Hierarchical nanostructure stabilizing high content coherent nanoprecipitates in Al-Cr-Fe-Ni-V high-entropy alloy

Abstract

High-density coherent nanoprecipitates have been widely introduced into the design of new structural materials to achieve a superior strength-ductility balance. However, the thermal instability of nanostructures limits their fabrication and application. In this study, we investigated the temporal evolution of nanoprecipitates in coherent nanoprecipitation-strengthened Al0.5Cr0.9FeNi2.5V0.2 high-entropy alloy during isothermal aging. When annealed at 600 °C for more than 100 h, we found that its nanoprecipitates were invariably stable, with no obvious changes occurring in terms of morphology and distribution. The excellent stability was mainly attributed to the restricted state of interface migration and diffusion owing to the hierarchical nanostructure. The Cr-enriched nano-lamellar BCC phase divided the Cr-depleted FCC(L12) matrix, forming barriers to long-range diffusion and resulting in a kinetically slow coarsening rate. As the nano-lamellar BCC phase spheroidized as the aging temperature increased to 700 °C, the diffusion barriers were destroyed. Remarkable coarsening occurred after that, which further verified the significant effect of the nano-lamellar BCC phase on the microstructural stability. These results provide a paradigm for designing alloys stabilized via hierarchical nanostructure, achieving good strength-ductility synergy while excellent thermal stability.