsening are studied by combining in-situ radiography observations with numerical and analytical models. A flat sample of a Ga-In alloy is partially solidified and then held isothermally in a vertical temperature gradient. The evolving dendritic microstructure is visualized using synchrotron X-ray imaging at the BM20 (ROBL) beamline at ESRF, France. During the coarsening stage, the temporal evolution of the geometrical features of sidebranches is captured by automated image processing. This data is then used to quantify the dynamics of two basic evolution mechanisms for sidebranches: retraction and pinch-off. The universal dynamics of sidearm necks during pinch-off are exploited to determine the product of liquid diffusivity and capillarity length D d 0 , as a parameter that is crucial in the calibration of quantitative models. By employing an idealized phase-field model for the evolution of a single sidebranch, the behavior of selected sidebranches is reproduced from the experiments in a consistent way.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.056
      Issue No: Vol. 146 (2018)
       
  • A critical evaluation on efficacy of recrystallization vs. strain induced
           boundary migration in achieving grain boundary engineered microstructure
           in a Ni-base superalloy
    • Authors: T.S. Prithiv; P. Bhuyan; S.K. Pradhan; V. Subramanya Sarma; S. Mandal
      Pages: 187 - 201
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): T.S. Prithiv, P. Bhuyan, S.K. Pradhan, V. Subramanya Sarma, S. Mandal
      A critical evaluation of recrystallization vs. strain induced boundary migration (SIBM) as the mechanism responsible behind the grain boundary engineering (GBE) in alloy 617 is made. Towards this hot deformation processing (in the temperature range of 1173–1473 K and strain rates of 0.001–10 s−1) as well as GBE-type iterative processing is performed on solution-annealed specimen. GBE-quantifying parameters such as Σ3n fraction, triple junction distribution, twin-related grain size ratio, twin-related domain (TRD) parameters and fractal dimension have been utilized to quantify the extent of GBE in the processed microstructures. Occurrence of dynamic recrystallization (DRX) during hot deformation processing does not induce notable multiple twinning leading to a microstructure analogous to that of solution-annealed condition even after complete DRX. Following GBE-type iterative processing, GBE microstructure has only been achieved when prolific multiple twinning occurred due to activation of SIBM leading to a high fraction of Σ3n boundaries (∼79%) with most of the twins being part of the grain boundary network. Consequently, an increasing proportion of J2 and J3 triple junctions with a concomitant increase in both the average number of grains per TRD and twin-related grain size ratio are achieved. However, occurrence of static recrystallization during GBE-type processing has led to smaller TRD size and copious random boundaries connectivity as confirmed by fractal analysis thus clearly suggesting that SIBM, not recrystallization catalyzes the profusion of Σ3n boundaries. As GBE relies on the multiple twinning mechanisms promoted via SIBM, recrystallization needs to be circumvented for successful attainment of GBE microstructure.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.045
      Issue No: Vol. 146 (2018)
       
  • Multiple electrical response and enhanced energy storage induced by
           unusual coexistent-phase structure in relaxor ferroelectric ceramics
    • Authors: Yong Li; Ningning Sun; Xiaowei Li; Jinhua Du; Liming Chen; Hongcheng Gao; Xihong Hao; Maosheng Cao
      Pages: 202 - 210
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Yong Li, Ningning Sun, Xiaowei Li, Jinhua Du, Liming Chen, Hongcheng Gao, Xihong Hao, Maosheng Cao
      The immense potential of energy storage materials applied in high energy storage devices has promoted the development of various materials and the exploration of structure. Here, we present the original observations on an unusual coexistent-phase structure in relaxor ferroelectric ceramics 0.2Pb(Mg1/3Nb2/3)O3-0.8Pb(Sn x Ti1-x )O3 (PMN-PST). The novel structure containing independent ferroelectric and relaxor phase in a single grain induces multiple electric response. More importantly, PMN-PST ceramics with the coexistent-phase structure exhibit the enhanced energy storage based on a high voltage pulse measurement. The maximum energy density reaches up to 0.85 J/cm3 at 70 kV/cm for x = 0.48, nearly three times of that of the ferroelectric ceramic (x = 0.36). It is proposed that the coexistent-phase structure generates domain switching and gives rise to coexistent-phase charge coupling that are together responsible for the excellent energy storage performance. This work opens a new pathway to develop relaxor ferroelectric materials in high energy storage fields.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.048
      Issue No: Vol. 146 (2018)
       
