Investigation of phase constitution and stability of gas-atomized Al0.5CoCrFeNi2 high-entropy alloy powders

In this study, Al0.5CoCrFeNi2 high-entropy alloy powders with uniform elements distribution were prepared by gas atomization process. The effects of heat treatment on the microstructure, phase transformation and mechanical properties were discussed. The FCC phase was observed in the as-atomized powder. The In-situ XRD results indicated that phase transform occurred at around 900 degrees C. Therefore, the annealing treatment was conducted at 1000 degrees C to observe the phase transformation. From the XRD of the different annealing times, additional diffraction peaks began to generate after 24 h. With SEM and EDS analyses, the results indicated the Cr element precipitated along the grain boundaries. The longer the annealing time, the more the precipitates. Afterward, EBSD was carried out to identify the crystal orientation of the phase. It was found that the FCC phase was aligned with the Ni3Al crystal structure. On the other hand, the Cr-rich phase formed after the annealing process tended to form the BCC crystal structure of NiAl and Fe, observed from the results of EDS and EBSD. Furthermore, the average hardness of as-atomized powder was 4.46 GPa. After the annealing process, the lattice distortion was eliminated. As a result, the hardness of FCC matrix was significantly decreased to 2.6 GPa. Whereas the hardness of segregated Cr-rich phase was 12.74 GPa. Thus, the average hardness of annealed powder was increased to 6.93 GPa due to the precipitation strengthening resulting from Cr-rich region.