Effects of Co and sintering method on microstructure and mechanical behavior of a high-entropy Al0.6NiFeCrCo alloy prepared by powder metallurgy

The effect of Co on the microstructure and mechanical behavior of a high-entropy alloy (HEA), Al0.6NiFeCrCo, consolidated via spark plasma sintering (SPS) or hot pressing (HP) from powders, was studied in detail. The microstructure of the mechanically alloyed (MA'ed) Al0.6NiFeCrCo HEA consisted of a primary body-centered cubic (BCC) solid-solution phase (∼61 vol.%) and a face-centered cubic (FCC) solid-solution phase (∼39 vol.%). In contrast, elimination of Co in the Al0.6NiFeCr HEA led to the formation of a primary BCC solid-solution phase (∼85 vol.%) with a small amount of FCC solid-solution phase (∼15 vol.%) present. The microstructure of the SPS'ed and HP'ed Al0.6NiFeCrCo HEAs contained mostly an FCC phase (∼88 vol.%) and a small amount of BCC phase (∼12 vol.%), whereas the SPS'ed and HP'ed Al0.6NiFeCr contained a primary FCC phase (∼69 vol.%) with a BCC phase (∼31 vol.%) present. Eliminating Co from the Al0.6NiFeCrCo HEA increased the volume fraction of the BCC phase, both in the MA'ed powders and the consolidated bulk materials. When compared to the SPS'ed Al0.6NiFeCrCo alloy, the HP'ed Al0.6NiFeCrCo alloy possessed slightly lower strength and Vickers hardness, but slightly higher plasticity. The influence of sintering method on mechanical behavior is identical for both Al0.6NiFeCr and Al0.6NiFeCrCo HEAs. Moreover, the SPS'ed and HP'ed Al0.6NiFeCr samples exhibited lower strength and Vickers hardness, but slightly higher plasticity, as compared to those of Al0.6NiFeCrCo. The mechanisms responsible for the observed influence of Co on phase composition and mechanical behavior of the Al0.6NiFeCrCo HEA are discussed in detail.