Local Ordering in Materials and Its Impacts on Mechanical Behaviors, Radiation Damage, and Corrosion : Session I
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 8:30 AM
March 20, 2023
Room: Sapphire 411A
Location: Hilton

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


8:30 AM  Invited
Short Range Order and the Evolution of Deformation Mechanisms in Both High and Low Entropy Alloys: Andrew Minor1; 1University of California-Berkeley
    This talk will describe our recent results utilizing energy filtered diffraction, 4D-STEM and in situ TEM nanomechanical testing that provide insight into multiscale deformation phenomena in α-titanium and the CrCoNi medium entropy alloy. Using energy-filtered TEM and HRSTEM techniques it is possible to directly image, and therefore quantitatively assess, SRO and its effect on mechanical properties. In order to understand the effect of SRO in terms of the evolution of plasticity at different stages, the technique of 4D-STEM was used during in situ deformation and fracture experiments. In addition to using these techniques to investigate deformation in medium entropy alloys, this talk will also describe effects related to SRO in pure Ti (a very low entropy alloy) and Ti-Al alloys.

9:00 AM  
Assessing Local Order in Alloys Using Total Scattering, Pair Distribution Functions and Reverse Monte Carlo Simulations: Lewis Owen1; 1University of Sheffield
     Over the years, the assessment of local structure within metallic systems has been largely ignored, due to the lack of suitable probes capable of capturing and characterising these effects. Total scattering, whereby both Bragg and diffuse scattering are considered simultaneously, has been demonstrated as a key technique for exploring local structure in a host of interesting chemical systems (e.g functional oxides, metal-organic frameworks and network glasses). Recently, we demonstrated how it can be successfully applied to the study of local structure within metallic systems.In this talk, I will discuss the application of total scattering for the elucidation of local order within metallic systems. The nature of the experiments will be discussed, along with data processing, analysis (focussing on the reverse Monte Carlo algorithm) and numerical quantification of order present within a system.

9:20 AM  Invited
Ordering in Ti-V-Nb-Hf Refractory High-Entropy Alloys with Al Alloying: C. Tasan1; Shaolou Wei1; Michael Xu1; James Lebeau1; 1Massachusetts Institute of Technology
    Search for refractory high-entropy alloys (RHEAs) with optimal property combinations is one of the most active domains of alloy design research in the last decade. Ti-V-Nb-Hf system exhibits, even in the as cast condition, high tensile ductility, a rare behavior among investigated RHEAs without extensive thermo-mechanical processing. Further exploring the property design space of this particular RHEA system, we here discuss the strengthening effects due to Al alloying, by experimentally studying role of ordering, and plasticity mechanisms.

9:50 AM  
Comparing Short-range Ordering Behavior in Novel Austenitic Steels via Fluctuation Electron Microscopy Techniques: Po-Cheng Kung1; Jian-Min Zuo1; Toshihiro Tsuchiyama2; Brian Somerday1; Petros Sofronis1; Jessica Krogstad1; 1University of Illinois at Urbana-Champaign; 2Kyushu University
    Detecting and identifying heterogeneous SRO domains via highly localized Scanning Electron Nano-Beam Diffraction (SEND), or 4D-STEM, is more readily facilitated by the addition of the Fluctuation Electron Microscopy (FEM) method. Because the heterogeneous SRO domains are embedded in a homogeneous matrix, the calculated normalized variance of weak diffraction intensity in the series of diffraction patterns can be used to differentiate signals arising from small domains from those that stem from the matrix. In the current study we will compare the short range ordering behavior in one commercial, and four novel austenitic stainless steel compositions. While the averaged diffraction patterns are nearly identical for all alloys, the variance maps reveal the changes in heterogeneous SRO diffraction from alloy to alloy, which are further exacerbated in the presence of hydrogen. These differences hint at how SRO structure and morphology are modified by minor adjustments to alloy composition and thermal/environmental history.

