Microstructure and Mechanical Properties of New AlCoxCrFeMo0.5Ni High-Entropy Alloys

Effects of Co content on microstructures and hot hardness of a new high-entropy alloy system, AlCoxCrFeMo0.5Ni (x = 0.5 to 2.0) were investigated. As cobalt content increases, the microstructure changes from dendrite to polygrain type and the constituent phases change from BCC + sigma at x = 0.5 to BCC + FCC + sigma at x = 2.0. The alloy hardness varies from Hv 788 at x = 0.5 to Hv 596 at x = 2.0. This can be explained with the relative amount of hard or phase, medium hard BCC phase and soft FCC phase. All the AlCoxCrFeMo0.5Ni alloys possess higher hardness level than that of Ni-based super-alloys In 718/In 718 H from room temperature to 1273 K. They obey the Westbrook equation presenting the normal heating behavior. Both alloys of x = 0.5 and 1.0 exhibit a transition temperature higher than that of Co-based alloy T-800 by about 200 K. They also have a high hot hardness of Hv 347 at 1273 K, which is higher than those of In 718 and In718 H by Hv 220. The strengthening mechanism for their superiority is proposed. The AlCoxCrFeMo0.5Ni alloy system has great potential in high-temperature applications.