  • Bulk tracer diffusion in CoCrFeNi and CoCrFeMnNi high entropy alloys
    • Authors: M. Vaidya; K.G. Pradeep; B.S. Murty; G. Wilde; S.V. Divinski
      Pages: 211 - 224
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): M. Vaidya, K.G. Pradeep, B.S. Murty, G. Wilde, S.V. Divinski
      High entropy alloys (HEAs) have emerged as a promising class of equiatomic or near equiatomic multicomponent alloys, which garner fundamental curiosities and interest in high temperature applications. Understanding diffusion kinetics of HEAs is critical to assess their phase stability and deformation behaviour, particularly at elevated temperatures. For the first time, bulk tracer diffusion coefficients of Co, Cr, Fe and Mn are determined in polycrystalline CoCrFeNi and CoCrFeMnNi HEAs using the radiotracer method in the temperature interval of 1073–1373 K. Material homogeneity and the absence of any phase decomposition in CoCrFeNi and CoCrFeMnNi HEAs were established by electron microscopy and atom probe tomography investigations. Both bulk and grain boundary diffusion contributions to penetration profiles are observed for diffusion of Co, Cr, Fe and Mn tracers in both HEAs. The temperature dependencies of bulk diffusion for all tracers show Arrhenius behaviour. The corresponding activation energies (Q) and the logarithm of pre-exponential factors (D 0) show a linear relationship, thus following the “compensation rule”. An increase of the configurational entropy leads to reduced diffusion rates only when a homologous temperature scale is used for comparison. The increase of activation energy barrier and lower frequency factors both contribute to the decreased diffusion rates. A cross-over temperature (Tc = 1020 K) is observed for Co diffusion (on slight extrapolation of Arrhenius plot) in CoCrFeNi and CoCrFeMnNi HEAs, while Cr and Fe exhibit almost parallel Arrhenius lines. Above Tc, the Co diffusivity is higher in CoCrFeMnNi than in CoCrFeNi, which suggests that diffusion in HEAs need not be assumed to retard with an increasing number of elements. The existence of a cross-over temperature correlates with the change in binding energy (or enthalpy) of the constituents from CoCrFeNi to CoCrFeMnNi.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.052
      Issue No: Vol. 146 (2018)
       
  • Study of the TiC1-x – TiO2 reactive interface
    • Authors: F. Réjasse; G. Trolliard; J. Léchelle; O. Rapaud; P. Carles; O. Grauby; H. Khodja
      Pages: 225 - 236
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): F. Réjasse, G. Trolliard, J. Léchelle, O. Rapaud, P. Carles, O. Grauby, H. Khodja
      The TiC1-x–TiO2 reactive interface obtained at 1100 °C has been studied by local structural investigations using Transmission Electron Microscopy (TEM) and X-rays microdiffraction. These results were completed by coupling complementary chemical characterizations by Nuclear Reaction Analysis (NRA), Rutherford Backscattering (RBS) and STEM (Scanning Transmission Electron Microscopy). A complex pattern of reactive interlayers of various thicknesses was formed as a result of the crossed redox reactions between TiO2 and TiC1-x. Our results show that the TiC1-x carbide is not oxidized by forming TiCxOy oxycarbides in spite of the complete solid solution existing between TiC1-x and TiO. TiC1-x is in fact destabilized by the oxygen liberated during the transformation of TiO2 into Magnéli type compounds (TinO2n-1) to form the most reduced Magnéli sub-oxides following the reaction: 3 TiCxOy + (5-3y)/2 O2 = Ti3O5 + 3x C. Starting from TiO2, a progressive reduction is achieved showing spatially distributed TinO2n-1 Magnéli phases characterized by high values of n close to TiO2 and lower values of n as reaching TiC1-x. The most reduced term is shown to be Ti3O5. Ti3O5 thus represents the final product of the reaction at 1100 °C both for the reaction of reduction of TiO2 and for the reaction of oxidation of TiC1-x.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.055
      Issue No: Vol. 146 (2018)
       
