Integration between Modeling and Experiments for Crystalline Metals: From Atomistic to Macroscopic Scales: Session I
Program Organizers: M Arul Kumar, Los Alamos National Laboratory; Irene Beyerlein, University of California, Santa Barbara; Levente Balogh, Queen's University; Josh Kacher, Georgia Institute of Technology; Caizhi Zhou, Missouri University of Science and Technology; Lei Cao, University of Nevada, Reno

Monday 8:00 AM
September 30, 2019
Room: G132
Location: Oregon Convention Center

Session Chair: Abigail Hunter, Los Alamos National Laboratory; Tim Rupert, University of California, Irvine

8:00 AM  Invited
Coupled Experimental and Computational Studies of Amorphous Grain Boundary Complexions: Timothy Rupert1; 1University of California, Irvine
    Complexions are phase-like features that are stable only at material defects. They represent an intriguing materials design opportunity, as the local structure of interfaces can be manipulated in a controlled manner. In this talk, we describe a combination of experimental and computational studies of disordered complexions (i.e., amorphous intergranular films). First, we discuss how alloy choice for both binary and ternary alloys can drive amorphous complexion formation and determine film thickness. We also explain how these features differ from bulk amorphous phases, with gradients in short range order observed. Finally, we show that these features can control a number of important properties of nanostructured materials. Behaviors of interest include activated sintering, strain localization, fatigue failure, radiation damage, and transport properties such as diffusion.

8:30 AM  Invited
Development of Microstructure-sensitive and Mechanism-based Modeling Tools by Integrating Experimental Characterization with Multiscale Modeling: Longsheng Feng1; Pengyang Zhao1; Steve Niezgoda1; Michael Mills1; Yunzhi Wang1; 1Ohio State University
    Location-specific component design requires modeling capabilities that incorporate specific deformation mechanisms operating in different locations of a component having different compositions and microstructures under different service conditions. In this presentation, we focus on how to use the phase-field method at different length scales to address this challenge and develop mechanism-based and microstructure-sensitive modeling tools. In particular, using creep deformation in Ni-base superalloys as an example, we demonstrate how to integrate modeling with experimental characterization and use phase-field to bridge ab initio calculations and crystal plasticity (CP) simulations to (a) identify deformation mechanisms and quantify their activation pathways, (b) provide “mechanism maps” and microstructure-sensitive constitutive laws for dislocation – microstructure interaction and co-evolution, and (c) develop an integrated phase-field + full-field FFT-based CP modeling framework for collective behavior of grain / precipitate microstructure and dislocation density during recrystallization and creep deformation. The work is supported by NSF under the DMREF program.

9:00 AM  
Discovery of a Wide Variety of Linear Complexions in Metallic Alloys: Vladyslav Turlo1; Timothy Rupert1; 1University of California, Irvine
    Linear complexions are new promising nanoscale-size defect states recently discovered at dislocations in Fe-9 at.% Mn. However, a systematic experimental investigation of linear complexion formation is extremely difficult and existing thermodynamic models require many parameters to be defined ahead of time, both slowing down the discovery of new types of linear complexions. Using atomistic simulations, we overcome such limitations and discover a variety of stable nanoscale-size structural and chemical states that are comprised of equilibrium and/or metastable phases confined to regions near dislocations. A wide variety of bcc and fcc metals are investigated. Depending on the considered system and applied conditions, these linear complexions maintain, partially modify, or replace the original defects that spawned them. Moreover, by considering different temperatures and compositions, we are able to construct “linear complexion phase diagrams” that are similar to real phase diagrams and define the necessary conditions for complexion formation.

9:20 AM  
Linking Strongest Grain Size to Underlying Deformation Mechanisms in Nanocrystalline Materials: Ankit Gupta1; Gregory Thompson2; Garritt Tucker1; 1Colorado School of Mines; 2University of Alabama
    The engineering interest in nanocrystalline (NC) materials stems from the potential to improve mechanical properties. In NC materials, grain boundaries (GB) act as a source of dislocation emissions that propagate via grains to accommodate strain. Several studies have established the presence of inverse hall patch regime in NC materials. In this regime, the governing deformation mechanism changes from dislocation based to GB sliding and migration. In this study, the strength of NC materials is studied as a function of grain size using atomistic simulations. The contribution of grains and GBs-based deformation mechanisms to the overall strain accommodation is quantified via continuum-based kinematic metrics. It is shown that NC materials exhibit a maximum in strength at a grain-size at which the strain is equally accommodated within the grain and grain-boundary deformation mechanisms. Finally, the potential to engineer materials with strongest grain size by tuning the underlying deformation mechanisms is discussed.

