Refractory high-entropy nanoalloys with exceptional high-temperature stability and enhanced sinterability

Abstract

Nanocrystalline alloys (nanoalloys) are prone to grain growth. It is known that grain boundary segregation and precipitation can stabilize nanoalloys, but the stabilization becomes less effective at high temperatures and adding grain growth inhibitors often reduces sinterability. Herein, we have simultaneously achieved exceptional high-temperature stability and improved sinterability for a class of TiNbMoTaW-based refractory high-entropy nanoalloys (RHENs). Bulk pellets of RHENs were fabricated through ball milling and spark plasma sintering, achieving 93–96% relative densities with 50–100 nm grain sizes for three compositions. For example, Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 sintered at 1300 °C attained ~ 96% relative density with ~ 55 nm mean grain size. Moreover, these RHENs exhibited exceptional stability at 1300 °C. Both Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 and Ti18.8Nb18.8Mo18.8Ta18.8W18.8Ni6 retained < 150 nm grain sizes after five hours annealing at 1300 °C. Notably, the addition of Ni, a well-known sintering aid for activated sintering of refractory metals such as W and Mo, in high-entropy TiNbMoTaW can promote sintering while maintaining high-temperature stability against rapid grain growth. This may be explained by hypothesized high-entropy grain boundary (HEGB) effects, while we recognize the possible (additional) effects of compositional inhomogeneity and secondary phase (Zener) pinning. These RHENs possess some of the highest temperature stability achieved for nanoalloys and ultrafine-grained metals.