Deformation and Transitions at Grain Boundaries VII: Grain Boundary Structure: FCC and Hexagonal
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Computational Materials Science and Engineering Committee
Program Organizers: Saryu Fensin, Los Alamos National Laboratory; Thomas Bieler, Michigan State University; Shen Dillon, University of California, Irvine; Douglas Spearot, University of Florida; Jian Luo, University of California, San Diego; Jennifer Carter, Case Western Reserve University

Monday 2:30 PM
February 24, 2020
Room: 5B
Location: San Diego Convention Ctr

Session Chair: Srikanth Patala, SASA Institute; Shen Dillon, University of California, Irvine


2:30 PM  
Structure, Deformation Response and Diffusion in Random [110] Tilt Grain Boundaries in FCC Alloys: Diana Farkas1; 1Virginia Polytechnic Institute
    We report atomistic simulation studies of grain boundary structure, diffusion and deformation response in FCC materials. The simulations are based on empirical interatomic potentials and use massively parallel molecular dynamics techniques at the atomistic level. The studies address a series of grain boundaries with random misorientations around the [110] crystallographic axis. We analyze the relaxed structures using various models and visualization techniques including the dislocation content of the boundary region. We find that that the random boundaries contain a varying amount and types of dislocations as part of the structure. These in turn influence the deformation response and the diffusion properties of the boundaries.

2:50 PM  
Anisotropic Mobility in Faceted Σ11 <110> tilt FCC Grain Boundaries and the Effect of Subsequent Doping: Megan McCarthy1; Timothy Rupert1; 1University of California, Irvine
    Faceted grain boundaries, where grain boundary area is increased to produce low-energy segments, have been shown to demonstrate new and unexpected migration trends. For example, several faceted boundaries have demonstrated anti-thermal and thermally damped mobility trends. In this work, we use molecular dynamics to demonstrate how faceted Σ11 <110> tilt bicrystal grain boundaries migrate in various FCC metals and also uncover the effect of dopant addition on this motion. In pure metals, a number of these bicrystals exhibit anisotropic mobility, in which boundary migration velocity is dependent on the direction of an applied driving force. One mechanism controlling their mobility is grain boundary dislocation emission. Upon alloying, a critical dopant concentration can alter the ability to create and re-absorb grain boundary dislocations, which introduces mobility anisotropy in several boundaries which were originally isotropic. The discovery of this effect highlights how microscopic boundary structure can influence grain boundary migration.

3:10 PM  
Interplay of Chemistry and Faceting at Grain Boundaries in an Al-alloy : Huan Zhao1; Liam Huber1; Wenjun Lu1; Nicolas Peter1; Dayong An1; Frédéric De Geuser1; Dirk Ponge1; Baptiste Gault1; Dierk Raabe1; 1Max-Planck-Institut Fur Eisenforschung G
    The boundary between two crystalline grains can decompose into arrays of facets with distinct crystallographic character. Faceting occurs to minimize the system’s free energy, i.e. when the total interfacial energy of all facets is below that of the topologically shortest interface plane. In a model engineering Al-alloy, we show that faceting occurs at selected coincident-site lattice boundaries and these facets strongly interact with the local chemistry. This faceting-chemistry interplay acts differently for different types of facets and leads to the formation of periodic segregation patterns, or to the asymmetric precipitation. This study shows that segregation-faceting interplay is not limited to bicrystals but may exist in engineering Al-alloys and hence affect their performance.

3:30 PM  
Quantifying and Predicting a "local" Stacking Fault Energy in Multi-principal Element Alloys: Carlyn LaRosa1; Maryam Ghazisaeidi1; 1Ohio State University
    Here, we quantify the effects of composition on energy fluctuations in FCC multi-principal element (MPE) alloys, examine how it relates to nanoscale deformation modes, and present a model which can reliably predict “local” stacking fault energy (SFE) as a function of the local atomic environment based on bond energies. The concept of a generalized SFE is well-suited for pure metals and dilute alloys. However, it is inadequate for quantifying the same phenomena in MPE alloys (e.g. medium- and high entropy alloys), where local compositional fluctuations can cause significant deviations from the average SFE. We apply our model to the binary and ternary subsets of the CrMnFeCoNi alloy. The outputs of our model can be used in strengthening models, with the added benefit of being significantly more cost effective than direct density functional theory (DFT) calculations. We anticipate extending our model to include the effects of local ordering.

