Grain Boundaries and Interfaces: Metastability, Disorder, and Non-Equilibrium Behavior: Alloying, Solute Segregation, and Precipitation: Part II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Computational Materials Science and Engineering Committee, TMS: Chemistry and Physics of Materials Committee, TMS: Phase Transformations Committee
Program Organizers: Yue Fan, University of Michigan; Liang Qi, University of Michigan; Jeremy Mason, University of California, Davis; Garritt Tucker, Colorado School of Mines; Pascal Bellon, University of Illinois at Urbana-Champaign; Mitra Taheri, Johns Hopkins University; Eric Homer, Brigham Young University; Xiaofeng Qian, Texas A&M University
Wednesday 2:00 PM
March 2, 2022
Location: Anaheim Convention Center
Session Chair: Yue Fan, University of Michigan, Ann Arbor; Xiaofeng Qian, Texas A&M University
2:00 PM Invited
Disordered Interfacial Features as Local Equilibrium States Capable of Modifying Nanocrystalline Metals: Timothy Rupert1; 1University of California, Irvine
To play devil’s advocate with title of this symposium, we show here that grain boundaries naturally prefer disordered states and that this behavior can be emphasized with proper alloy design. Nanocrystalline metals demonstrate this principle, as we explore the usage of structurally-disordered features to improve these materials. Cu-rich alloys with varying degrees of chemical complexity at the boundary are used to study the stabilization of amorphous complexions. We isolate both the local descriptors of these features, including short range order and chemical heterogeneities, as well as the kinetics associated with ordered-disordered complexion transitions. Al-rich alloys are then used to show how disordered features can be incorporated into a hierarchical nanostructure. Both amorphous grain boundary complexions and elongated nanoscale precipitates with disordered interiors are observed, with disorder again tied to improved mechanical properties. As a whole, this work shows that disorder can be an opportunity.
The Role of Grain Boundary Metastability in Solute Segregation: Insights from Atomistic and Machine Learning Studies: Fadi Abdeljawad1; Yasir Mahmood1; Enrique Martinez1; 1Clemson University
Even minute amounts of elemental species at GBs greatly influence boundary dynamics. The challenge in describing the migration of doped GBs lies in the fact that GBs exhibit a plethora of possible atomic configurations, i.e., GB metastable states. While GB solute segregation is an area of active research, most studies assume equilibrium boundary structures; these are the lowest energy GB configurations. In this work, we investigate the role of GB metastability in solute segregation and boundary migration in a model Al-Mg system. Metastable structures for several  and  tilt GBs in Al are generated. Then, atomistic simulations are used to quantify the impact of boundary metastability and site dependency on Mg segregation. Gaussian process modeling is used to derive a predictive model relating the segregation energy to the local atomic environment, where GB metastability is treated as a source of variability.
Exploring the Structure and Chemistry Contributions to Interfacial Segregation in NbMoTaW with High-throughput Atomistic Simulations: Ian Geiger1; Timothy Rupert1; 1University of California Irvine
The chemical complexity of multi-principal element alloys makes interfacial segregation difficult to predict. In this study, we use atomistic simulations of bicrystals to sample a variety of interfacial sites in NbMoTaW and explore emergent trends in local interfacial behavior. We study hundreds of bicrystals, using equilibrium and metastable configurations from pure materials as templates for subsequent doping, and study tens of thousands of grain boundary sites, to isolate a variety of local states. General segregation trends are reported, but more importantly, deviations from the mean behavior signal situations where local structural and chemical driving forces lead to interesting behavior. For example, incomplete depletion of Ta and W at low angle boundaries provides evidence of chemical pinning of these elements due to B2 ordering at low temperatures. Overall, this work highlights the complex interplay between local boundary structure and chemical short range ordering in multi-principal element alloys.
Using Grain Boundary Segregation Spectra to Design for Nanocrystalline Stability: Malik Wagih1; Christopher Schuh1; 1Massachusetts Institute of Technology
The segregation of solute atoms at grain boundaries (GBs) is a key route to thermodynamically stabilize nanocrystalline alloys. To date, the standard approach to design and screen for nanocrystalline stability uses a simplified representation that treats the GB network as a “single” entity, with an “averaged” value to quantify the tendency of solute atoms to segregate at the GB i.e. an “averaged” segregation energy. This simplification, however, ignores the variety of local atomic environments or site-types available at the GB in polycrystals, which results in a spectrum of segregation energies. In this talk, we remove this simplification, and outline enthalpic screening criteria to design for nanocrystalline stability, which takes into account the spectrality of the GB network. We proceed to apply the developed criteria to screen over 200 binary alloys for nanocrystalline stability, using segregation spectra obtained from our recently developed machine learning framework, which we briefly discuss as well.
