Influence of increasing Al concentration on phase, microstructure and mechanical behaviors of Ni1.5CoFeCu1−xAlxV0.5 high entropy alloys

The present work systematically investigated the influence of increasing Al concentration on the phase composition, microstructure evolution and mechanical behaviors of Ni1.5CoFeCu1−xAlxV0.5 (x = 0.1, 0.2, 0.3 mol) high entropy alloys (HEAs) synthesized by mechanical alloying (MA) followed by spark plasma sintering (SPS). In the MA powders, FCC phase reduced while BCC phase increased with increase in Al content. In the alloys consolidated by SPS, BCC phase disappeared with the formation of vanadium carbides as well as aluminum oxide due to carbon and oxygen contaminations. Microstructure characterization suggests that three bulk HEAs all displayed the bimodal grain microstructure, consisting of coarse-grained (CG) regions as well as fine-grained (FG) regions with uniform dispersion of aluminum oxide nanoparticles. Furthermore, with the increase of Al concentration in the alloy, the volume fraction of FG region increased gradually and the average grain sizes of CG and FG regions reduced dramatically. Since the microstructure refinement resulted in the enhancement of grain boundary strengthening, the yield strength of alloy was improved significantly with increasing Al. In addition, the increase of Al concentration remarkably changed the strain-hardening behavior of bulk HEA by influencing the strain-hardening stages during compression.