  • Multi-technique characterization of the precipitates in thermally aged and
           neutron irradiated Fe-Cu and Fe-Cu-Mn model alloys: Atom probe tomography
           reconstruction implications
    • Authors: Shipeng Shu; Brian D. Wirth; Peter B. Wells; Dane D. Morgan; G. Robert Odette
      Pages: 237 - 252
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Shipeng Shu, Brian D. Wirth, Peter B. Wells, Dane D. Morgan, G. Robert Odette
      Cu precipitation in Fe is technologically and scientifically important. Atom probe tomography (APT) often reports high Fe in copper rich precipitates (CRPs). Some argue that this is an APT artifact. Here, Fe-0.80 at. % Cu (Fe-Cu) and Fe-0.78 at. % Cu-1.05 at.% Mn (Fe-Cu-Mn) alloys were characterized by several microanalytical techniques, following thermal aging and neutron irradiation that produced a high density of nanoscale CRPs. The CRP number densities, mole fractions and sizes characterized by different techniques are reasonably consistent, but not the compositions in the case of APT. A new small-angle neutron scattering method was used, involving measuring the temperature dependence of magnetic scattering (T-SANS), which proved that the CRPs are non-magnetic at ambient temperature. Nuclear scattering mass balance constraints show that CRPs do not contain a significant amount of Fe. Positron annihilation spectroscopy and SANS magnetic-to-nuclear scattering ratio measurements are also consistent with very small amounts, if any, of Fe in the CRPs.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.006
      Issue No: Vol. 146 (2018)
       
  • Reversible phase transformation phenomenon in titanium dioxide films:
           Evidence beyond interface-nucleation and dissolution-precipitation
           kinetics
    • Authors: Subodh K. Gautam; Jitendra Singh; D.K. Shukla; E. Pippel; P. Poddar; Fouran Singh
      Pages: 253 - 264
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Subodh K. Gautam, Jitendra Singh, D.K. Shukla, E. Pippel, P. Poddar, Fouran Singh
      The re-crystallization kinetics and rutile to anatase reversible phase transformation (PT) in nano-crystalline titanium dioxide (TiO2) are reported. Initially, an amorphous TiO2 film is used for the present study and in situ isothermal annealing dependent nucleation and growth kinetics of anatase and rutile phase is studied at low temperature (∼523 K) and well explained using Johnson–Mehl–Avrami-Kolmogorov (JMAK) model. The anatase nanocrystallite (NCs) transformation into rutile phase is reported with isothermal annealing for longer time and temperature dependent annealing in lower temperature range 523 K–673 K and explained using interface-nucleation mechanism. Furthermore, the thermodynamic stability of rutile NCs and lattice stress-induced reversible PT in nano-sized rutile TiO2 are confirmed in moderate temperature range (623 K - 973 K) and well explained using x-ray diffraction, micro-Raman spectroscopy and near edge x-ray absorption fine structure spectroscopy studies. However, annealing at higher temperature (1123 K - 1323 K) induces the growth of anatase NCs and their natural transform into rutile phase are explained by well-known dissolution–precipitation mechanism. Activation energy of rutile PT is quantified and found higher for dissolution-precipitation mechanism than that for interface nucleation at earlier stage. Thus, overall PT kinetics at different temperature range is well understood by invoking in three step mechanism: I) early stage anatase-to-rutile transformation is dominated by interface-nucleation, II) then intermediate stage reversible rutile-to-anatase PT and, III) at later stages, anatase-to-rutile PT is controlled by dissolution–precipitation mechanism.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.050
      Issue No: Vol. 146 (2018)
       
  • Origins and dissociation of pyramidal dislocations in
           magnesium and its alloys
    • Authors: Zhigang Ding; Wei Liu; Hao Sun; Shuang Li; Dalong Zhang; Yonghao Zhao; Enrique J. Lavernia; Yuntian Zhu
      Pages: 265 - 272
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Zhigang Ding, Wei Liu, Hao Sun, Shuang Li, Dalong Zhang, Yonghao Zhao, Enrique J. Lavernia, Yuntian Zhu
      Alloying magnesium (Mg) with rare earth elements such as yttrium (Y) has been reported to activate the pyramidal <c + a> slip systems and improve the plasticity of Mg at room temperature. However, the origins of such dislocations and their dissociation mechanisms remain poorly understood. Here, we systematically investigate these mechanisms using dispersion-inclusive density-functional theory, in combination with molecular dynamics simulations. We find that <c + a> dislocations form more readily on the pyramidal I plane than on the pyramidal II plane in Mg. The addition of Y atoms in Mg facilitates the dissociation of <c + a> dislocations on pyramidal II, leading to the easier formation of the pyramidal II than pyramidal I in Mg-Y alloy. Importantly, in pyramidal II slip plane, a flat potential-energy surface (PES) exists around the position of stable stacking fault energy (SFE), which allows cooperative movement of atoms within the slip plane. Alloying Mg with Y atoms increases the range of the PES, and ultimately promotes different sliding pathways in the Mg-Y alloy. These findings are consistent with experimentally observed activation of the pyramidal II <c + a> slip system in Mg-Y alloys, and provide important insight into the relationship between dislocation structure and macroscopic enhancement of plasticity.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.049
      Issue No: Vol. 146 (2018)
       
