Hot deformation behavior and microstructure evolution of non-equimolar Ti2ZrHfV0.5Ta0.2 refractory high-entropy alloy

Abstract

This work aims to elucidate the hot deformation behavior and microstructure evolution of a non-equimolar TiZrHf-rich refractory high-entropy alloy (RHEA) with a composition of Ti2ZrHfV0.5Ta0.2 (at. %). The as-cast Ti2ZrHfV0.5Ta0.2 alloy composed solely of a body-centered cubic phase showed superior tensile mechanical properties with a yield strength of 865 MPa and a total elongation of 22.5%. The uniaxial compression tests were performed at temperatures ranging from 900 to 1100 °C and strain rates varying from 10−3 to 10−1 s−1. The relation among the flow stress, deformation temperature, and strain rate was represented by the Arrhenius-type constitutive equations. The calculations show a comparatively low apparent activation energy of 144–119 kJ/mol over the entire range of strains, revealing a noticeable contradiction between the concentration of group IV elements (Ti, Zr, Hf) and the high-temperature strength. The excessive addition of Group IV elements tends to deteriorate the high-temperature strength due to the relatively low melting points and self-diffusion activation energies of Group IV elements. The misorientation analysis based on electron back-scattered diffraction indicated that the dynamic recrystallization mechanism depends strongly on the thermo-mechanical processing conditions. A low deformation temperature and high strain rate results in an abnormal discontinuous dynamic recrystallization mechanism. Additionally, the dominant role played by the edge dislocation in RHEAs during deformation is supposed to be the reason for the abnormal discontinuous dynamic recrystallization mechanism. A series of stress drops were observed under low deformation temperatures and high strain rates. Based on an interrupted hot-compression experiment and a strain aging experiment, the stress drop can be attributed to the unlocking of dislocations pinned by the Cottrell atmosphere or short-range ordering.