Experimental investigation and phase diagram of CoCrMnNi–Fe system bridging high-entropy alloys and high-alloyed steels

Abstract

An investigation of equiatomic and non-equiatomic high-entropy alloys (HEAs) and medium-entropy alloys (MEAs) was carried out by performing both thermodynamic calculations and experiments. The design strategy of the alloys was based on a constant valence electron concentration (VEC) of 8 while varying the content of Fe in a Fex(CoCrMnNi)100−x where x = 00, 20, 40, and 60. The X-ray diffraction results showed that the alloys have a single face-centered cubic structure at 900 °C. The tensile test results revealed that both the yield strength and ultimate tensile strength decrease as the amount of Fe is increased but the uniform elongation increases. Additionally, the strain-hardening rate is significantly enhanced at higher Fe concentrations due to the activation of deformation twinning as an additional deformation mechanism. Interestingly, the critical twinning stress is significantly reduced as the Fe content is increased from Fe00 to Fe60. As a result, the deformation twins were easily activated in a specimen deformed up to a true strain of 20% for Fe60, unlike the other alloys, which exhibited no deformation twinning at the same true strain. Furthermore, it was revealed that the frequency of deformation twinning increases proportionally with increase in the Fe content.