  • Phonon excitations in a single crystal Mg85Zn6Y9 with a synchronized
           long-period stacking ordered phase
    • Authors: Shinya Hosokawa; Koji Kimura; Jens Rüdiger Stellhorn; Koji Yoshida; Koji Hagihara; Hitoshi Izuno; Michiaki Yamasaki; Yoshihito Kawamura; Yoji Mine; Kazuki Takashima; Hiroshi Uchiyama; Satoshi Tsutsui; Akihide Koura; Fuyuki Shimojo
      Pages: 273 - 279
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Shinya Hosokawa, Koji Kimura, Jens Rüdiger Stellhorn, Koji Yoshida, Koji Hagihara, Hitoshi Izuno, Michiaki Yamasaki, Yoshihito Kawamura, Yoji Mine, Kazuki Takashima, Hiroshi Uchiyama, Satoshi Tsutsui, Akihide Koura, Fuyuki Shimojo
      Inelastic X-ray scattering (IXS) experiments were carried out at room temperature at BL35XU of the SPring-8 on a single crystal Mg85Zn6Y9 alloy with a synchronized long-period stacking ordered (LPSO) phase of purely 18R type. The IXS measurements were carried out in both the longitudinal and transverse directions along the Γ -K and Γ -M (momentum vector q ⊥ lattice axis c ), and Γ -A ( q ∥ c ) directions for the energy transfers ω below 40 meV. Peaks arising from longitudinal and transverse acoustic (LA and TA) modes were clearly observed in the IXS spectra. The dispersion relations of the LA and TA modes in the Mg85Zn6Y9 LPSO alloy are mostly similar to those of previous INS data on single crystal pure Mg. New impurity-derived dispersion-less excitation modes are observed along all of the crystal directions at about 5, 10, and 17 meV. Partial vibrational density of states (vDOS) was obtained using an a b i n i t i o molecular dynamics simulation, and it was found that the ∼ 10 meV peak mainly originates from the phonon excitations inside the L12-type Zn6Y8 clusters and the ∼ 17 meV peak from those between the cluster and the host Mg atoms, and a small shoulder at 5 meV consists of motions of all the elements. These findings clarify that the dispersion-less modes mainly correspond to localized stretching and twisting modes around the impurity clusters in the LPSO alloys.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.053
      Issue No: Vol. 146 (2018)
       
  • Correlating the microstructure, growth mechanism and magnetic properties
           of FeSiAl soft magnetic composites fabricated via HNO3 oxidation
    • Authors: Dong Liu; Chen Wu; Mi Yan; Jian Wang
      Pages: 294 - 303
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Dong Liu, Chen Wu, Mi Yan, Jian Wang
      Nitric acid oxidation has been used to fabricate insulation coatings of the FeSiAl soft magnetic composites (SMCs). Growth mechanism of the coatings obtained with varied HNO3 concentration has been systematically studied based on careful analysis of the coating thickness, microstructure and composition. The oxidation process for the powders reacted with 10 wt% HNO3 corresponds to the steady-state passivation, while pitting corrosion occurs with raised HNO3 concentration of 30 wt%. Origin of the pitting corrosion has been analyzed based on the point defect model. Evolution of the coatings has been revealed under different growth conditions and correlated to the magnetic performance of the SMCs. The samples oxidized with 10 wt% HNO3 mainly form a thin and compact oxidation layer with dominating Al2O3, AlO(OH) and a small amount of Fe3O4, while increased HNO3 concentration (30 wt%) gives rise to mixed Al2O3, AlO(OH), Fe2O3 and Fe3O4 in the thicker coating layers. Enhanced magnetic performance of the FeSiAl SMCs can be achieved by optimized thickness and composition of the coatings (μ e = 180.4, P cv = 327.2 mW/cm3 for the SMCs oxidized by 10 wt% HNO3 and μ e = 157.3, P cv = 241.2 mW/cm3 via oxidization by 30 wt% HNO3 for 5 min).
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2018.01.001
      Issue No: Vol. 146 (2018)
       
