Showing papers by "Mark Asta published in 2018"

Tunable stacking fault energies by tailoring local chemical order in CrCoNi medium-entropy alloys.

Abstract: High-entropy alloys (HEAs) are an intriguing new class of metallic materials due to their unique mechanical behavior. Achieving a detailed understanding of structure-property relationships in these materials has been challenged by the compositional disorder that underlies their unique mechanical behavior. Accordingly, in this work, we employ first-principles calculations to investigate the nature of local chemical order and establish its relationship to the intrinsic and extrinsic stacking fault energy (SFE) in CrCoNi medium-entropy solid-solution alloys, whose combination of strength, ductility, and toughness properties approaches the best on record. We find that the average intrinsic and extrinsic SFE are both highly tunable, with values ranging from -43 to 30 mJ⋅m-2 and from -28 to 66 mJ⋅m-2, respectively, as the degree of local chemical order increases. The state of local ordering also strongly correlates with the energy difference between the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases, which affects the occurrence of transformation-induced plasticity. This theoretical study demonstrates that chemical short-range order is thermodynamically favored in HEAs and can be tuned to affect the mechanical behavior of these alloys. It thus addresses the pressing need to establish robust processing-structure-property relationships to guide the science-based design of new HEAs with targeted mechanical behavior.

Dipolar cations confer defect tolerance in wide-bandgap metal halide perovskites

Abstract: This publication is based in part on work supported by the US Office of Naval Research (Grant Award No.: N00014-17-1-2524), by an award (OSR-2017-CPF-3321-03) from the King Abdullah University of Science and Technology (KAUST), by the Ontario Research Fund Research Excellence Program, and by the Natural Sciences and Engineering Research Council (NSERC) of Canada. The work of H.T. was also supported by a Rubicon grant (680-50-1511) from the Netherlands Organization for Scientific Research (NWO) and by the Thousand Talent Program for Young Outstanding Scientists in China. D.B. and M.A. acknowledge the support by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, and Materials Sciences and Engineering Division, under Contract No. DE-AC02-05-CH11231. Materials Project program KC23MP. M.I.S. acknowledges the Government of Canada’s Banting Postdoctoral Fellowship Program for financial support. The DFT simulation is supported by the IBM Canada Research and Development Center through the Southern Ontario Smart Computing Innovation Platform (SOSCIP). A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory.

Spatial correlation of elastic heterogeneity tunes the deformation behavior of metallic glasses

Abstract: Metallic glasses (MGs) possess remarkably high strength but often display only minimal tensile ductility due to the formation of catastrophic shear bands. Purposely enhancing the inherent heterogeneity to promote distributed flow offers new possibilities in improving the ductility of monolithic MGs. Here, we report the effect of the spatial heterogeneity of elasticity, resulting from the inherently inhomogeneous amorphous structures, on the deformation behavior of MGs, specifically focusing on the ductility using multiscale modeling methods. A highly heterogeneous, Gaussian-type shear modulus distribution at the nanoscale is revealed by atomistic simulations in Cu64Zr36 MGs, in which the soft population of the distribution exhibits a marked propensity to undergo the inelastic shear transformation. By employing a mesoscale shear transformation zone dynamics model, we find that the organization of such nanometer-scale shear transformation events into shear-band patterns is dependent on the spatial heterogeneity of the local shear moduli. A critical spatial correlation length of elastic heterogeneity is identified for the simulated MGs to achieve the best tensile ductility, which is associated with a transition of shear-band formation mechanisms, from stress-dictated nucleation and growth to structure-dictated strain percolation, as well as a saturation of elastically soft sites participating in the plastic flow. This discovery is important for the fundamental understanding of the role of spatial heterogeneity in influencing the deformation behavior of MGs. We believe that this can facilitate the design and development of new ductile monolithic MGs by a process of tuning the inherent heterogeneity to achieve enhanced ductility in these high-strength metallic alloys.

