Effects of Co and Ti on microstructure and mechanical behavior of Al0.75FeNiCrCo high entropy alloy prepared by mechanical alloying and spark plasma sintering

Abstract

The effects of Co removal and Ti addition on the microstructure and mechanical behavior of a high-entropy alloy (HEA), Al0.75FeNiCrCo, were studied systematically. Al0.75FeNiCrCo, Al0.75FeNiCr and Al0.75FeNiCrCoTi0.25 were successfully prepared by the combination of mechanical alloying (MA) and spark plasma sintering (SPS). During the MA process, a primary body-centered cubic (BCC) supersaturated solid-solution phase and a face-centered cubic (FCC) supersaturated solid-solution phase were formed in each of the three investigated alloys. Removing Co from the Al0.75FeNiCrCo HEA or adding Ti into the Al0.75FeNiCrCo HEA could facilitate increasing the content of BCC phase both during the MA process and after the SPS process. Following SPS, bulk Al0.75FeNiCrCo was composed of a major FCC phase (∼79vol%) and a minor BCC phase (∼21vol%). However, with Co removal, bulk Al0.75FeNiCr alloy exhibited a main BCC phase (∼60vol%) and an FCC phase (∼40vol%). With Ti addition, bulk Al0.75FeNiCrCoTi0.25 alloy consisted of a major FCC phase (∼77vol%) and a minor BCC phase (∼23vol%). The EDS/TEM results of the bulk Al0.75FeNiCrCo alloy revealed that the BCC phase was enriched in Al–Ni, while the FCC phase was enriched in Fe–Cr–Co. Meanwhile, a small fraction of nanoscale twins were present in the bulk Al0.75FeNiCrCo alloy. The bulk Al0.75FeNiCrCo alloy exhibited high strength and high hardness, mainly attributed to solid-solution strengthening, twin-boundary strengthening and grain-boundary strengthening. With Co removal, the bulk Al0.75FeNiCr alloy showed lower strength, hardness and ductility in comparison with the Al0.75FeNiCrCo alloy. With Ti addition, compared to the Al0.75FeNiCrCo alloy, the bulk Al0.75FeNiCrCoTi0.25 alloy exhibited higher strength and hardness with a slightly lower ductility.