Computational Thermodynamics and Kinetics: Grain Boundaries and Defects II
Sponsored by: TMS: Chemistry and Physics of Materials Committee, TMS: Computational Materials Science and Engineering Committee
Program Organizers: Niaz Abdolrahim, University of Rochester; Stephen Foiles, Sandia National Laboratories; James Morris, Oak Ridge National Laboratory; Raymundo Arroyave, Texas A & M University

Thursday 8:30 AM
March 2, 2017
Room: 11B
Location: San Diego Convention Ctr

Session Chair: Timofey Frolov, Lawrence Livermore National Laboratory; Bilge Yildiz, Massachusetts Institute of Technology

8:30 AM  Invited
Defect Equilibria in Semiconducting Oxides under Thermodynamic Forces: Bulk and Interfaces: Mostafa Youssef1; Jing Yang1; Krystyn Van Vliet1; Bilge Yildiz1; 1Massachusetts Institute of Technology
    The functionality of oxides can be improved by controlling the underlying defect equilibria. This control can be achieved via the various thermodynamic forces including temperature, chemical potential and stress as prominent drivers. Starting from first principles we developed a framework to analyze the defect equilibria in semiconducting oxides under such thermodynamic forces. Validation of this framework was performed on a prototypical model system, ZrO2. We expanded this framework to account for hetero-interfaces (e.g. ZrO2/Cr2O3) by considering interfacial segregation energies, band alignment, and equilibration of the chemical potentials of electrons and species. Starting from bulk equilibria as boundary conditions, space charge and core zones are evaluated by coupling of the Poisson and the drift-diffusion equations. The richness of the defect equilibria in ternary systems such as SrTiO3 is also exposed by this framework indicating a broad room for tuning its ionic and electronic defects by stress, electrochemical potential and electric field.

9:00 AM  
Design of Interfaces between Transition Metal Carbide and Nitride Precipitates and Matrix in Austenitic Steels: Oleg Kontsevoi1; Gregory Olson1; 1Northwestern University
    We investigate the role of interfacial adhesion between nano-scale sized transition metal carbide and nitride precipitates and matrix in the design of high performance precipitation-strengthened fully austenitic steels. First-principles density functional theory calculations with the highly-precise FLAPW method were employed to investigate the interfacial adhesion strength as the main property relevant to the optimization of steel grain refining dispersions for resistance to microvoid-driven shear localization in ductile fracture. Binary, ternary and quarternary precipitate phases mixed on both metal and non-metal sublattice were considered in order to find the compositions that allow to maximally strengthen the interface bonding between matrix and these carbides and nitrides, and thus delay the microvoid softening. Given the high level of Ni and Cr inherent in the austenitic alloy designs, the role of Ni and Cr segregation near the interface is investigated as a potential further source of adhesion enhancement.

9:20 AM  
Thermodynamic Stabilization of High Concentrations of Planar Faults in Near-stoichiometric NiTi Shape Memory Alloys: Sascha Maisel1; Blazej Grabowski1; Jrg Neugebauer1; 1MPIE
    Near-stoichiometric NiTi exhibits a martensitic phase transition from its low temperature phase to its B2-ordered state, which is responsible for its shape-memory properties. Depending on which phase diagram is consulted, the B2 phase extends over a considerable concentration range. We present a configurationally exhaustive screening of off-stoichiometric Ni-Ti on its bcc-based lattice both a T=0 and T=1200 K by means of a cluster-expansion Hamiltonian based on ab-initio input. We show that a multitude of defect structures derived from the ideal compound are either stable ground-states or very close to the equilibrium ground-state lines. These defect structures are exclusively characterized by planar faults along the <100> (bcc) planes. This is in contrast to other B2-forming systems like Ni-Al, which can tolerate an exceedingly high number of point defects. We discuss implications of these faults both from a thermodynamic and an application-oriented perspective.