  • Mechanically enhanced grain boundary structural phase transformation in Cu
    • Authors: Mohammad Aramfard; Chuang Deng
      Pages: 304 - 313
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Mohammad Aramfard, Chuang Deng
      It has been previously proposed that grain boundary can be treated as a two-dimensional phase in materials with possible transformations between its multiple metastable states under external stimuli such as high temperature. In this work, the influences of another type of common external stimuli, i.e. mechanical load, on grain boundary structural phase transformation (GBSPT) are studied by molecular dynamics simulations. Two types of high angle symmetrical grain boundaries in Cu are used as model systems to investigate their structural transformation under both constant and cyclic mechanical loading. While in general GBSPT can be enhanced or weakened under a constant tensile or compressive stress as compared to that caused by temperature only, the influences of tension and compression are not the same. For this reason, cyclic loading consisting of symmetric tensile/compressive half-cycles can significantly influence the overall GBSPT. Furthermore, it is found that the macroscopic strain in the material caused by GBSPT under compression/tension can be well described by Coble creep, which implies that GBSPT could be a grain boundary diffusional process during the creep of polycrystalline materials that has been overlooked before. Additionally, it is shown that together with shear coupled grain boundary motion and sources of free volume such as voids or cracks, cyclic shear loading can also cause GBSPT between its metastable structures of different atomic density, which can be possibly utilized to heal damage or microcracks in engineering materials.
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      PubDate: 2018-02-05T09:11:44Z
      DOI: 10.1016/j.actamat.2017.12.062
      Issue No: Vol. 146 (2018)
       
  • Phase stability and transformation in a light-weight high-entropy alloy
    • Authors: Rui Feng; Michael Gao Chuan Zhang Wei Guo Jonathan Poplawsky
      Abstract: Publication date: March 2018
      Source:Acta Materialia, Volume 146
      Author(s): Rui Feng, Michael C. Gao, Chuan Zhang, Wei Guo, Jonathan D. Poplawsky, Fan Zhang, Jeffrey A. Hawk, Joerg C. Neuefeind, Yang Ren, Peter K. Liaw
      Light-weight high-entropy alloys (HEAs) with a vast alloy-design space have offered new avenues to explore novel low-cost, high strength-to-weight ratio structural materials. Studying their phase stability and possible transformations is critical for designing microstructures for optimal material properties. However, the complex local atomic environment of HEAs poses challenges to the fundamental understanding of phase stability and transformation behaviors. The present study investigates the phase stability and transformation behaviors of a newly-designed light-weight Al1 . 5CrFeMnTi HEA by integrated experimental and theoretical approaches. The coherent precipitation of the L21 phase within the body-centered-cubic (BCC) matrix at intermediate temperatures was observed, and the size, shape, coherency, and spatial distribution of the L21 phase were subsequently altered through selected annealing treatments. Moreover, the CALculation of PHAse Diagrams (CALPHAD) and first-principle calculations successfully optimize the compositions of light-weight HEAs with a coherent BCC/L21 two-phase structure. Shed light by the present study, novel light-weight HEAs, featuring the lower density (below 6 g/cm3) and cost, can be designed for high-temperature applications.
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      PubDate: 2018-02-05T09:11:44Z
       
  • Editors for Acta Materialia
    • Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145


      PubDate: 2018-02-05T09:11:44Z
       
  • 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.
      Graphical abstract image

      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.
      Graphical abstract image

      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.
      Graphical abstract image

      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.
      Graphical abstract image

      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.
      Graphical abstract image

      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)
       
