Mechanical and thermal properties of high/medium entropy materials

Abstract

In recent years, high/medium entropy materials have attracted increasing attention due to their high performance of strength and radiation resistance. However, due to the complexity of the atomic environment, the formation and migration behaviors of defects, as well as the strengthening mechanisms for traditional metals and alloys are not applicable to these materials. Therefore, understanding the underlying mechanisms of these materials is crucial for further optimizing and improving their performance characteristics. Although some recent theories have been proposed to reveal the unique principles governing these materials, they are still to be further developed. In this thesis, we first focus on the formation and migration energies of vacancies and interstitials in a body-centered cubic high entropy alloy using the first principles theory. Our results demonstrate that the average formation energies of point defects are higher than those in corresponding pure metallic systems. By analyzing the atomic environment, we observe that vacancies in VCrMnFe0.33 tend to migrate to lattice sites with more V atoms as neighbors, while interstitials prefer to form [110] dumbbells with Mn atoms. Next, we investigate the effects of local chemical ordering on the dislocation morphology, local stacking fault energy, and strength characteristics of the VCrTaW high entropy alloy. Dislocations are observed to exhibit curvature due to the presence of local pinning sites with relatively high local fault energies. Although the local chemical ordering becomes increasingly pronounced at low annealing temperatures, the diffuse antiphase boundary energy diminishes to zero after approximately five successive glides. Performing a strength analysis by considering factors such as the Peierls stress, generalized fault energy, and fault energy fluctuation-induced strength reveals that the strength contribution from the generalized fault energy τ¯γ increases as the local chemical ordering increases. Conversely, the local fault energy fluctuations in the ordering samples lead to less wavy dislocations and consequently contribute less to the overall strength than that in the random sample. The interplay between these two factors determines the final strengthening or softening effects of local chemical ordering. Furthermore, we present a novel manifestation of entropy in medium entropy alloys by reimaging the statistical distributions of the dislocation line tension energies. In the random VCoNi alloy sample, we find that the self-energy, u, fits well with the exponential distribution of βMe−βM, which maximizes the system entropy. However, in samples with local chemical ordering, the dislocation self-energy deviates from the maximum entropy distribution due to the changes in the atomic environment caused by annealing. The average energy < u > decreases after the introduction of local chemical ordering, as does the dislocation waviness. Moreover, this theory is also applied to different alloy systems and high temperatures, and in both cases, we observe good agreement with our findings. In addition, we study the effects of doping and local chemical ordering on thermal transport in an entropy-regulated thermoelectric material PbSe0.5Te0.25S0.25. The lattice thermal conductivity of PbSe decreases from 1.87 Wm−1K−1 to 0.76 Wm−1K−1 for PbSe0.5Te0.25S0.25. Moreover, our findings reveal that local chemical ordering can enhance thermal transport by 14%, primarily due to the reduced phonon scattering in the frequency range of 0-2 THz.