Computational Thermodynamics and Kinetics: Grain Boundaries and Defects I
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

Tuesday 8:30 AM
February 28, 2017
Room: 11B
Location: San Diego Convention Ctr

Session Chair: Elizabeth Holm, Carnegie Mellon University; James Morris, Oak Ridge National Laboratory

8:30 AM  Invited
MPMC Discrete Thermodynamic Simulations of Grain Growth in Nanotwinned Polycrystalline Films: Elizabeth Holm1; Philip Goins1; 1Carnegie Mellon University
    Grain growth in polycrystalline films can be influenced by a variety of physical parameters, including grain boundary, interface, and surface energies; volumetric energies due to strain, stress, or twinning; and kinetic processes including boundary mobility and film grooving. At a given temperature, which of these factors dominates depends on film thickness. In order to understand texture evolution in nanotwinned Ag films, we use the Material Point Monte Carlo (MPMC) method, an efficient multiphysics model that operates on a dynamic random lattice of material points, to perform a parametric study of grain growth at various film thicknesses. We compare the resulting grain structures, crystallographic textures, and growth kinetics to experimental studies of polycrystalline Ag films in order to help distinguish between proposed texture evolution models.

9:00 AM  
Construction of Grain Boundary “Phase” Diagrams with Atomistic Simulation: Shengfeng Yang1; Naixie Zhou1; Jian Luo1; 1University of California, San Diego
    The formation of interfacial phases at grain boundaries (GBs) and the occurrence of GB phase-like (complexion) transitions can affect the microstructural development and materials properties. In this study, we first modify a genetic algorithm to predict GB atomic structure by effectively searching for the lowest-energy configuration through the energy landscape at T=0K. Subsequently, a hybrid molecular dynamics and Monte Carlo method is implemented to predict equilibrium GB structures and transitions at finite temperatures. The results from atomistic simulations are used to construct GB “phase” diagrams, which reveal the GB excess, effective interfacial width, structural disorder, and GB diffusivity as functions of the bulk concentration and temperature. Examples of alloys (Mo-Ni) and ceramics (Y2O3-stablized ZrO2, perovskite oxides) will be discussed. The influences of GB complexion formation and transition on the mechanical and other properties are studied. The atomic simulation results are critically compared with density functional theory calculations and experimental observations.

9:20 AM  
Grain Growth in Thin Films as a Truly Three-dimensional Problem: A Simulation Study: Dana Zöllner1; Ahu Öncü1; 1Otto von Guericke University Magdeburg
    Historically, metallography has been the two-dimensional characterization of materials microstructures by optical microscopy. A consequential problem has long been known: A two-dimensional section through a three-dimensional object gives us often only a very poor idea about size and form of the object. Therefore, many attempts have been made to gain three-dimensional information experimentally. Nevertheless, in simulations and also analytical theories thin films are commonly still treated as two-dimensional objects making comparisons with 3D experimental data rather hard. In the present work, grain growth in thin films is investigated by three-dimensional Potts model simulations focusing particularly on the transition from bulk-like growth to columnar microstructures.

9:40 AM  
Interaction of Shear-coupled Grain Boundary Motion with Crack Studied by Molecular Dynamics Simulations: Aramfard Mohammad1; Chuang Deng1; 1University of Manitoba
    In this research, the interaction of shear coupled grain boundary motion (SCGBM) in face-centered cubic metals with crack has been studied by using molecular dynamics simulations in simple bicrystal models. The influences of metal type, temperature, grain boundary structure, and crack geometry have been examined systematically. Three types of microstructural evolution have been identified, namely, crack healing, grain boundary decohesion, and sub-grain formation. The underlying atomistic mechanisms for each type of SCGBM-crack interaction, particularly grain boundary decohesion and crack healing, have been explored. It is found that crack healing can be enhanced by applying cyclic shear loading, which is accompanied by a structural transformation in the grain boundary by rearranging the basic structural units. The healed structure is stable and strong, which sheds some light for healing severely damaged polycrystalline materials.

10:00 AM Break

10:15 AM  Invited
Stochastic Grain Boundary Dynamics in a DSC Model for Shear Coupling: Jian Han1; Vaclav Vitek1; David Srolovitz1; 1University of Pennsylvania
    The coupling between shear and grain boundary (GB) migration has been widely studied over the past 15 years and several models have been proposed to explain its origin. In this presentation, we present a model for shear coupling based on a disconnection (DSC dislocations plus steps) description. We then apply this model to provide a self-consistent description of GB shear coupling, GB sliding and GB roughening and their temperature dependence. Our approach is based upon bicrystallography and both molecular dynamics and Monte Carlo simulations.

