Local Ordering in Materials and Its Impacts on Mechanical Behaviors, Radiation Damage, and Corrosion : Session II
Sponsored by: TMS Structural Materials Division, TMS: Chemistry and Physics of Materials Committee, TMS: Nuclear Materials Committee, TMS: Corrosion and Environmental Effects Committee
Program Organizers: Penghui Cao, University of California, Irvine; Yang Yang, Pennsylvania State University; Fadi Abdeljawad, Lehigh University; Irene Beyerlein, University of California, Santa Barbara; Enrique Lavernia, University of California, Irvine; Robert Ritchie, University of California, Berkeley

Monday 2:00 PM
March 20, 2023
Room: Sapphire 411A
Location: Hilton

Session Chair: Irene Beyerlein, University of California, Santa Barbara; Yang Yang, The Pennsylvania State University; Robert Ritchie, University of California, Berkeley; Penghui Cao, University of California, Irvine


2:00 PM  Invited
Multi-scale Investigation of Chemical Short-range Order and Dislocation Glide in the MoNbTi and TaNbTi Refractory Multi-principal Element Alloys: Shyue Ping Ong1; Hui Zheng1; Lauren Fey2; Xiang-Guo Li1; Yong-Jie Hu3; Liang Qi4; Shuozhi Xu2; Irene Beyerlein2; 1University of California, San Diego; 2University of California, Santa Barbara; 3Drexel University; 4University of Michigan, Ann Arbor
    Refractory multi-principal element alloys (RMPEAs) are promising materials for high-temperature structural applications. In this talk, I will discuss the role of chemical short-range ordering (CSRO) on dislocation glide in two RMPEAs --- TaNbTi and MoNbTi --- using a multi-scale modeling approach. We show that MoNbTi exhibits a much greater degree of SRO than TaNbTi and the local composition has a direct effect on the unstable stacking fault energies. From mesoscale phase-field dislocation dynamics simulations, we find that the gliding dislocations experience significant hardening due to pinning and depinning caused by random compositional fluctuations, with higher SRO decreasing the degree of USFE dispersion and hence, amount of hardening. Finally, we show how the morphology of an expanding dislocation loop is affected by the applied stress, with higher SRO requiring higher applied stresses to achieve smooth screw dislocation glide.

2:30 PM  
Symmetry Considerations for Ordering in High-entropy Alloys : Flynn Walsh1; Robert Ritchie1; Mark Asta1; 1Lawrence Berkeley National Laboratory
    The formation of local chemical order in high-entropy alloys is examined from the perspective of the mean-field static concentration wave formalism. Motivating the approach, existing theory for binary alloys is used to explain experimental observations regarding the pseudo-binary ordering of the CrCoNi medium-entropy alloy. In particular, an effort is made to distinguish among the the types of chemical order that give rise to diffuse intensities in diffraction experiments, i.e. incipient long-range order, short-range order representable by continuously forming concentration waves, and nominally unfavorable ordering motifs arising from geometric considerations. The theory is then more rigorously extended to many-component alloys and applied to well studied systems, including the fcc Cantor alloy and bcc refractory high-entropy alloys.

2:50 PM  
Short Range Order Prediction in High Throughput from First Principles: A Descriptor-based Screening Process: Nathan Smith1; Chris Wolverton1; 1Northwestern University
    Multi-principal element alloys (MPEAs) represent a vast compositional space with exciting prospects for improved performance over traditional alloys. Computational studies in the literature have made efforts to predict single-phase stability of MPEAs in high throughput; however, most work on short range order (SRO) has been limited to select alloy systems and has employed methods, like the cluster expansion, which do not easily lend themselves to high throughput study. We here present a descriptor-based approach to determine qualitatively the sign of the Warren-Cowley SRO parameter from simpler density functional theory calculations. This method compares three key energies: (1) the ordered state, readily available via databases such as the OQMD, (2) the disordered state, approximated from special quasi-random structures, and (3) the energy of coherent phase separation, obtainable from small unit cell calculations. We investigate the relationship between SRO and corrosion resistance via the creation of complex diffusion pathways for passivating elements.

