Ultrahard bulk nanocrystalline VNbMoTaW high-entropy alloy

Abstract

Coarse-grained high-entropy alloys (HEAs) have such excellent mechanical properties as high hardness and strength. The well-known Hall-Petch relation suggests that this high hardness should be further increased if the crystallite size of HEAs is decreased to nanometer scale. We prepared single-phased nanocrystalline (NC) refractory VNbMoTaW HEAs powders by mechanical alloying (MA). The achieved NC HEAs have a body-centered cubic (BCC) structure and an average grain size of approx. 6 nm. We further used a high-pressure/high-temperature (HPHT) technique to consolidate the NC powders to achieve bulk NC HEAs. The evolution of phases, microstructure and mechanical properties of the consolidated bulk NC HEAs were studied as a function of consolidating temperature. The bulk NC VNbMoTaW HEAs consolidated at an optimized temperature of 1150 °C have an average grain size of ∼30 nm and a hardness of 11.4 GPa, ∼ twice that of coarse-grained refractory HEAs. This ultrahigh hardness has three origins: solid solution, grain boundary and dislocation hardening.