Developing a single-phase and nanograined refractory high-entropy alloy ZrHfNbTaW with ultrahigh hardness by phase transformation via high-pressure torsion

TL;DR

This study develops a single-phase, nanograined refractory high-entropy alloy ZrHfNbTaW with ultrahigh hardness of 860 Hv through phase transformation induced by high-pressure torsion, demonstrating superior hardness due to multiple strengthening mechanisms.

Contribution

Introduces a novel single-phase refractory HEA with ultrahigh hardness achieved via phase transformation and severe plastic deformation, surpassing previous alloys.

Findings

Achieved ultrahigh hardness of 860 Hv in ZrHfNbTaW alloy.

Demonstrated phase transformation from dual-phase to single-phase BCC structure.

Identified multiple mechanisms contributing to hardness, including lattice distortion, nanograin formation, and dislocation strengthening.

Abstract

High-entropy alloys (HEAs) are potential candidates for applications as refractory materials. While dual-phase refractory HEAs containing an ordered phase exhibit high hardness, there is high interest in developing intermetallic-free and single-phase refractory HEAs with high hardness. In this study, a new equiatomic HEA ZrHfNbTaW with an ultrahigh hardness of 860 Hv is developed. The alloy is first synthesized with a dual-phase structure via arc melting and further homogenized to a single body-centered cubic (BCC) structure by phase transformation via high-pressure torsion (HPT), using the concept of ultra-severe plastic deformation process. The ultrahigh hardness of the alloy, which is higher than those reported for refractory alloys and single-phase HEAs, is attributed to (i) solution hardening by severe lattice distortion, (ii) Hall-Petch grain boundary hardening by the formation of nanograins with 12 nm average size, and (iii) dislocation hardening confirmed by high-resolution transmission electron microscopy.