UCLA

Refractory multi-element alloys (RMEA) with body-centered cubic (bcc) structure have been the object of much research over the last decade due to their high potential as candidate structural materials for applications in harsh environments such as power plants, aerospace industry, or nuclear sectors. However, theories are yet to be developed to fully explain their exceptional strength at elevated temperatures, as well as to either verify or refute whether they are as irradiation-tolerant as they are considered to be. A a good starting point to dig in is that macroscopic irradiation behavior of the alloys is known to connected with properties at atomic-level through the roles of point defects, i.e., self-interstitial atoms (SIA) and vacancies. As for high-temperature strength, it is hypothesized that global chemical fluctuations in these complex alloys are major causes of strengthening through interactions with screw dislocation that significantly alter dislocation energetics and mechanisms which are known to control the plasticity of classical bcc metals and dilute alloys.

Among all members of the RMEA family, the equiatomic Nb-Mo-Ta-W system has become a model alloy of the RMEA group due to its phase stability, microstructural simplicity, and mixture of elements with high lattice distortion, making it an ideal experimental and computational test bed to study fundamental behavior. Therefore, to explore the above points and implement the modern theories, we use atomistic simulations to study the properties vacancies as well as self-interstitial atoms in the quaternary equiatomic Nb-Mo-Ta-W refractory alloy. To investigate the bcc RMEA plasticity, we develop a kinetic Monte Carlo (kMC) model to simulate screw dislocation motion in RMEAs and perform simulations to study the relationship between dislocation mechanisms and alloy strength and quantify the contribution to the total strength due to screw dislocation.

In our investigation of vacancy properties in Nb-Mo-Ta-W, we calculate their energetics in the equiatomic Nb-Mo-Ta-W alloy, especially vacancy formation and migration energies, using molecular statics calculations based on a spectral neighbor analysis potential specifically developed for Nb-Mo-Ta-W. We consider vacancy properties in bulk environments including the effect of short-range order (SRO) by preparing supercells through Metropolis Monte-Carlo relaxations, and temperature on the calculation. The nudged elastic band (NEB) method is applied to study vacancy migration energies. Our results show that both vacancy formation energies and vacancy migration energies are statistically distributed with a wide spread, on the order of 1.0 eV in some cases, and display a noticeable dependence on SRO. Moreover, the large spread in vacancy formation energies results in an asymmetric thermal sampling of the formation energy distribution towards lower values. This gives rise to effective vacancy formation energies that are noticeably lower than the distribution averages. We study the effect that this phenomenon has on the vacancy diffusivity in the alloy and discuss the implications of our findings on the structural features of Nb-Mo-Ta-W.

In our study of SIA properties in Nb-Mo-Ta-W, it is found that the <111> orientation to be the most common among all split configurations. Chemically, these SIA defects adopt a variety of structures involving all pairs of atoms, including --surprisingly-- a relatively high occurrence of octahedral SIA. In terms of their diffusivities, we find two clearly distinguished regimes at and below 600 K and above it, where the SIA diffusion changes dimensionality from 1D to 3D. We calculate the migration energies and diffusion pre-factors in both regions, from which we extract the translational and rotational components of the defect migration. We find values of 0.25 eV and pre-factors of ∼10^−11 m^2·s^−1 in the low temperature regime,and 0.57 eV and ∼10^−8 m^2·s^−1 in the high temperature one, and estimate the rotational energy barrier at 0.37 eV

Our simulations of screw dislocation kinetics indicate, in agreement with molecular dynamics simulations, that chemical energy fluctuations along the dislocation line lead to measurable concentrations of kinks in equilibrium in a wide temperature range. A fraction of these form cross-kink configurations, which are ultimately found to control screw dislocation motion and material strength. It is found that the self-pinning stress remains even at high temperatures due to the balance of two competing effects: strengthening due to higher concentrations of kinks on multiple glide planes, and softening associated with the thermal dissolution of cross-kinks.