Melts of CrCoNi-based high-entropy alloys: Atomic diffusion and electronic/atomic structure from ab initio simulation

Abstract: High-entropy alloys (HEAs) are an emerging class of advanced structural alloys under extensive research; yet, the properties of the liquid states of these materials, which are relevant to their processing, have been far less explored In this work, we utilize ab initio molecular dynamics simulations to investigate the melt properties of a representative HEA—the Cantor alloy—and its derivatives: CrMnFeCoNi, CrFeCoNi, and CrCoNi The atomic dynamics of these melts at various temperatures are investigated, specifically to analyze their electronic and atomic structures, including charge transfer, pair distribution functions, and short-range order Results are compared with existing information for the liquids of metallic glasses, which also typically contain multiple principal elements, but retain the amorphous state under moderate to fast cooling rates The present results provide insights into the structural and bonding factors favoring solidification to single-phase solid solutions in HEAs

Dynamics of Nanoscale Dendrite Formation in Solution Growth Revealed Through in Situ Liquid Cell Electron Microscopy

Abstract: Formation mechanisms of dendrite structures have been extensively explored theoretically, and many theoretical predictions have been validated for micro- or macroscale dendrites. However, it is challenging to determine whether classical dendrite growth theories are applicable at the nanoscale due to the lack of detailed information on the nanodendrite growth dynamics. Here, we study iron oxide nanodendrite formation using liquid cell transmission electron microscopy (TEM). We observe "seaweed"-like iron oxide nanodendrites growing predominantly in two dimensions on the membrane of a liquid cell. By tracking the trajectories of their morphology development with high spatial and temporal resolution, it is possible to explore the relationship between the tip curvature and growth rate, tip splitting mechanisms, and the effects of precursor diffusion and depletion on the morphology evolution. We show that the growth of iron oxide nanodendrites is remarkably consistent with the existing theoretical predictions on dendritic morphology evolution during growth, despite occurring at the nanoscale.

Free energy of grain boundary phases: Atomistic calculations for Σ 5 ( 310 ) [ 001 ] grain boundary in Cu

Abstract: Atomistic simulations are employed to demonstrate the existence of a well-defined thermodynamic phase transformation between grain boundary (GB) phases with different atomic structures. The free energy of different interface structures for an embedded-atom-method model of the $\mathrm{\ensuremath{\Sigma}}5(310)[001]$ symmetric tilt boundary in elemental Cu is computed using the nonequilibrium Frenkel-Ladd thermodynamic integration method through molecular dynamics simulations. It is shown that the free-energy curves predict a temperature-induced first-order interfacial phase transition in the GB structure in agreement with computational studies of the same model system. Moreover, the role of vibrational entropy in the stabilization of the high-temperature GB phase is clarified. The calculated results are able to determine the GB phase stability at homologous temperatures less than 0.5, a temperature range particularly important given the limitation of the methods available hitherto in modeling GB phase transitions at low temperatures. The calculation of GB free energies complements currently available 0 K GB structure search methods, making feasible the characterization of GB phase diagrams.

Free energy of grain boundary phases: Atomistic calculations for $\Sigma$5(310)[001] grain boundary in Cu

Abstract: Atomistic simulations are employed to demonstrate the existence of a well-defined thermodynamic phase transformation between grain boundary (GB) phases with different atomic structures. The free energy of different interface structures for an embedded-atom-method model of the $\Sigma 5 (310) [001]$ symmetric tilt boundary in elemental Cu is computed using the nonequilibrium Frenkel-Ladd thermodynamic integration method through molecular dynamics simulations. It is shown that the free-energy curves predict a temperature-induced first-order interfacial phase transition in the GB structure in agreement with computational studies of the same model system. Moreover, the role of vibrational entropy in the stabilization of the high-temperature GB phase is clarified. The calculated results are able to determine the GB phase stability at homologous temperatures less than $0.5$, a temperature range particularly important given the limitation of the methods available hitherto in modeling GB phase transitions at low temperatures. The calculation of GB free energies complements currently available $0\,\mathrm{K}$ GB structure search methods, making feasible the characterization of GB phase diagrams.