10:10 AM Break

10:25 AM  Invited
Solute-strengthening in Alloys with Short-range Order: William Curtin1; Shankha Nag2; Xin Liu1; 1Epfl Sti Igm Lammm; 2TU Darmstadt
    The yield strength of high entropy alloys is primarily controlled by solute-dislocation interactions that are dominated by solute misfit volume interactions with the dislocation pressure field. Solute-solute interactions often play a secondary role. We present an extended theory that includes strengthening due to short-range order (SRO). The theory reveals that SRO affects the strengthening of the misfit interactions, in addition to the average strengthening due to destruction of the SRO by slip, and can lead to a reduction in strength. We validate this result in model BCC Mo-Nb-Ta-W alloys by computing the energy barriers for edge dislocations using the NEB method for alloys with controlled levels of SRO, confirming that SRO can increase or decrease the energy barriers and strengthening due to misfit interactions. The misfit strengthening due to SRO depends only on the SRO parameters and so can be assessed without direct knowledge of solute-solute interactions.

10:55 AM  
Dislocation Motions in Refractory High-entropy Alloys and Effects of Chemical Order and Disorder: Xinyi Wang1; Francesco Maresca2; Penghui Cao1; 1University of California, Irvine; 2Engineering and Technology Institute Groningen, Faculty of Science and Engineering, University of Groningen
    The origin of extraordinary mechanical properties in body-centered cubic refractory high entropy alloys (HEAs) can be traced back to dislocation motion in concentrated solid solutions because the locally high chemical fluctuations in HEAs inevitably influence its motion mechanisms. In this talk, we will present the atomistic mechanisms and the associated energy barriers governing screw and edge dislocation motion in bcc refractory HEAs using a set of atomistic simulation techniques. The screw dislocation migration pathway reveals a hierarchical and multilevel structure potential energy landscape with a collection of small basins nested in large metabasin. By introducing chemical short-range order (SRO), the energy barriers for individual mechanisms all increase but to different extents. Finally, we will discuss the effect of SRO on edge dislocation migration barrier, and our preliminary results indicate a possible SRO-strengthening to softening crossover, depending on temperature and strain rate.

11:15 AM  Invited
Neural-network Based Atomistic Simulation on Chemical Order Formation Kinetics in Medium Entropy Alloys: Shigenobu Ogata1; Jun-ping Du1; 1Osaka University
    We studied the chemical ordering in CrCoNi medium-entropy-alloys (MEA) using a large-scale atomic model with a neural network potential (NNP) trained by density functional theory and molecular dynamics/Monte-Carlo (MD/MC) hybrid simulated annealing method. The MD/MC simulations from high to low temperatures show a chemical ordering transition at 800 K from the chemical short-range order to the superlattice ordering consisting of Cr(110)/CoNi and Cr(100)/CoNi nano-sized superlattice domains. In addition, the kinetics of chemical ordering evolution through vacancy diffusion is studied using the NNP with kinetic Monte-Carlo (kMC) simulation method combined with a novel neural network technique of fast activation energy evaluation. The kMC simulations at various temperatures lead to a diagram of Time-Temperature-Chemical ordering (TTC) relation, which can be used to tune the degrees of chemical ordering by controlling annealing temperature and time.

11:45 AM  
Structural Transitioning in Near Boundary Segregation Zones due to Chemical Ordering in NbMoTaW: Doruk Aksoy1; Megan McCarthy2; Ian Geiger1; Timothy Rupert1; 1University of California, Irvine; 2Sandia National Laboratories
    Chemical ordering affects microstructural stability in environments where several elements compete for the same atomic sites. Moreover, interfaces introduce additional instability through increased variation of local atomic environments. In this work, the effect of interface structure on chemical ordering is investigated in polycrystalline NbMoTaW using atomistic modeling. Hybrid Monte Carlo/molecular dynamics simulations are performed to obtain the segregation behavior and chemical ordering at and near boundaries by taking slices at different radial increments from the interface. Our results suggest that the transition regions between interface and inner grain require special consideration, because of the structural transitioning associated with the predominant structure at the interface. For instance, the A2-B2 transition occurs within these extended segregation zones, and zone width is dependent on grain size, temperature, and interface character. A better understanding of this transition could provide new avenues for complex concentrated alloy design and microstructure engineering.