Deformation and Transitions at Interfaces : Grain Boundary Structure
Sponsored by: TMS Functional Materials Division (formerly EMPMD), TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Thin Films and Interfaces Committee
Program Organizers: Saryu Fensin, Los Alamos National Laboratory; Thomas Bieler, Michigan State University; Rozaliya Barabash, OakRidge National Lab; Shen Dillon, Universe of Illinois; Jian Luo, University of California, San Diego; Doug Spearot, University of Florida

Monday 8:30 AM
February 27, 2017
Room: 23B
Location: San Diego Convention Ctr

Session Chair: Douglas Spearot, University of Florida

8:30 AM  Invited
Influence of Grain Boundary Structure and Character on the Deformation Mechanisms of Grain Boundaries: Diana Farkas1; Bryan Kuhr1; Ian Robertson2; Gary Was3; 1Virginia Tech; 2University of Wisconsin; 3University of Michigan
    Large scale MD was used to study the role of grain boundary structure on the deformation response of randomly generated grain boundaries in FCC materials. We report how variations in the local grain boundary structure can affect the generation of dislocations from grain boundaries and the interaction of grain boundaries with dislocations. The atomistic techniques provide critical information on how the local structure in a random boundary changes with applied stress and how it affects the overall deformation response of grain boundary networks.

8:50 AM  Invited
Quantifying Structure-Property Relationships of Grain Boundaries and Interfaces at the Atomic Scale for Design of Polycrystalline Materials: Mark Tschopp1; 1Army Research Laboratory
    Grain boundaries and interfaces play a commanding role in the bulk properties of polycrystalline materials, interacting with dislocations and/or cracks, absorbing defects and solute atoms, and moving with stress and/or temperature. Suffice it to say that understanding the structure-property relationships of grain boundaries and interfaces in metals and ceramics is critical to designing material systems for improved properties and performance. This presentation will introduce the different kinds of grain boundaries and interfaces in metals/ceramics and discuss some recent research thrusts to understand grain boundary/interfacial behavior, to model these grain boundaries/interfaces, and to experimentally tailor these grain boundaries/interfaces for real material systems. The understanding of interfaces, in particular the structure-property relationships, is a key component of the “Materials-By-Design” thrust – a concept related to national initiatives to develop integrated computational material models that aim to link chemistry and processing all the way to performance, with everything in between.

9:10 AM  
A Mesoscale Model of Grain Boundary Faceting: The Role of Facet Junctions: Fadi Abdeljawad1; Douglas Medlin1; Jonathan Zimmerman1; Khalid Hattar1; Stephen Foiles1; 1Sandia National Laboratories
    In grain boundaries with anisotropic interfacial energies, an initially flat grain boundary (GB) may facet resulting in a “hill-and-valley” morphology with well-defined planes and corners/edges connecting them. In general, dislocation-like defects exist at GB facet junctions due to differences in the translation states at these intersecting facets. Herein, we present a mesoscale model that reduces to classic thermodynamic treatments of faceting (due to Cabrera/Frank/Cahn) but goes beyond by accounting for the excess energy due to facet junctions and their non-local interactions. We consider the case of a Σ5 <001> tilt GB in BCC iron, where we investigated facets along {210} and {310} planes both experimentally and with the aid of atomistic calculations for the inclination-dependent GB energy, which was used as an input in our model. Linear stability analysis and simulation results highlight the role of junction energy and associated non-local interactions on the faceting instability and subsequent facet coarsening.

9:30 AM  Invited
Alloy Stabilization of Nanocrystalline Grain Structures: Case Study of Pt-Au: Stephen Foiles1; Christopher O'Brien1; Ping Lu1; Michael Chandross1; Nicholas Argibay1; Brad Boyce1; 1Sandia National Laboratories
    Nanocrystalline metals have desirable properties, but the grain microstructure is often insufficiently stable for engineering application. One route to stabilize the grain structure is alloying which can be imagined to have three broad effects: Zener pinning of boundaries by precipitates, the reduction of grain boundary energy due to segregation and the reduction of grain boundary mobility due to solute-boundary interactions. Here we will examine these potential influences for the model alloy platinum-gold. Off-lattice Monte Carlo simulations of the equilibrium segregation at both select bi-crystal boundaries and in nanoscale polycrystals predict the variability in segregation levels. Molecular dynamics simulations determine the response of select boundaries and grain microstructures to both thermodynamic driving forces and mechanical stresses. The atomistic predictions will be compared to electron microscopy experimental observation and quantification of the segregation during thermal annealing of platinum-gold thin films.

9:50 AM  Invited
Kinetic Monte Carlo Simulations of Grain Boundary Kinetic Events: Kathleen Alexander1; Christopher Schuh1; 1Massachusetts Institute of Technology
    Employing the activation-relaxation technique to search for transition pathways and a deterministic method for mapping an energy landscape, we have developed a kinetic Monte Carlo (KMC) method that can be used to study grain boundary (GB) kinetics. As a case study, we considered the Σ5 (210) GB in copper at temperatures between 25–900 C. We have found that the activation energy for intrinsic diffusivity at this GB is higher than bulk diffusivity and is similar to the reported values of activation energy for interstitial diffusion at this boundary. Analogous to the case of interstitial diffusion, we have observed that intrinsic diffusion along the GB plane is slightly favored over diffusion out of the GB plane. At low temperatures, cumulative simulation times reaching several seconds have been achieved. These methods can be extended to simulate other properties, GBs, and materials under conditions comparable to experimental studies.