Bistable Amphoteric Native Defect Model of Perovskite Photovoltaics.

Abstract: The past few years have witnessed unprecedented rapid improvement of the performance of a new class of photovoltaics based on halide perovskites. This progress has been achieved even though there is no generally accepted mechanism of the operation of these solar cells. Here we present a model based on bistable amphoteric native defects that accounts for all key characteristics of these photovoltaics and explains many idiosyncratic properties of halide perovskites. We show that a transformation between donor-like and acceptor-like configurations leads to a resonant interaction between amphoteric defects and free charge carriers. This interaction, combined with the charge transfer from the perovskite to the electron and hole transporting layers results in the formation of a dynamic n-i-p junction whose photovoltaic parameters are determined by the perovskite absorber. The model provides a unified explanation for the outstanding properties of the perovskite photovoltaics, including hysteresis of J–V characte...

Quantum effects on dislocation motion from ring-polymer molecular dynamics

Abstract: Quantum motion of atoms known as zero-point vibration was recently proposed to explain a long-standing discrepancy between theoretically computed and experimentally measured low-temperature plastic strength of iron and possibly other metals with high atomic masses. This finding challenges the traditional notion that quantum motion of atoms is relatively unimportant in solids comprised of heavy atoms. Here we report quantum dynamic simulations of quantum effects on dislocation motion within the exact formalism of Ring-Polymer Molecular Dynamics (RPMD). To extend the reach of quantum atomistic simulations to length and time scales relevant for extended defects in materials, we implemented RPMD in the open-source code LAMMPS thus making the RPMD method widely available to the community. We use our RPMD/LAMMPS approach for direct calculations of dislocation mobility and its effects on the yield strength of α-iron. Our simulation results establish that quantum effects are noticeable at temperatures below 50 K but account for only a modest (≈13% at T = 0 K) overall reduction in the Peierls barrier, at variance with the factor of two reduction predicted earlier based on the more approximate framework of harmonic transition state theory. Our results confirm that zero-point vibrations provide ample additional agitation for atomic motion that increases with decreasing temperature, however its enhancing effect on dislocation mobility is largely offset by an increase in the effective atom size, an effect known as quantum dispersion that has not been accounted for in the previous calculations. Large ring-polymer molecular dynamics can accurately simulate quantum effects on dislocation motion. A team led by Rodrigo Freitas at the University of California, Berkeley and Lawrence Livermore National Laboratory, U.S.A., investigated the effect of atomic quantum motion on the Peirels stress, i.e., the low-temperature resistance to dislocation motion. While classical molecular dynamics yielded the expected overestimation of the Peirels stress compared to experiments, ring-polymer molecular dynamics on 150,000 atoms using parallel computing showed a smaller discrepancy between simulations and experiments. This indicated that earlier quantum corrections overestimated the agitation effect of zero-point-vibrations and downplayed the effect of atomic neighbor confinement. Implementing efficient ring-polymer molecular dynamics can help us study extended defects in materials, while accurately accounting for quantum corrections to atom dynamics.

Electronic and Polar Properties of Vanadate Compounds Stabilized by Epitaxial Strain

Abstract: Recent experimental and computational studies have demonstrated pressure and epitaxial stabilization of polar PbVO3 phases with perovskite-derivative crystal structures. In this study, we demonstra...

Phase transformation pathways of ultrafast-laser-irradiated Ln 2 O 3 (Ln =Er -Lu )

