Influence of Ti addition and sintering method on microstructure and mechanical behavior of a medium-entropy Al0.6CoNiFe alloy

The influence of Ti addition and sintering method on the microstructure and mechanical behavior of a medium-entropy alloy, Al0.6CoNiFe alloy, was studied in detail. Alloying behavior, microstructure, phase evolution and mechanical properties of Al0.6CoNiFe and Ti0.4Al0.6CoNiFe alloys were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), as well as by mechanical testing. During the mechanical alloying (MA) process, a supersaturated solid solution consisting of both BCC and FCC phases was formed in the Al0.6CoNiFe alloy. With Ti addition, the Ti0.4Al0.6CoNiFe alloy exhibited a supersaturated solid solution with a single FCC phase. Following hot pressing (HP), the HP sintered (HP'ed) Al0.6CoNiFe bulk alloy was composed of a major BCC phase and a minor FCC phase. The HP'ed Ti0.4Al0.6CoNiFe alloy exhibited a FCC phase, two BCC phases and a trace unidentified phase. Nanoscale twins were present in the HP'ed Ti0.4Al0.6CoNiFe alloy, where deformation twins were observed in the FCC phase. Our results suggest that the addition of Ti facilitated the formation of nanoscale twins. The compressive strength and Vickers hardness of HP'ed Ti0.4Al0.6CoNiFe alloy were slightly lower than the corresponding values of the HP'ed Al0.6CoNiFe alloy. In contrast with HP'ed Al0.6CoNiFe alloy, spark plasma sintered (SPS'ed) Al0.6CoNiFe alloy exhibited a major FCC phase and a minor BCC phase. Moreover, the SPS'ed Al0.6CoNiFe alloy exhibited a lower compressive strength and Vickers hardness, but singificantly higher plasticity, as compared to those of the HP'ed counterpart material.