Computational Thermodynamics and Kinetics: Diffusion and Kinetics 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 2:00 PM
February 28, 2017
Room: 11B
Location: San Diego Convention Ctr

Session Chair: Christine Geers, Chalmers University of Technology; Dallas Trinkle, University of Illinois, Urbana-Champaign

2:00 PM  Invited
Surface Reaction and Transport in Oxides Formed on FeCrAl Alloys in High Temperature Nitridation Environments: Christine Geers1; Vedad Babic1; Itai Panas1; Lars-Gunnar Johansson1; 1Chalmers Technical University
    An extended experimental study on commercial FeCrAl and FeNiCrAl alloys was leading to fundamental questions on the nitrogen adsorption and diffusion possibilities at high temperatures. These questions were approached by a combination of state-of-the-art microscopy combined with DFT calculations and computational thermodynamic for different oxides, different oxide combinations and oxide in contact with the underlying alloy. A comprehensive understanding of the on-going processes was achieved.

2:30 PM  
Accelerated Analysis of Beta Phase Ti-Nb-Al Ternary Diffusion via Optimization Fitting of Interdiffusion Coefficients in Three-Alloy Diffusion Multiples: James Haley1; Kaka Ma2; Aparna Tripathi3; Kaustubh Kulkarni3; Anil Sachdev4; Enrique Lavernia1; 1University of California, Irvine; 2Colorado State University; 3India Institute of Technology, Kanpur; 4General Motors
    A new approach is proposed to investigate composition dependent ternary interdiffusion coefficients in solid systems for an extensive part of a single-phase field in a ternary isotherm with a single diffusion multiple experiment. The present work aims to explore the feasibility of using optimization as an accelerated analysis tool for studying ternary diffusion. A diffusion multiple consisting of three terminal single-phase alloys is fabricated to obtain concentration profiles in diffusion zones, which form a triangle region in the ternary composition space. Interdiffusion coefficients are represented with a series of composition dependent parameters which are optimized to minimize differences in the composition profiles of the simulated and experimental data. First, the approach is evaluated against concentration profiles generated with a hypothetical set of interdiffusion coefficients. Subsequently, the optimization fitting is applied to experimental data from a diffusion multiple in the high temperature body-centered-cubic (BCC) beta phase in the Ti-Nb-Al system.

2:50 PM  
Evaluation of Silver and Tin Diffusion Mobility in Magnesium Alloys: Ian Parker1; Michele Manuel1; 1University of Florida
    Magnesium alloys are of significant interest for transportation applications due to its low density and high specific strength. Silver is added to some magnesium alloys containing rare earth elements in order to improve precipitation strengthening. Similar effects have been observed when silver is added to magnesium-tin alloys. This work aims to evaluate the mobility of silver in magnesium as well as examine the effect of silver concentration on tin diffusion. Diffusion mobilities are determined experimentally with diffusion couples and evaluated with Boltzmann-Matano analysis. This data is then integrated into a computational mobility database for predictive multicomponent diffusion simulations. This project is supported by the National Science Foundation under grant number DMR-1410883.

3:10 PM  
Modeling Alloying Effects on Hydrogen Evolution Reaction Kinetics for Decelerated Magnesium Corrosion: Krista Limmer1; Joseph Labukas1; Jan Andzelm1; 1U.S. Army Research Laboratory
    The corrosion behavior of magnesium has long been correlated with the hydrogen evolution reaction (HER) on the magnesium surface. Owing to the relatively slow anodic kinetics of pure magnesium, reducing the overall corrosion rate of magnesium requires a reduction in the HER kinetics. Alloying additions and impurities generally accelerate the cathodic kinetics through the formation of intermetallics, insoluble impurity islands, or the enrichment of noble elements on the magnesium surface. Alloying with arsenic has recently been observed experimentally to poison the cathodic kinetics of magnesium, although the mechanism of this reduction is uncertain. In this study the HER kinetic barriers for alloyed magnesium were evaluated using first principles density functional theory. The HER was examined on the basal (0001) surface of dilute binary Mg-X alloys following a recent atomistic mechanism model of hydrogen recombination and evolution. The adsorption energy of hydrogen atoms and the H2 molecule on the Mg-X surface was used as a method to rapidly survey more than 20 alloying additions as possible cathodic poisons. The full hydrogen recombination and evolution kinetic barriers were then examined in detail for selected alloying elements (e.g. As and Ge). This mechanistic understanding of how alloying additions contribute to the poisoning of magnesium corrosion may be used to develop magnesium alloy systems with reduced corrosion rates.