  • Strain-induced indium clustering in non-polar a-plane InGaN quantum wells
    • Authors: Ja Kyung Lee; Bumsu Park; Kyung Song; Woo Young Jung; Dmitry Tyutyunnikov; Tiannan Yang; Christoph T. Koch; Chan Gyung Park; Peter A. van Aken; Young-Min Kim; Jong Kyu Kim; Junhyeok Bang; Long-Qing Chen; Sang Ho Oh
      Pages: 109 - 122
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Ja Kyung Lee, Bumsu Park, Kyung Song, Woo Young Jung, Dmitry Tyutyunnikov, Tiannan Yang, Christoph T. Koch, Chan Gyung Park, Peter A. van Aken, Young-Min Kim, Jong Kyu Kim, Junhyeok Bang, Long-Qing Chen, Sang Ho Oh
      In conventional light-emitting diodes the epitaxial strain and related piezoelectric polarization arising along the polar [0001] growth direction of the InGaN/GaN quantum wells (QWs) induce internal fields which adversely affect the radiative recombination of electron-hole pairs therein. Growing the quantum wells along a nonpolar orientation can, in principle, avoid this problem but seems to face with another problem associated with indium clustering. In this study, we present experimental evidence that supports the inhomogeneous distribution of indium in non-polar a-plane InGaN QWs by using dark-field inline electron holography as well as atom probe tomography measurements and discuss the possible origin by density functional theory calculation. A model non-polar a-plane QW structure with 10 nm-thick In0.1Ga0.9N double QWs was investigated and compared with the polar c-plane QWs with the same QW structure. Unlike the random distribution in the polar QWs, the indium atoms in the non-polar QW exhibit inhomogeneous distribution and show a tendency of periodic clustering. We suggest the dipole interaction energy and the strain energy associated with indium substitution could have a substantial influence on the local composition of strained InGaN QWs and, particularly, triggers In clustering in the non-polar a-plane QW structure. Accompanying phase field modeling rationalizes that In clustering can also modify the in-plane polarization through piezoelectric effects, preventing the electrostatic potential from diverging along the in-plane polar direction.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.039
      Issue No: Vol. 145 (2017)
       
  • Nano-phase separation sintering in nanostructure-stable vs. bulk-stable
           alloys
    • Authors: Mansoo Park; Tongjai Chookajorn; Christopher A. Schuh
      Pages: 123 - 133
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Mansoo Park, Tongjai Chookajorn, Christopher A. Schuh
      Accelerated sintering through nanoscale-phase separation is explored in the Cr-Ni binary system. When the processing requirements of both supersaturation and nanocrystallinity are met in the initial powders, Cr-Ni alloys show an onset of sintering at a low temperature and a rapid rate of densification. Independent characterization techniques, namely x-ray diffraction, thermomechanical analysis, and electron microscopy, confirm that Ni-rich phases develop upon sintering in a way that enhances consolidation. However, this system is shown to be unique in that the Ni addition facilitates rapid nano-phase separation sintering but does not promote stability of a nanoscale grain structure; the nanostructure is a transient feature of the system and the fully consolidated material is microcrystalline with a phase-separated structure. A thermodynamic stability analysis explains this structural evolution, and clarifies the roles of alloying elements that stabilize nanostructure and those that accelerate sintering in nanostructured systems. This work may broaden the applicability of nano-phase separation sintering and inform alloy design based on sinterability.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.030
      Issue No: Vol. 145 (2017)
       
  • High-quality TiN/AlN thin film heterostructures on c-sapphire
    • Authors: A. Moatti; J. Narayan
      Pages: 134 - 141
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): A. Moatti, J. Narayan
      We have developed TiN/AlN/c-sapphire epitaxial heterostructures and compared it with TiN/c-sapphire epitaxial heterostructures, needed for GaN-based LEDs and lasers. AlN is used as a buffer layer to provide a high misfit strain and facilitate the 2D growth on sapphire. The large misfit strain between sapphire and AlN makes this substrate a great candidate for GaN-based devices because it guarantees a full relaxation of AlN thin films through domain matching epitaxy paradigm. TiN can also act as an excellent contact and bottom electrode for Ⅲ-Ⅴ nitrides. Also, the introduction of TiN as a buffer layer decreases the critical thickness beyond which dislocations can grow in GaN thin films due to higher misfit strain compared to sapphire, which also improves the quality of potential GaN thin films. The selected-area-electron-diffraction patterns, scanning transmission electron microscopy, and transmission Kikuchi diffractions along with atomic arrangement simulations revealed that films are epitaxial with the following relationships: TiN<101>‖AlN[ 1 ¯ 2 1 ¯ 0]‖sapphire[01 1 ¯ 0] (in-plane), and TiN<111>‖AlN[0001]‖sapphire[0001] (out-of-plane). This is equivalent to a 30° rotation of Al basal plane in AlN with respect to that in sapphire. In TiN/c-sapphire epitaxial platforms, there is a 30° rotation: TiN<101>‖sapphire[01 1 ¯ 0] (in-plane), and TiN<111>‖sapphire[0001] (out-of-plane). It is shown that these heterostructures are fully relaxed in terms of misfit strains and only thermal strain stays as unrelaxed. The domain matching epitaxy paradigm is used to rationalize the epitaxial growth. The details of dislocations nucleation and glide in these heterostructures were studied and the results also discussed to elucidate the mechanism of strain relaxation.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.044
      Issue No: Vol. 145 (2017)
       
