Creep in a nanocrystalline VNbMoTaW refractory high-entropy alloy

Abstract

Refractory high-entropy alloys (RHEAs) are considered to be a promising candidate for elevated temperature applications. Nanocrystalline (NC) RHEAs are supposed to exhibit many different high-temperature mechanical behaviors in comparison with their coarse-grained (CG) and ultrafine-grained (UFG) counterparts. However, the creep behaviors of NC RHEAs, which must be well evaluated for high-temperature applications, are largely unknown because it is difficult to produce bulk quantities of NC RHEAs for creep tests. In the present work, an equiatomic bulk NC VNbMoTaW RHEA with an average grain size of 67 ± 17 nm was synthesized by mechanical alloying (MA) and the subsequent high-pressure/high-temperature sintering. The creep tests were performed on bulk specimens by compression at high temperatures (973 and 1073 K) under different stresses (70–1100 MPa). The creep resistance of the bulk NC VNbMoTaW is slightly lower than that of the bulk CG VNbMoTaW, but much higher than that of previously reported CG and UFG HEAs. The derived activation volume, stress exponent, and activation energy of bulk NC VNbMoTaW indicate that the creep deformation is dominated by grain boundary diffusion. The creep deformation is controlled by the diffusion of Mo and Nb elements, which have the two slowest grain boundary diffusivities among the five alloying elements. The present work provides a fundamental understanding of the creep behavior and deformation mechanism of NC RHEAs, which should help design advanced creep-resistant RHEAs.