The FCC to BCC phase transformation kinetics in an Al_0.5CoCrFeNi high entropy alloy

In this paper, thermal expansion method is used to characterize the phase transformation process of Al_0.5CoCrFeNi high entropy alloy (HEA), and the FCC-BCC phase transformation kinetics, microstructure and mechanical properties are investigated. Results show that during continuous heating process, three stages of phase transformation process for Al_0.5CoCrFeNi HEA are characterized by thermal expansion curves, and the third stage is corresponding to FCC-BCC phase transformation. The FCC-BCC phase transformation kinetics is analyzed using the thermal expansion experiments carried out at different heating rates. The FCC-BCC phase transformed fraction, f, as a function of temperature is determined from thermal expansion curve. The activation energies, Q, determined by KAS model show an increasing trend which is from 144.4 kJ/mol at initial stage (f = 0.2) to 209.2 kJ/mol when f = 0.8, indicating the barrier for transformation is increasing. The Avrami exponent, n, determined by modified JMA model, varies with transformed fraction, indicating the FCC-BCC phase transition firstly is interface controlled with decreasing nucleation rate (f < 0.05), and changed to diffusion controlled with an increasing nucleation rate (f < 0.15), then diffusion controlled with decreasing nucleation rate. The microstructure and mechanical properties of the corresponding FCC-BCC phase transformation process are also studied.