9:40 AM  Invited
Predicting Phase Behavior of Interfaces with Evolutionary Algorithms: Qiang Zhu1; Robert Rudd2; Timofey Frolov2; 1University of Nevada Las Vegas; 2Lawrence Livermore National Laboratory
    Recent years have seen a rapid growth of evidence suggesting that materials interfaces are capable of first-order structural transformations in which the interface properties undergo discontinuous changes. Experiments have linked these transitions to abnormal grain growth in ceramics, activated sintering and liquid metal embrittlement and raised a number of fundamental questions concerning the atomic structures and kinetic properties of these interface phases. Using improved simulation methodology recent modeling efforts in relatively simple metallic systems discovered new multiple structural states of grain boundaries and demonstrated first order transitions between them. Same studies also showed that grain boundary structures can be very complex and current modeling capabilities are too limited to predict structures of grain boundaries in complex materials. We developed a new computational tool based on USPEX code that uses evolutionary algorithms to predict complex interface structures in multicomponent systems. Applications of this tool to several materials systems are discussed.

10:10 AM Break

10:30 AM  Invited
Effect of Bicrystallography on Thermal Resistance of Grain Boundaries: J. Hickman1; Yuri Mishin1; 1George Mason University
    Thermal resistance of grain boundaries (GBs) may strongly affect the heat transport in polycrystalline materials. Phonon scattering by GBs is largely responsible for the strong effect of microstructure on the efficiency of thermoelectric materials and devices. We apply non-equilibrium molecular dynamics simulations with a new highly-optimized atomistic potential to study the effect of GB structure and bicrystallography on thermal resistance of GBs in silicon. The relations between the thermal resistance and GB characteristics obtained in this work may guide the design of optimized microstructures in Si solar cells and other applications of phonon engineering.

11:00 AM  
Ab Initio Study of Point Defects in Heusler Alloys: Consequences for Magnetocaloric Properties: Biswanath Dutta1; Vijaya Begum1; Tilmann Hickel1; Jrg Neugebauer1; 1Max-Planck-Institut fr Eisenforschung GmbH
    The effect of chemical ordering on magnetocaloric properties is an area of intense research since it has direct consequences for functional applications. Recent experiments indicated the importance of vacancies, which strongly influence the ordering kinetics. Within this study, we combine density functional theory with Monte Carlo (MC) simulations to understand the impact of vacancies on the phase stability and transformation behavior in Ni-Mn-based Heusler alloys. They are discussed in terms of chemical potentials, which are constrained by the formation of various secondary phases. Our calculations yield highest vacancy concentrations for the Ni sublattice, while the most likely diffusion path involves in addition the Mn sublattice. Our results explain the experimental trends in the ordering kinetics of Heusler alloys. Using MC simulations we further predict the impact of these point defects on the magnetocaloric entropy change in these materials.

11:20 AM  
A Non-Schmid Crystal Plasticity Finite Element Approach to Multi-scale Modeling of Nickel-based Superalloys: Shahriyar Keshavarz1; Andrew Reid1; Stephan Langer1; Somnath Ghosh2; 1NIST; 2JHU
    This study develops non-Schmid crystal plasticity constitutive models at two length scales, and bridges them in a multi-scale framework. The constitutive models address thermo-mechanical behavior of Nickel-based superalloys for a large temperature range, viz. 300 Ke1223 K, and include orientation dependencies and tension-compression asymmetry. The orientation dependencies result in tension- compression asymmetry for almost all orientations on the standard unit triangle. However simulations show different trends for the stronger direction (tension or compression) in terms of yield stress and hardening. The multi-scale framework includes two sub-grain and homogenized grain scales. The homogenized model develops functional forms of constitutive parameters in terms of characteristics of the sub-grain two-phase microstructural morphology including precipitates shape, volume fraction and size in the sub-grain microstructure. This homogenized model can significantly expedite crystal plasticity FE simulations due to the parametrized representation, while retaining accuracy.