An official website of the United States government
Abstract Entropic stabilization has evolved into a strategy to create new oxide materials and realize novel functional properties engineered through the alloy composition. Achieving an atomistic understanding of these properties to enable their design, however, has been challenging due to the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calculations and linear regression algorithms to investigate the role of local configurational and structural disorder on the thermodynamics of vacancy formation in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the electrical properties. We find that the cation-vacancy formation energies decrease with increasing local tensile strain caused by the deviation of the bond lengths in ESOs from the equilibrium bond length in the binary oxides. The oxygen-vacancy formation strongly depends on structural distortions associated with the local configuration of chemical species. Vacancies in ESOs exhibit deep thermodynamic transition levels that inhibit electrical conduction. By applying the charge-neutrality condition, we determine that the equilibrium concentrations of both oxygen and cation vacancies increase with increasing Cu mole fraction. Our results demonstrate that tuning the local chemistry and associated structural distortions by varying alloy composition acts an engineering principle that enables controlled defect formation in multi-component alloys.
more »
« less
This content will become publicly available on July 28, 2025
The thermodynamic effects of solute on void nucleation in Mg alloys
Replica exchange transition interface sampling simulations in Mg–Al alloys with high vacancy concentrations indicate that the presence of a solute reduces thermodynamic barriers to the clustering of vacancies and the formation of voids. The emergence of local minima in the free energy along the reaction coordinate suggests that void formation may become a multi-step process in the presence of a solute. In this scenario, vacancies agglomerate with solute before they coalesce into a stable void with well-defined internal surfaces. The emergence of vacancy–solute clusters as intermediate states would imply that classical nucleation theory is unlikely to adequately describe void formation in alloys at high vacancy concentrations, a likely precursor for alloy strengthening through nanoscale precipitation.
more »
« less
- Award ID(s):
- 2320355
- PAR ID:
- 10528622
- Publisher / Repository:
- AIP
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 161
- Issue:
- 4
- ISSN:
- 0021-9606
- Page Range / eLocation ID:
- 044509
- Subject(s) / Keyword(s):
- Free energy landscapes, Molecular dynamics, Monte Carlo methods, Potential energy surfaces, Metallurgy, Crystallographic defects, Alloys, Reaction rate constants
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Additions of solute that trap vacancies slow down vacancy diffusion and promote point-defect recombination in alloys subjected to irradiation. Such selective alloying can thus help to minimize the detrimental consequences resulting from point defect fluxes. The current work investigates the effect of solute additions on the recombi- nation rate using kinetic Monte Carlo simulations for a model alloy system, which was parametrized to Cu-Ag in the dilute limit, but with an increased solubility limit, ≈0.86 at.% at 300 K. As the solute concentration was increased above 0.1 at.%, solute clustering was observed and led to a strong increase in recombination rate. The beneficial effects of solute clustering on reducing vacancy mobility, and reducing solute drag, were analyzed by calculating relevant transport coefficients using the KineCluE code (Schuler et al., Computational Materials Science (2020) 172,109,191). Moreover, it was observed in the KMC simulations that large recombination rates resulted in a shift of steady-state distributions of solute cluster sizes to smaller clusters compared to equilibrium distributions in the solid solution. This shift is rationalized as resulting from the irreversible character of the interstitial-vacancy recombination reaction. These results suggest a novel irradiation effect on phase stability where a high recombination rate increases the solubility limit of a solute at steady state over its equilibrium value.more » « less
-
We describe the triboluminescence response of undoped (BaAl2Si2O8, h–BAS) and Eu-doped (h–BAS:Eu) barium hexacelsian powders and show that the triboluminescence behavior is dependent on the formation of barium vacancies. X-ray photoelectron spectroscopy of the h–BAS:Eu powders confirms the presence of Eu3+ and Eu2+ in the compound, leading to the formation of significant vacancy point defects in excess of those found in h–BAS as a result of the charge imbalance caused by the substitution of Eu3+ in Ba2+ sites. From electron paramagnetic resonance measurements and density functional theory (DFT) calculations, we demonstrate that the vacancy defects correspond to singly ionized barium vacancies. DFT-calculated thermodynamic transitions and electronic structure calculations reveal deep energy levels within the compound’s energy band gap, with a strong emission at 3.33 eV correlated to an electron exchange between the conduction band minimum and a barium vacancy center. Time-resolved triboluminescence spectra show that the increased concentration of barium vacancies in h–BAS:Eu enhances the signal by about 75% compared to the signal from h–BAS. These results play an important role in the understanding of fundamental mechanisms behind the triboluminescence response of ceramic materials as well as the role of different types of defects in this process.more » « less
-
Abstract This study employs a high-fidelity numerical framework to determine the plastic material flow patterns and temperature distributions that lead to void formation during friction stir welding (FSW), and to relate the void morphologies to the underlying alloy material properties and process conditions. Three aluminum alloys, viz., 6061-T6, 7075-T6, and 5053-H18, were investigated under varying traverse speeds. The choice of aluminum alloys enables the investigation of a wide range of thermal and mechanical properties. The numerical simulations were validated using experimental observations of void morphologies in these three alloys. Temperatures, plastic strain rates, and material flow patterns are considered. The key results from this study are as follows: (1) the predicted stir zone and void morphology are in good agreement with the experimental observations, (2) the temperature and plastic strain rate maps in the steady-state process conditions show a strong dependency on the alloy type and traverse speeds, (3) the material velocity contours provide a good insight into the material flow in the stir zone for the FSW process conditions that result in voids as well as those that do not result in voids. The numerical model and the ensuing parametric studies presented in this study provide a framework for understanding material flow under different process conditions in aluminum alloys and potentially in other alloys. Furthermore, the utility of the numerical model for making quantitative predictions and investigating different process parameters to reduce void formation is demonstrated.more » « less
-
null (Ed.)The Kitaev honeycomb model has attracted significant attention due to its exactly solvable spin-liquid ground state with fractionalized Majorana excitations and its possible materialization in magnetic Mott insulators with strong spin-orbit couplings. Recently, the 5d-electron compound H3LiIr2O6 has shown to be a strong candidate for Kitaev physics considering the absence of any signs of a long-range ordered magnetic state. In this work, we demonstrate that a finite density of random vacancies in the Kitaev model gives rise to a striking pileup of low-energy Majorana eigenmodes and reproduces the apparent power-law upturn in the specific heat measurements of H3LiIr2O6. Physically, the vacancies can originate from various sources such as missing magnetic moments or the presence of nonmagnetic impurities (true vacancies), or from local weak couplings of magnetic moments due to strong but rare bond randomness (quasivacancies). We show numerically that the vacancy effect is readily detectable even at low vacancy concentrations and that it is not very sensitive either to the nature of vacancies or to different flux backgrounds. We also study the response of the site-diluted Kitaev spin liquid to the three-spin interaction term, which breaks time-reversal symmetry and imitates an external magnetic field. We propose a field-induced flux-sector transition where the ground state becomes flux-free for larger fields, resulting in a clear suppression of the low-temperature specific heat. Finally, we discuss the effect of dangling Majorana fermions in the case of true vacancies and show that their coupling to an applied magnetic field via the Zeeman interaction can also account for the scaling behavior in the high-field limit observed in H3LiIr2O6.more » « less
