Microstructure and magnetic properties of the FeCoNi(CuAl)0.8Ga0.06 high-entropy alloy during the phase transition

The FeCoNi(CuAl)0.8Ga0.06 high-entropy alloy (HEA) was prepared by the vacuum arc melt casting method, then annealed at different temperatures for 1 h. The microstructure, phase constitution, and magnetic and mechanical properties of the FeCoNi(CuAl)0.8Ga0.06 HEA during the phase transition were investigated. The experimental results showed that the as-cast alloy consisted of FCC and BCC duplex phases, and there were numerous nano-precipitates uniformly distributed in the BCC matrix. After annealing at different temperatures, the phase transformed from FCC to BCC. The relative volume fraction of the BCC phase increased, and the size of the nano-precipitates in the BCC matrix were larger after annealing than that of the as-cast sample. STEM-EDS images showed that there was a Cu-rich zone in the phase boundary region between FCC and BCC phases, and annealing may have significant influence on this region. Based on XRD, SEM, TEM and STEM-EDS results, a phase transition mechanism was proposed, which may have a close relationship with the Cu-rich nano-precipitates in the BCC matrix and the Cu-rich zone in the phase boundary region. In addition, it was found that the volume fraction of BCC (or FCC) phase can govern the magnetic properties. This study may help to deepen the fundamental understanding of the phase transition mechanism in HEAs.