3:30 PM Break

3:45 PM  Invited
Kinetic Monte Carlo Enabled Modeling of Diffusion Assisted Plastic Deformation: James Martino1; Srinath Chakravarthy1; 1Northeastern University
    Many phenomena such as solute strengthening in lightweight alloys and alloyed high strength steels are controlled by quantum mechanical interactions of solutes or impurities with defects (dislocations, grain boundaries, interfaces, or cracks). The local stresses driving these atomic-scale processes are determined by behavior occurring at much larger spatial and temporal scales. To allow atomistic resolution at large spatial scales, we will use a “Coupled Atomistic/Discrete-Dislocation” (CADD) to represent defects while handling dislocation plasticity at both atomistic and continuum scales. Kinetic Monte Carlo (KMC) is used as the temporal scale buffer between the continuum and atomistic regions of the model as a general-purpose approach to couple the disparate temporal scales in concurrent multi-scale simulations. A domain with an initial Gaussian distribution of vacancies is considered and details are presented for the temporal evolution of vacancy concentrations for domains represented by (a) FEM/KMC, (b) KMC/MD and (c) FEM/KMC/MD.

4:15 PM  
Long-time Simulations of Cation Diffusion and Material Recovery in Disordered Gd2Ti2O7 Pyrochlore: Romain Perriot1; Blas Uberuaga1; Richard Zamora1; Danny Perez1; Arthur Voter1; 1Los Alamos National Laboratory
     We used molecular dynamics and parallel trajectory splicing (ParSplice) simulations to characterize the diffusion of cation defects in Gd2Ti2O7 pyrochlore as a function of the disorder on the microsecond timescale. We found that interstitials can quickly heal the disordered material, while vacancy-mediated annihilation occurs more slowly. However, we find that significant disorder allows for fast cation diffusion, which is then accompanied by antisite annihilation. The cation diffusivity is therefore not constant, but decreases with the material’s evolution. We speculate the existence of an antisite percolation network above a disorder threshold, which allows for the fastdiffusion of cations. These results highlight the dynamic interplay between fast cation diffusion and the recovery of disorder and has important implications for understanding radiation damage evolution, sintering and aging.

4:35 PM  
First-Principles Computational Study of Charged Vacancy Diffusion in Alpha-Al2O3 and Alpha-Cr2O3: Guofeng Wang1; Yinkai Lei1; Corinne Gray1; 1University of Pittsburgh
    Alumina (Al2O3) and chromia (Cr2O3) are thermally grown oxides that efficiently enable alloys to withstand high operating temperatures and oxidizing environments. The functionality of these oxide scales are closely related to the diffusion process of ions. To acquire knowledge of diffusion mechanisms and predict diffusion coefficients, we have calculated the energies of charged vacancies diffusion through the bulk crystal and grain boundaries of Al2O3 and Cr2O3 using the first-principles density functional theory method. In this study, we predict that the migration energy of vacancy diffusion strongly depends on the charge state of the vacancy involved. Importantly, we reveal that this charge-dependent vacancy diffusion in alumina/chromia is directly related to the electron occupancy and energy level change of the defect states of the charged vacancy. Moreover, we predict that the addition of reactive elements to the scales would reduce the ion diffusion due to both blocking effect and electronic effect.

4:55 PM  
Automated Diffusivity Theory without Kinetic Monte Carlo: Solute Diffusivity from First Principles: Dallas Trinkle1; 1University of Illinois, Urbana-Champaign
    Mass transport controls both materials processing and properties, such as ionic conductivity, in a wide variety of materials. While first-principles methods compute activated state energies, upscaling to mesoscale mobilities requires the solution of the master equation. For all but the simplest cases of interstitial diffusivity, calculating diffusivity directly is a challenge. Traditionally, modeling has taken two paths: uncontrolled approximations that map the problem onto a simpler (solved) problem, or a stochastic method like kinetic Monte Carlo, which is difficult to converge for strong correlations. Moreover, uncertainty quantification or derivatives of transport coefficients are complicated without analytic or semi-analytic solutions. A new automated Green function approach for transport both determines the minimum set of transition states to calculate from symmetry and computes the dilute-limit transport without additional approximations. We compute diffusivity in a variety of systems to showcase the flexibility and accuracy of the approach.

5:15 PM  
The Effects of Quantum Dynamics of Atomic Motion on Dislocation Mobility: Rodrigo Freitas1; Mark Asta2; Vasily Bulatov3; 1University of California Berkeley and Lawrence Livermore National Laboratory; 2University of California Berkeley; 3Lawrence Livermore National Laboratory
    Recent studies suggested that zero-point vibrations should have important effects on dislocation motion even in heavy refractory metals, explaining long standing discrepancies between experimental estimates and theoretical predictions of the Peierls stress of body-centered cubic crystals at low temperatures. Here we employ Path-Integral Molecular Dynamics to examine the effects of the quantum dynamics of atomic motion on the thermally activated motion of dislocations in α-iron.