Grain growth and Hall-Petch relationship in a refractory HfNbTaZrTi high-entropy alloy

Understanding the effect of temperature variation on the microstructural evolution is particularly important to refractory high-entropy alloys (RHEAs), given their potential high-temperature applications. Here, we experimentally investigated the grain-growth behavior of the HfNbTaZrTi RHEAs during recrystallization at temperatures from 1,000 to 1,200 degrees C for varied durations, following cold rolling with a 70% thickness reduction. Following the classical grain-growth kinetics analysis, two activation energies are obtained: 205 kJ/mol between 1,000 and 1,100 degrees C, and 401 kJ/mol between 1,100 and 1,200 degrees C, which suggests two mechanisms of grain growth. Moreover, the yield strength - grain size relation was found to be well described by the Hall-Petch relation in the form of sigma(y) = 942+ 270D(-0.5). It was revealed that the friction stress, 942 MPa, in the HfNbTaZrTi HEA is higher than that of tungsten alloys, which indicates the high intrinsic stress in the BCC-RHEA. The coefficient, 270 MPa/mu m(-1/2), is much lower than that in the 316 stainless steel and Al0.3CoCrFeNi HEAs, which indicates low grain-boundary strengthening. (C) 2019 Elsevier B.V. All rights reserved.