Pressure-stabilized dual-BCC polymorphism in a rhenium-based high-entropy alloy

TL;DR

This study demonstrates how high pressure can induce and stabilize a dual-BCC microstructure in a rhenium-based high-entropy alloy, enabling new metastable phase engineering pathways.

Contribution

It reveals a pressure-driven, diffusionless transformation creating a dual-BCC microstructure in a refractory high-entropy alloy, a novel approach for metastable phase design.

Findings

Pressure induces a diffusionless transformation of hexagonal to BCC phase.

The pressure-stabilized BCC phase is kinetically trapped upon decompression.

The resulting dual-BCC microstructure exhibits contrasting elastic properties.

Abstract

Accessing metastable structural states in high-entropy alloys offers a promising route to tailor material properties, yet the use of high pressure to engineer such states remains underexplored. Here, we report the pressure-driven synthesis of a unique metastable dual-BCC microstructure in a near-equimolar ReNbTiZrHf alloy. Starting from an ambient two-phase mixture of hexagonal (C14-derived) and body-centered cubic (BCC) phases, compression induces a selective, diffusionless transformation of the hexagonal constituent into a second, crystallographically distinct BCC polymorph, while the original BCC phase remains stable. Upon decompression, the pressure-induced BCC phase is kinetically trapped, yielding a dual-BCC state that is inaccessible via conventional thermal processing. The pressure-stabilized BCC polymorph is Re-enriched and inherits the exceptional stiffness of its hexagonal parent (bulk modulus ~290 GPa), creating a composite microstructure with pronounced elastic and mechanical contrast relative to the softer original BCC matrix (~180 GPa). These findings demonstrate that pressure can effectively navigate the flat free-energy landscapes of chemically complex alloys, establishing a robust pathway for polymorph engineering and metastable phase design in refractory HEAs.