Heterogeneous grain size and enhanced hardness by precipitation of the BCC particles in medium entropy Fe–Ni–Cr alloys

We report the chemical-composition-dependent precipitation of Cr-rich BCC particles and their role in the hardening of Fe–Cr–Ni medium entropy alloys (MEAs). Three alloys with different chemical compositions: 33Fe–32Cr–35Ni MEA (33Fe), 40Fe–30Cr–30Ni MEA (40Fe), and 45Fe–30Cr–25Ni MEA (45Fe) (at%), were investigated. After annealing at 800 and 950 °C, all three alloys had a heterogeneous grain size distribution, except for 40Fe annealed at 950 °C. The samples annealed at 800 °C showed a more heterogeneous grain size distribution, smaller average grain size, and lower degree of recrystallization than the samples annealed at 950 °C. This difference is ascribed mainly to the contents and precipitation kinetics of Cr-rich BCC particles at the two temperatures. The hardness decreased with the increase in annealing temperature for all the samples. The 33Fe sample exhibited higher hardness than 40Fe and 45Fe because of the joint effect of grain boundary strengthening and precipitation strengthening by the Cr-rich BCC particles. Different peak-load-dependent hardness behaviors were observed in 33Fe-A800 with relatively coarse BCC particles and 40Fe-A800 with fine BCC particles. The fine particles led to higher local hardness, whereas coarse particles resulted in higher microhardness owing to the hetero-deformation-induced strengthening effect. These results provide new insights into the strength optimization of medium- and high-entropy alloys through the dispersion of coarse Cr-rich BCC particles.