3:50 PM  Invited
Simulating Grain Boundary Structures with DFT Accuracy Through Active Learning of Interatomic Potentials: Tolga Akiner1; Srikanth Patala1; 1North Carolina State University
    Grain boundaries (GBs) influence a wide array of physical properties in polycrystalline materials and play an important role in governing microstructural evolution under extreme environments. Therefore, to develop robust predictive models of structural alloys, the structures and energetics of GBs, with DFT accuracy, are desired. Traditionally, ab initio techniques can only be used to simulate highly-symmetric GB structures and are limited to system sizes of the order of hundreds of atoms. In this talk, we present a machine learning approach based on Moment Tensor Potentials and active learning (AL) to simulate complex GB structures. Using the AL strategy, we develop an interatomic potential that is specifically suited for the atomic configurations of interest – the interface structures. We validate this method for the general GB structures of Aluminum and discuss the potential applications of this technique for simulating GBs in multi-component alloy systems with DFT accuracy.

4:10 PM Break

4:30 PM  
Study on Effect of Symmetric Tilt Grain Boundaries on Twin Nucleation in Ti: Deepesh Giri1; Christopher Barrett1; Haitham El Kadiri1; 1Mississippi State University
    Determining a favorable site for nucleation of a twin variant in Hexagonal Close Packed (HCP) metals remains a daunting task. Site-specific twin nucleation is sensitive to the prior atomic structure of the lattice and hence twin embryos form in regions where the lattice transformation energy is minimum. This work probes the effects of symmetric tilt grain boundaries (STGBs) in nucleation of twin embryos in HCP metals. We use Molecular Statics (MS) simulations to explore the roles of [10-10] and [1-210] STGBs during twin formation in Ti. Twin nucleation process is affected by the presence of grain boundaries and defects like vacancies and dislocations. We use a noble method called Nudged Elastic Band (NEB) method to consider these factors and quantify the twinning phenomena. This allows direct comparisons between various cases with respect to twin nucleation and hence provides a measure of the material plasticity.

4:50 PM  
Vacancy-mediated Solute/Twin Boundary Interactions in HCP Alloys: Mohammad Shahriar Hooshmand1; Maryam Ghazisaeidi1; 1Ohio State University
    Twinning is an important mechanism during the plastic deformation of materials with hexagonal- close-packed crystal structure. Strength and formability are critical properties for this family of structural materials and can consequently be improved by controlling twin nucleation and growth. Here, we study the effect of substitutional alloying elements on strain rate and temperature dependence of twin growth in Mg alloys. Without using any fitting parameters, we derive an analytical model which takes atomic scale calculated parameters as input and predicts the equilibrium concentration of segregated solutes and strengthening due to solute-twin interactions. Our findings shed light on the underlying mechanisms governing the effect of substitutional solutes transport through vacancy-mediated diffusion to the twin boundary and the plastic deformation of hcp alloys.

5:10 PM  
The Role of Microstructure and Loading Parameters on Deformation Twinning in Nanocrystalline Mg at High Strain Rates: Sergey Galitskiy1; Garvit Agarwal2; Avinash Dongare1; 1University of Connecticut; 2Argonne National Laboratory
    Classical MD simulations are carried out to investigate the twinning/detwinning mechanisms in HCP Mg microstructures under high strain rate loading conditions. The role of grain size (varying from 25 to 100 nm), orientation, and strain rate (at order of 10^8 -10^9 s-1) is examined. Recently developed Extended-CNA method is utilized to characterize the type and volumetric evolution of twins. The analysis is performed for individual grains to understand the effects of grains orientation . The results suggest that twinning behavior is highly anisotropic under both tension and compression. In addition, low angle grains twin more easily under tensile loading, whereas high angle grains tend to twin under compression. MD simulations suggest that strain rate significantly affects the type, growth rate and volume fraction of twins. The role of grain size and strain rates of loading on the evolution of number and volume fractions of twins will be presented.

5:30 PM  
Database and Predictive Model of Grain Boundary Properties of Elemental Metals: Hui Zheng1; Xiang-Guo Li1; Richard Tran1; Chi Chen1; Matthew Horton2; Donny Winston2; Kristin Persson2; Shyue Ping Ong1; 1University of California, San Diego; 2Lawrence Berkeley National Laboratory
    The structure and energy of grain boundaries (GBs) are essential to explore the GB character distribution (GBCD) which strongly affects a polycrystalline material’s mechanical properties such as hardness, brittleness and fracture mechanism. In this work, we present the largest Grain Boundary Database (GBDB) using DFT calculations. The database currently encompasses 338 GBs of 59 elemental metals. We covered 10 types of common twist and symmetric tilt GBs for body-centered cubic and face-centered cubic systems, and the ∑7(0001) twist GB for hexagonal close-packed systems. A scaled-structural template approach used in our high-throughput GB calculation substantially reduces the computational cost for GBs with more distortions. We rigorously validated the DFT grain boundary energies and work of separation against previous experimental and computational data. We also developed a predictive model for GB energy from the cohesive energy and shear modulus.