Abstract: Ultrafast laser irradiation causes intense electronic excitations in materials, leading to transient high temperatures and pressures. Here, we show that ultrafast laser irradiation drives an irreversible cubic-to-monoclinic phase transformation in ${\mathrm{Ln}}_{2}{\mathrm{O}}_{3}\phantom{\rule{4pt}{0ex}}(\mathrm{Ln}=\mathrm{Er}--\mathrm{Lu})$, and explore the mechanism by which the phase transformation occurs. A combination of grazing incidence x-ray diffraction and transmission electron microscopy are used to determine the magnitude and depth-dependence of the phase transformation, respectively. Although all compositions undergo the same transformation, their transformation mechanisms differ. The transformation is pressure-driven for $\mathrm{Ln}=\mathrm{Tm}--\mathrm{Lu}$, consistent with the material's phase behavior under equilibrium conditions. However, the transformation is thermally driven for $\mathrm{Ln}=\mathrm{Er}$, revealing that the nonequilibrium conditions of ultrafast laser irradiation can lead to novel transformation pathways. Ab initio molecular-dynamics simulations are used to examine the atomic-scale effects of electronic excitation, showing the production of oxygen Frenkel pairs and the migration of interstitial oxygen to tetrahedrally coordinated constitutional vacancy sites, the first step in a defect-driven phase transformation.

Epitaxial phase diagrams of SrTiO 3 , CaTiO 3 , and SrHfO 3 : Computational investigation including the role of antiferrodistortive and A -site displacement modes

Abstract: Ground-state epitaxial phase diagrams are calculated by density functional theory (DFT) for ${\mathrm{SrTiO}}_{3}, {\mathrm{CaTiO}}_{3}$, and ${\mathrm{SrHfO}}_{3}$ perovskite-based compounds, accounting for the effects of antiferrodistortive and $A$-site displacement modes. Biaxial strain states corresponding to epitaxial growth of (001)-oriented films are considered, with misfit strains ranging between $\ensuremath{-}4%$ and 4%. Ground-state structures are determined using a computational procedure in which input structures for DFT optimizations are identified as local minima in expansions of the total energy with respect to strain and soft-mode degrees of freedom. Comparison to results of previous DFT studies demonstrates the effectiveness of the computational approach in predicting ground-state phases. The calculated results show that antiferrodistortive octahedral rotations and associated $A$-site displacement modes act to suppress polarization and reduce the epitaxial strain energy. A projection of calculated atomic displacements in the ground-state epitaxial structures onto soft-mode eigenvectors shows that three ferroelectric and six antiferrodistortive displacement modes are dominant at all misfit strains considered, with the relative contributions from each varying systematically with the strain. Additional $A$-site displacement modes contribute to the atomic displacements in ${\mathrm{CaTiO}}_{3}$ and ${\mathrm{SrHfO}}_{3}$, which serve to optimize the coordination of the undersized $A$-site cation.

Transmission Electron Microscopy (TEM) Study of the Oxide Layers Formed on Fe-12Cr-4Al Ferritic Alloy in an Oxygenated Pb-Bi Environment at 800°C

Abstract: Liquid lead–bismuth eutectic (LBE) can serve as a heat transfer fluid for advanced nuclear applications as well as concentrated solar power but poses corrosion challenges for the structural materials at elevated temperatures. Oxide passivation of the surfaces of these materials during exposure to liquid LBE can inhibit such material degradation. In this study, transmission electron microscopy of oxides formed on Fe-Cr-Al alloy during exposure to low-oxygenated LBE at 800°C has been performed. A complex structure of the oxide film has been revealed, consisting of a homogeneous inner layer of mostly Al2O3 and a heterogeneous outer layer.

Bistable Amphoteric Native Defect Model of Perovskite Photovoltaics

Abstract: The past few years have witnessed unprecedented rapid improvement of the performance of a new class of photovoltaics based on halide perovskites. This progress has been achieved even though there is no generally accepted mechanism of the operation of these solar cells. Here we present a model based on bistable amphoteric native defects that accounts for all key characteristics of these photovoltaics and explains many idiosyncratic properties of halide perovskites. We show that a transformation between donor-like and acceptor-like configurations leads to a resonant interaction between amphoteric defects and free charge carriers. This interaction, combined with the charge transfer from the perovskite to the electron and hole transporting layers results in the formation of a dynamic n-i-p junction whose photovoltaic parameters are determined by the perovskite absorber. The model provides a unified explanation for the outstanding properties of the perovskite photovoltaics, including hysteresis of J-V characteristics and ultraviolet light-induced degradation.