9:40 AM  
Computation of Embrittling Potencies of Sulfur for a Range of Nickel Pure Tilt Grain Boundaries via Atomistic Simulation Methods: Doruk Aksoy1; Rémi Dingreville2; Douglas E. Spearot1; 1University Of Florida; 2Sandia National Labs
    Segregation induced embrittlement of intrinsically ductile polycrystalline metals is not fully understood in terms of its contributing mechanisms. Several researchers have presented models to explain embrittlement behavior from energetic, chemical and structural points of view, via experiments and theoretical studies. One material system that is highly susceptible to embrittlement is the Ni-S binary system. Despite rigorous research in this material system, the role of the grain boundary structure is often overlooked. In this work, utilizing a newly developed embedded-atom method potential tailored for the S induced embrittlement of Ni, an extensive study of pure tilt grain boundaries is conducted to calculate the embrittling potencies associated with the segregation of S impurities. Furthermore, experimentally observed ordered and disordered grain boundary structures at high temperatures are analyzed using atomistic modeling techniques to categorize and characterize grain boundaries by their embrittling potencies and corresponding innate structural properties.

10:00 AM Break

10:20 AM  Invited
Integration between Modeling and Experiments at the Micron Scale: Laurent Capolungo1; Aaron Tallman1; Aaron Kohnert1; Darshan Bamney2; Douglas Spearot2; Ricardo Lebensohn1; 1Los Alamos National Laboratory; 2University of Florida
     Micron-scale and single crystal models in which line defects are represented explicitly, necessitate validation against experimental characterization. Further, these modeling techniques can also be used to complement characterization methods in order to quantify the dislocation content in deformed samples. Two such techniques are discussed: transmission electron microscopy (TEM) and diffraction line profile analysis (LPA).First, Discrete Dislocation Dynamics (DDD) is used to quantify microstructure evolutions due to sample preparation for TEM observations such as to correlate bulk deformed microstructure with those observed in TEM foils. By studying more than 100 configurations in five material systems, the differences between bulk microstructures and those observed in TEM foils are discussed from the statistical viewpoint. Second, the DDD is used to generate a database of diffraction patterns. More than 500 whole line diffraction profiles are generated. Using data analytics, a reduced order model is proposed to correlate dislocation content from diffraction profiles.

10:50 AM  Invited
Phase Field Dislocation Dynamics (PFDD) for Nanoscale Metals: Xiaoyao Peng1; Nithin Mathew2; Irene Beyerlein3; Kaushik Dayal1; Abigail Hunter2; 1Carnegie Mellon University; 2Los Alamos National Laboratory; 3University of California, Santa Barbara
    The purpose of this research is to investigate nanoscale deformation processes through the development and application of a 3D phase field dislocation dynamics (PFDD) model. The phase field approach is centered on energy minimization. The total system energy is comprised of several contributions that can depend on the problem of interest. Most common are elastic contributions that take into consideration dislocation-dislocation interactions, interactions with an applied stress, and a generalized stacking fault energy term. The latter term is used to model extended stacking faults and partial dislocations. In this case, the 3D PFDD model can be informed directly by atomistic simulations in order to incorporate a dependence on the entire material γ-surface. Additionally, this energy term can be used to account for the directional motion of dislocations in body-centered cubic (bcc) metals, where edge dislocations are easier to move than screw dislocations due to differences in the dislocation core structure.

11:20 AM  
Implementing Grain Boundaries in Phase-field Dislocation Dynamics: Tengfei Ma1; Pranay Chakraborty1; Lei Cao1; 1University of Nevada
     Grain boundaries are one of the most important microstructure features in crystalline materials, which determine the deformation process and mechanical properties. The incorporation of grain boundaries is essential for the predictive capability of materials modeling. Recent advances in implementing grain boundaries and partial dislocations in phase-field dislocation dynamics model are presented. The model can predict the nucleation of partial dislocations from grain boundaries and the formation of extended stacking faults lagging behind. Dislocation activities are found to be significantly changed by the average grain size and applied strain rate. The simulations also capture a jerky stress-strain response, which indicates dislocation avalanches when the external loading speed is slow and the grain size is large. In addition, statistical analysis of the avalanche events reveals that the dislocationavalanches are time and space correlated.

11:40 AM  
Growth of Twin Embryos by Disconnection Propagation in Mg: Molecular Dynamics and Continuum Modeling: Yang Hu1; Vladyslav Turlo1; Subhash Mahajan2; Irene Beyerlein3; Enrique Lavernia1; Julie Schoenung1; Timothy Rupert1; 1University Of California, Irvine; 2University Of California, Davis; 3University Of California, Santa Barbara
    Understanding twinning deformation physics is essential for improving the ductility of Mg and Mg alloys, yet twin embryo growth is often neglected because it is difficult to uncover the details of the rapid expansion process. Using molecular dynamics simulations, we observe the emission of twinning disconnections from the basal-prismatic interfaces. Based on our atomistic findings, we propose a phenomenological model for twin embryo growth driven by propagating disconnections and obtain analytical expressions for the evolution of twin size with time. Application to the simulation data allows for the extraction of the velocities of the twin tips and twinning disconnections. This model connects twin shape to applied stress and even makes predictions that are consistent with experimental data at lower stresses. As a whole, this work fills an important gap in the understanding of the twinning process by providing a clear description of the initial growth of the twin embryo.