3:30 PM Break
3:45 PM Invited
Unraveling Mechanisms of Interface Diffusion and Interfacial Creep in Metals and Metal-ceramic Composites: Ian Chesser1; Raj Koju1; Yuri Mishin1; 1George Mason University
Interface diffusion and interfacial slip are two key processes controlling the high-temperature creep deformation of metal-matrix composites. Fundamental understanding of these processes remains poor. Atomistic simulations are a promising route toward understanding, but reliable methodology for measuring kinetic coefficients and disentangling competing mechanisms has not been developed in this context. In this work, we compare interface diffusion and sliding coefficients in pure Al and Si grain boundaries to those in AlSi phase boundaries using direct molecular dynamic simulations at high and intermediate homologous temperatures. Interface diffusion and interface slip are shown to be distinct, but correlated processes with characteristic fractal dimensions, cluster size distributions, and nucleation and growth dynamics. Diffusion and sliding mechanisms are analyzed over a large parameter space including multiple interface geometries, temperatures and strain rates.
Solute Drag in Regular Solution Alloys: Self-similarity and the Role of Grain Boundary Structure: Fadi Abdeljawad1; Malek Alkayyali1; 1Clemson University
Grain boundary (GB) solute segregation has been experimentally shown to mitigate grain growth and thermally stabilize the grain structures of nanocrystalline (NC) alloys. However, most studies are focused on the thermodynamic aspect of solute segregation, and the role of solute drag remains poorly understood. Herein, we develop a sharp-interface solute drag model of regular solution NC alloys, which explicitly incorporates solute-solute interactions in both the bulk grains and GBs. To account for the GB structure, an indicator function is used to describe solute-GB interactions within the GB region, which is found to capture effects such as monolayer, multilayer, and nonsymmetrical segregation. GB solute drag is shown to exhibit a self-similar behavior, where the peak solute drag scales with the GB heat of mixing. A universal solute drag-velocity relation is proposed that can be used in screening the alloy design space for the most stable NC systems.
Segregation-assisted Concentration Modulation within Grain Boundaries: Longsheng Feng1; Di Qiu2; Kamal Kadirvel1; Pengyang Zhao3; Suliman Dregia1; Yunzhi Wang1; 1The Ohio State University; 2Shanghai University; 3Shanghai Jiao Tong University
Recent experimental observations found spinodal-like concentration modulation in grain boundary (GB) planes. We propose different pathways that could be responsible for achieving such concentration modulation within the GB, including those mediated by intrinsic heterogeneity of GB structures, some transient states during segregation transition at GBs, and two-step aging heat treatment. The intrinsic periodicity of GB structure is modeled by phase field simulations at nanometer scale, and the segregation pattern is calculated through a 3-dimensional segregation isotherm. The thermodynamics and kinetics of GB segregation and segregation transition are also studied through phase field simulations and the possibilities of true spinodal decomposition at GB are discussed. The work is supported by NSF DMREF program and AFOSR FA9550-20-1-0015.
Machine Learning-assisted Prediction of Interfacial Segregation in a Refractory Multi-principal Element Alloy: Doruk Aksoy1; Timothy Rupert1; 1University of California, Irvine
The evolution of models explaining segregation behavior of solutes continues at a rapid pace. A complexity that requires elucidation is the co-segregation behavior in multi-principal element alloys (MPEAs). Co-segregation depends not only on the chemical ordering in the MPEA, but also on the interface structure and its evolution during the segregation process. In this work, we categorize solute-solute interactions using dimensionality reduction techniques for a NbMoTaW refractory MPEA polycrystal. We investigate both the dilute limit and chemically complex environments for the solutes. The evolution between these states is also studied via hybrid Monte Carlo-Molecular Dynamics simulations. Finally, we utilize all the segregation energies and local atomic environment vectors and feed them into an artificial neural network algorithm to predict segregation behavior in the complex alloy. The diversity of structural and chemical motifs present in the analysis further validates the complexity of the problem and assist the interfacial design of MPEAs.