10:10 AM Break

10:30 AM  Invited
The Role of Collective Atomic Motion on Interface Migration and Deformation: Hao Zhang1; 1University of Alberta
    It has long been hypothesized that grain boundaries have features in common with glass-forming (GF) liquids. Emergent collective atomic motion and dynamic heterogeneity are symptomatic of diverse types of amorphous condensed materials, which have been identified in recent molecular dynamics (MD) simulations in a wide range of GF liquids. In addition, collective motion arises in systems that are not normally considered to be GF liquids. Our recent MD simulations have indicated that collective motion is prevalent in grain boundaries. It has also been observed in simulations of homogeneous melting of bulk crystalline Ni, the interfacial dynamics of bulk crystals, and dynamic relaxation of Cu-Zr metallic glasses. Apparently, collective atomic motion is important to understand the dynamic heterogeneity and dynamic response in variety systems. In this talk, we will discuss the effects of collective atomic motion on grain boundary migration, grain boundary self-diffusion, and formation of shear bands in metallic glasses.

10:50 AM  Invited
Grain Boundaries, Disorder, and Mass Transport in Complex Oxides: Blas Uberuaga1; Romain Perriot1; 1Los Alamos National Laboratory
    The relationship between interfaces, including grain boundaries, and mass transport is complex in the simplest of materials. In complex oxides the situation is even more confounding. In the bulk, mass transport is intricately related to the level of cation disorder. Here, using pyrochlore as a model system, we examine the relationship between grain boundary structure, cation disorder, and mass transport. We find that, for pyrochlores exhibiting ordered cation sublattices, grain boundaries tend to enhance oxygen transport and that enhancement depends on the nature of the boundary. However, as cation disorder is increased, there is a transition from grain boundary to bulk dominated diffusion. In the later regime, as the role of the grain boundaries is completely overwhelmed by the bulk contributions, the character of the grain boundaries is irrelevant. These results have implications for the properties of grain boundaries in complex oxides generally and mass transport in pyrochlore more specifically.

11:10 AM  
Non-Arrhenius Grain Growth, Interfacial Complexion Transitions and the Grain Boundary Character Evolution in SrTiO3: Madeleine Kelly1; Gregory Rohrer1; Wolfgang Rheinheimer2; Michael Hoffmann2; 1Carnegie Mellon University; 2KIT
    Strontium titanate exhibits interesting, non-Arrhenius grain growth within the temperature range of 1300 C to 1550 C. The relative grain boundary energy was determined by measuring grain boundary thermal grooves using AFM in this temperature range. A complexion transition, as indicated by an abrupt change in grain boundary energy, was detected between 1350 C and 1400 C which is approximately where the grain size distribution begins to transition. Orientation information by EBSD was collected by Xe-ion plasma FIB and five parameter grain boundary character distributions were determined at temperatures before non-Arrhenius grain growth behavior began, within the transition region and after the transition. The temperature dependence of grain growth, complexion transitions and populations of grain boundary planes will be discussed for strontium titanate in a temperature range of 1300 C to 1550 C.

11:30 AM  Invited
The Impact of Irradiation Dose Rate and Temperature on Grain Structure Evolution in Nuclear Fuel: Michael Tonks1; 1Pennsylvania State University
    Grain boundaries (GBs) act as sinks for defects, reducing defect energies in irradiated materials. However, drastic changes to the grain structure can occur in fuel materials. Irradiated materials are driven systems, with defects constantly being generated, and the microstructure evolves to attempt to control this influx of defects. At high temperature, defects are mobile and can segregate to GBs, though the grains grow, increasing the required migration distance for segregation. At lower temperatures, migration is too slow for segregation to counteract defect production, so grain subdivision occurs creating a structure with a two orders of magnitude smaller grain size. At even lower temperatures, grain subdivision is not sufficient to control the defects and amorphization occurs. In this presentation we use simple models of the system free energy and the defect evolution to begin to define the transition conditions between grain growth, grain subdivision, and amorphization in irradiated nuclear fuel.

11:50 AM  Invited
The Effect of Interface Elastic Fields on Interface Sink Strengths: Aurelien Vattre1; Thomas Jourdan1; Hepeng Ding2; Cosmin Marinica1; Michael Demkowicz2; 1CEA; 2Texas A&M University
    We show that elastic interactions between point defects and semicoherent interfaces lead to a marked enhancement in interface sink strength. Our conclusions stem from simulations that integrate object kinetic Monte Carlo with anisotropic elasticity calculations of interface stress fields. Elastic interactions of interfaces with vacancies and interstitials are characterized using elastic dipole tensors computed from first principles. Surprisingly, the enhancement in sink strength is not due primarily to increased thermodynamic driving forces, but rather to reduced defect migration barriers, which induce a preferential drift of defects towards interfaces. The sink strength enhancement is highly sensitive to the detailed character of interfacial stresses, suggesting that “super-sink” interfaces may be designed by optimizing interface stress fields. Such interfaces may be used to create materials with unprecedented resistance to radiation-induced damage.

12:10 PM  Invited
Virtual Diffraction of Grain Boundaries: Characterize, Optimize, and Drive Motion: Shawn Coleman1; 1U.S. Army Research Laboratory
    Virtual diffraction from atomistic simulations of grain boundaries and interfaces can make meaningful connections to experiments and provide further insights into simulations. The diffraction patterns in this work, are computed using a highly parallelized algorithm (using LAMMPS) to compute kinematic diffraction intensities across a three-dimensional mesh of reciprocal space. The diffraction algorithm can explore of a multitude of materials classes and very complex simulation geometries because it specifically accounts for different elements and does not require any assumption of the modeled structures. Within grain boundary and interface models, virtual selected area diffraction patterns are used to characterize the underlying periodic structure of relaxed structures enabling direct connections to experiments. When constructing complex interfaces, three dimensional diffraction maps are shown to narrow the search for ideal orientation relationships. Additionally, virtual diffraction information computed for individual atoms provide the orientation information needed to induce grain boundary motion using a synthetic driving force.