3:10 PM  Invited
Strengthening Mechanisms In Refractory High Entropy Alloys Based on Athermal Atomic-sized Dislocation Imperfections: Jaime Marian1; 1University of California, Los Angeles
    Refractory high entropy alloys (RHEA) are promising candidates as high-temperature structural materials due to an outstanding combination of strength, fracture toughness and creep resistance. However, while certain RHEA have been seen to retain strengths of over 500 MPa well above 1000C, the fundamental causes behind this display of high-temperature strength are still under active investigation. Here we present a theoretical and computational model based on the presence of atomic-level defects as the controlling elements of the alloy plastic response. The model solves the elastic/kinetic problem using dislocation dynamics within a kinetic Monte Carlo time evolution scheme, and captures the role played by screw and edge dislocations. We focus on one of the most popular quaternary systems, the equiatomic Nb-Mo-Ta-W alloy, described by suitable atomistic potentials, and from which we obtain all model parameters. Using the fully-parameterized model, we show very good agreement between model predictions and experimental yield-strength measurements

3:40 PM Break

3:55 PM  Invited
Local and Short-range Chemical Order in High/Medium Entropy Alloys: Evan Ma1; 1Xi'an Jiaotong University, China
    Among the local chemical order (LCO) that can develop in H/MEAs, chemical short-range order (CSRO) is arguably the most difficult to decipher, because its convincing identification demands not only irrefutable diffraction evidence, but also intricate chemical information on sub-nanometer length scale regarding the different preferences of the constituent species to occupy certain lattice planes/sites in the first and second nearest-neighbor atomic shell(s). Here we discover that under an appropriate zone axis, micro/nano beam diffraction, atomic-resolution imaging and chemical mapping in transmission electron microscope can reveal CSRO in face-centered-cubic VCoNi and CrCoNi MEAs, including its spatial extent, atomic packing configuration and preferential lattice occupancy by the species. Atomic strain mapping demonstrates the interactions of the CSROs with dislocations. See X. Chen et al., Nature 592 (2021) 712-716, and L. Zhou et al., Acta Mater. 224 (2022) 117490.

4:25 PM  
Computational and Characterization Methods to Quantify Chemical Ordering in Compositionally Complex Alloys: Annie Barnett1; Daniel Foley1; Elaf Anber1; Yevgeny Shlafstein1; Alejandro Perez2; Partha Das2; Douglas Spearot3; Irene Beyerlein4; Michael Falk1; Mitra Taheri1; 1Johns Hopkins University; 2NanoMEGAS SPRL; 3University of Florida; 4University of California Santa Barbara
    Non-uniform, compositionally complex alloy (CCA) landscapes are challenging to fully characterize. Difficulty arises in confirming ordering mechanisms, in quantitatively describing order, and in rigorously relating such order to experimental measurement of reciprocal space lattices patterns. Precession Electron Diffraction (PED) can extract quantitative ordering information in metallic systems, including slight changes in ordering that occur at high spatial resolution, but a need remains for more evidence of local behavior. Hybrid Monte-Carlo/Molecular Dynamics simulations were performed to generate experimentally comparable models to compliment conclusions about the ordering signatures previously found using PED. Short-range order (SRO) parameters are well-defined mathematically but do not directly provide any physical interpretation. Such parameters benchmark the chemical state of systems generated both experimentally and computationally. Modeling results, PED patterns, and SRO parameters illustrate the benefits of a combined methodology to facilitate quantification and characterization of the local nature of chemical ordering in complex metallic systems.

4:45 PM  
Observation of Fe and Mn Chemical Ordering in High Mn Steels by Neutron Diffraction: Lawrence Cho1; Pawan Kathayat1; Yuran Kong1; John Speer1; Kip Findley1; Donald Brown2; Bjorn Clausen2; Sven Vogel2; Samantha Lawrence2; 1Colorado School of Mines; 2Los Alamos National Laboratory
    While many austenitic alloys exhibit long-range-ordered phases, e.g. L12 and L’12, Fe-Mn alloys have long been known as chemically disordered solid solutions. Recent studies on Fe-Mn-C alloys have suggested the existence of short-range ordering, but the experimental evidence of ordering reactions in high Mn steels is still lacking in literature. In this study, we investigated ordering reactions in three high Mn (30 wt%) austenitic steels through neutron diffraction, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The neutron diffraction measurements detected, in addition to austenite peaks, extra peaks, which are interpreted to be associated with an L12-type superlattice, i.e. chemical ordering of Fe and Mn. The possible superlattice reflections were not detected in the TEM/XRD analyses because X-ray (or electron) scattering factors for Fe and Mn are similar. In contrast, Fe and Mn have significantly different scattering lengths for neutron diffraction, which explains the observation of the potential superlattice reflections.