Heterogeneous precipitation behavior and stacking-fault-mediated deformation in a CoCrNi-based medium-entropy alloy

Combining high strength and good ductility is highly-desired yet challenging for conventional structural materials. Newly emerging concentrated multi-component alloys with face-centered-cubic structure provide an ultra-ductile matrix, and the precipitation hardening based on these alloys provides a very effective way to achieve a superior strength-ductility combination. Here, we report a high-strength CoCrNi-based medium-entropy alloy hardened by nanoscale L12-(Ni, Co, Cr)3(Ti, Al)-type particles with mixing heterogeneous and homogeneous precipitation behaviors. Compared to the single-phase CoCrNi medium-entropy alloy, the yield strength and the tensile strength of the precipitation-strengthened CoCrNi medium-entropy alloy were increased by ∼70% to ∼750 MPa and ∼44% to ∼1.3 GPa, respectively, whereas a good ductility, ∼45%, was still achieved. The underlying deformation micro-mechanisms were systematically investigated using transmission electron microscope. In the single-phase CoCrNi medium-entropy alloy, the deformation mode was dominated by mechanical twinning. In the precipitation-hardened medium-entropy alloy, however, a high density of stacking faults prevailed. We revealed that the absence of mechanical twinning in this low stacking fault energy precipitation-strengthened medium-entropy alloy could relate to the increasing critical twinning stress affected by the channel width of the matrix. We further calculated that the increment of the yield strength was substantially from precipitation strengthening. Our present findings provide not only a fundamental understanding of the deformation micro-mechanism of the precipitation-strengthened CoCrNi medium-entropy alloy but also a useful guidance for the development of precipitation-hardened concentrated multi-component alloys in the future.