10:45 AM  
A Universal Discrete Dislocation Model for Thermal Activation and Diffusion-assisted Climb: Run Zhu1; Srinath Chakravarthy1; 1Northeastern University
    Given that thermal activation and climb of dislocation contribute significantly to high temperature plastic behavior, a thorough understanding of dislocation motion is extremely critical. A new discrete dislocation model is presented to determine equilibrium configurations of dislocations as a function of temperature and strain rate. The thermal activation mechanisms and diffusion-assisted climb are incorporated at appropriate time scales. Also, a new universal model for diffusion assisted climb is presented that is applicable and accurate for arbitrary choice of time scales. It involves solution of an auxiliary mechanical-diffusion boundary value problem by treating dislocations as continuous source/sink of vacancies. It enables computing the rate of climb of dislocations at arbitrary temporal scales. Several applications are shown for our new model from tension/bending of micron sized samples, to creep and fatigue at high temperature.

11:05 AM  
Non-Schmid Effects on Dislocation Core Structure and Influence on Dislocation Mobility in Titanium: Max Poschmann1; Daryl Chrzan1; Mark Asta1; 1UC Berkeley
    Titanium alloys are known to exhibit a tension-compression asymmetry in the critical resolved shear stress for slip of a-type dislocations. We discovered that non-shear stresses can have significant effects on the core structure of a-type screw dislocations in titanium and thereby influence the energy barrier to slip. Core structures of these dislocations are computed using both density functional theory and modified embedded atom method to achieve both high accuracy and understanding of cell size effects. The nudged elastic band method was used in combination with both techniques to calculate barrier energies and transition paths for dislocation slip. This allows us to determine how non-Schmid effects can influence dislocation mobility by changing the dislocation core structure. Our results suggest that the tension-compression asymmetry may be attributable to these effects. This information should be useful in improving dislocation kinetics models, especially in high-stress regimes. This research is supported by ONR grant N00014-12-1-0413.

11:25 AM  Cancelled
A Dislocation Density Approach to Determine Pipe Diffusivity: Chaoyi Zhu1; Tyler Harrington1; Kenneth Vecchio1; 1UC San Diego
    It has been an experimental challenge using radioactive tracers to determine self-diffusion coefficients along dislocations, especially at elevated temperature when phase and magnetic transformations complicates the diffusion kinetics. Consequently, pipe diffusivity data of, for example, ferrite iron at elevated temperature is inconsistent, which is detrimental to extrapolation of diffusivity data close to the austenitic transformation temperature. In this study, we present a new method to determine pipe diffusivity based on dislocation density measured through combined nanoindentation and EBSD based approaches on sintered pure nickel samples. This method is derived from that fact that volume diffusion through a dislocation pipe diffusion model dominates over grain boundary diffusion as a means to mass transfer for heavily compacted powder material. Using the theoretical model developed by Olevsky et al., which relates kinetics of shrinkage processes to dislocation density, it is possible to calculate pipe diffusivity over a wide range of temperatures.

11:45 AM  
Developing the Third Generation of Calphad Databases - Modelling Al as a Case Study: Sedigheh Bigdeli1; Albert Glensk2; Blazej Grabowski2; Alexandra Khvan3; Huahai Mao1; Malin Selleby1; 1KTH Royal Institute of Technology; 2Max-Planck-Institut für Eisenforschung GmbH; 3National University of Science and Technology MISIS
    In developing the next generation of Calphad databases, new models are used in which each term contributing to the Gibbs energy has a physical meaning. Harmonic vibrations of atoms are modelled using the Einstein temperature, anharmonic vibrations, electronic and magnetic contributions for solid phases each have specific terms. The two-state model is used for the liquid phase. To continue the development, a new description for unary aluminum is presented using the the new models. To be able to develop these new thermodynamic databases we are confined to using existing computational tools but at the same time we want to make the best possible use of atomistic methods and most recent experimental data. Therfore, DFT results for pure Al at finite temperatures are used together with new experimental measurements of the heat capacity. Strenghts and limitations of each method are discussed.