  • Phase, hardness, and deformation slip behavior in mixed HfxTa1-xC
    • Authors: Chase J. Smith; Xiao-Xiang Yu; Qianying Guo; Christopher R. Weinberger; Gregory B. Thompson
      Pages: 142 - 153
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Chase J. Smith, Xiao-Xiang Yu, Qianying Guo, Christopher R. Weinberger, Gregory B. Thompson
      A series of HfxTa1-xC atomic compositions, where x = 0.0, 0.13, 0.25, 0.50, 0.75, 0.83, and 1.0, were computationally and experimentally studied in terms of their phase, hardness, and dislocation emission behavior. Using an ab initio evolutionary algorithm, the B1 rocksalt structure was found to be the 0 K ground state phase for the binary carbides whereas the ternary compounds were a mixture of metal-site ordered monoclinic, trigonal, and cubic structures. However, all experimental carbides were found to exhibit a B1 structure, with two B1 phases forming for Hf-rich ternary compositions. A modest rise in the elastic constant computationally predicted hardness was found with experimental mean values showing a similar trend from nanoindentation tests. With the presence of Ta, even in small amounts, in the Hf-rich ternary compositions, <110>{111} slip was observed which is a change from the reported <110>{110} slip for HfC. This change is explained by the promotion of an intrinsic stacking fault in the {111} planes.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.038
      Issue No: Vol. 145 (2017)
       
  • Chemistry and three-dimensional morphology of martensite-austenite
           constituent in the bainite structure of low-carbon low-alloy steels
    • Authors: N. Takayama; G. Miyamoto; T. Furuhara
      Pages: 154 - 164
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): N. Takayama, G. Miyamoto, T. Furuhara
      The martensite-austenite constituent (MA) originates from carbon-enriched and untransformed austenite surrounded by bainite and is known to deteriorate the toughness of bainitic steel. The relationship between MA and bainite formation in low-carbon low-alloy steel was investigated with respect to the effects of cooling rate and Nb addition, as well as the relationship between the MA distribution and surrounding bainite. Nb addition enhanced the formation of MA over a wider range of cooling rates, and the fraction of MA became maximum at a cooling rate of 5 K/s. A microstructure consisting of carbide-free coarse bainite with few high angle boundaries is formed near the prior austenite grain boundary, and fine bainite with a high density of carbide and high angle boundaries is formed within the austenite grains. MA tends to be located in the coarse bainite region. Three-dimensional observations of the bainite and MA indicated MA is elongated along bainitic ferrites with the same growth direction or the same habit plane, while MA is blocky when the surrounding bainitic ferrites have different growth directions. Thus, the shape of MA is strongly affected by the crystallography of the surrounding bainite structure. The carbon content in austenite revealed that the carbon content of MA is close to the T0 composition at the transformation temperature, which suggests that MA formation is caused by a reduction of the driving force for transformation by carbon enrichment, which results in incomplete bainite transformation. Nb addition further inhibits the decomposition of austenite and promotes MA formation.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.036
      Issue No: Vol. 145 (2017)
       
  • Elasticity and strength of silica aerogels: A molecular dynamics study on
           large volumes
    • Authors: William Gonçalves; Julien Morthomas; Patrice Chantrenne; Michel Perez; Geneviève Foray; Christophe L. Martin
      Pages: 165 - 174
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): William Gonçalves, Julien Morthomas, Patrice Chantrenne, Michel Perez, Geneviève Foray, Christophe L. Martin
      The nanoporous structure and the mechanical properties of silica aerogels are studied by molecular dynamics simulations on large samples. For the first time, atomistic simulations are able to reproduce a pore size distribution, centered on 10 nm, comparable to experimental values. Using an unprecedented combination for silica aerogels of large volumes, large strains, and relatively small strain-rates, direct evidence of their peculiar mechanical behavior at the nanoscale is provided, from elasticity to fracture. The surface stress that silica ligaments experience produces a significant tension-compression asymmetry and an unusual discontinuity in the Poisson effect. The ductility of highly porous silica aerogels arises from an interplay between surface stress and a significant amount of reorganization in ligaments. Taking advantage of the large volumes accessible here, the very heterogeneous nature of low-density silica aerogels is reproduced, with an impact both on their elasticity and on their strength. In particular, a clear dependence between tensile strength and sample volume is uncovered, which opens perspectives for the elaboration of multi-scale models.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.005
      Issue No: Vol. 145 (2017)
       
