Effect of Cr addition on the alloying behavior, microstructure and mechanical properties of twinned CoFeNiAl0.5Ti0.5 alloy

The effect of Cr addition on alloying behavior, microstructure and mechanical properties of multicomponent CoFeNiAl0.5Ti0.5 high entropy alloy (HEA) was studied in detail. Non-equiatomic CoFeNiAl0.5Ti0.5 and CrCoFeNiAl0.5Ti0.5 alloys were fabricated by the combination of mechanical alloying (MA) and spark plasma sintering (SPS). Alloying behavior, microstructure, phase evolution and mechanical properties of the two alloys were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM), as well as by an Instron testing system. During MA, a supersaturated solid solution consisting of a metastable FCC phase was formed in CoFeNiAl0.5Ti0.5 alloy. With Cr addition, the alloy showed a supersaturated solid solution with an FCC phase and a BCC phase. After SPS, bulk CoFeNiAl0.5Ti0.5 alloy was composed of a main FCC phase, a minor BCC and a tiny unknown phase. The addition of Cr into CoFeNiAl0.5Ti0.5 alloy exhibited two FCC phases (FCC1 and FCC2) with similar lattice constant and a tiny unknown phase, meanwhile selected area electron diffraction (SAED) pattern of the FCC1 phase was the same as that of the FCC phase of CoFeNiAl0.5Ti0.5 alloy. Nanoscale twins presented in both of CoFeNiAl0.5Ti0.5 and CrCoFeNiAl0.5Ti0.5 alloys, but deformation twinning occurred only in the FCC phase of CoFeNiAl0.5Ti0.5 and the FCC1 phase of the Cr added alloy which displayed the same SAED pattern. Moreover, the addition of Cr lowered the formation ability of nanoscale twins evidently. The addition of Cr into CoFeNiAl0.5Ti0.5 alloy could decrease compressive strength and Vickers hardness slightly.