  • Coupling eutectic nucleation mechanism investigated by phase field crystal
           model
    • Authors: Can Guo; Jincheng Wang; Junjie Li; Zhijun Wang; Yunhao Huang; Jiwei Gu; Xin Lin
      Pages: 175 - 185
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Can Guo, Jincheng Wang, Junjie Li, Zhijun Wang, Yunhao Huang, Jiwei Gu, Xin Lin
      Eutectic solidification has been investigated for decades; however, our understanding of the mechanism of eutectic nucleation remains very limited. In this work, we investigated the regular eutectic solidification process by an atomistic simulation method, and a new eutectic nucleation scenario is proposed. We found that, near the eutectic composition, the nucleation of a metastable triangle phase is more energy favorable for diffusion-controlled binary eutectic systems, and the solidification often passes through a transient state. With the proceeding of solidification, alternated A-rich and B-rich domains will be generated at the interface between the transient phase and liquid phase by pseudospinodal decomposition. When the compositions of alternated domains are close to the respective equilibrium values, eutectic phases will be formed inside the domains simultaneously, which further develops into coupling eutectic colonies. Furthermore, the construction of solute interdiffusion field and the formation of interface curvature are explained self-consistently with our eutectic nucleation mechanism.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.012
      Issue No: Vol. 145 (2017)
       
  • On the origin of anomalous eutectic growth from undercooled melts: Why
           re-melting is not a plausible explanation
    • Authors: Andrew M. Mullis; Caroline R. Clopet
      Pages: 186 - 195
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): Andrew M. Mullis, Caroline R. Clopet
      Ag-Cu melt at the equilibrium eutectic composition has been undercooled using a melt fluxing technique. The resulting samples have been sectioned through the nucleation point and the spatial distribution of anomalous eutectic has been quantified. This is shown to be highly inhomogeneous, with the volume of anomalous material decreasing approximately linearly with distance from the nucleation site. In samples at low undercooling the volume fraction of anomalous eutectic near the nucleation site is around an order of magnitude greater than the calculated recalescence solid fraction. As such, any model for the origin of the anomalous eutectic invoking partial remelting of this initial solid is shown to be infeasible. An alternative model, based on a kinetic shift of the eutectic point during rapid solidification is proposed.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.12.016
      Issue No: Vol. 145 (2017)
       
  • Effect of ferrite-to-austenite phase transformation path on the interface
           crystallographic character distributions in a duplex stainless steel
    • Authors: N. Haghdadi; P. Cizek; P.D. Hodgson; V. Tari; G.S. Rohrer; H. Beladi
      Pages: 196 - 209
      Abstract: Publication date: 15 February 2018
      Source:Acta Materialia, Volume 145
      Author(s): N. Haghdadi, P. Cizek, P.D. Hodgson, V. Tari, G.S. Rohrer, H. Beladi
      The effect of the ferrite to austenite phase transformation route on the microstructure and interface plane character distributions was studied in a duplex stainless steel. Two markedly different austenite morphologies (i.e., equiaxed and Widmanstätten) were produced through diffusional (slow cooling) and semi-shear (air-cooling) transformations, respectively. Both austenite morphologies had textures similar to the as-received condition, which was attributed to a “texture memory” effect. The air-cooled microstructure displayed a significantly higher content of Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) interfaces (39%) compared with the slow-cooled one (16%), due to the change in the austenite nucleation and growth mechanism during the phase transformation. A five-parameter analysis of different interfaces revealed that for K-S/N-W orientation relationships, ferrite and austenite terminated on (110) and (111) planes, respectively, regardless of the transformation route. The population of these planes, however, increased as the transformation rate increased. A higher fraction of Σ3 boundaries was observed in the equiaxed austenite morphology compared with its Widmanstätten counterpart, which was mainly attributed to the different kinetics and the growth mode of austenite plates during the phase transformation. Σ9 boundaries were mostly formed where two Σ3 boundaries met and were largely of tilt character because of geometric constraints. The intervariant boundary plane distributions of both austenite microstructures displayed more frequent {111} orientations than other planes for a majority of the boundaries. This trend was markedly stronger for Widmanstätten austenite.
      Graphical abstract image

      PubDate: 2017-12-27T09:11:53Z
      DOI: 10.1016/j.actamat.2017.11.057
      Issue No: Vol. 145 (2017)
       
 
 
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