Transition of dominating roles from dislocations to stacking faults enables superior mechanical properties of CoCrNi alloys

Generally, balanced strength and ductility can be achieved by tailoring crystal defects for face-centered cubic (FCC) alloys, in which dislocations play a critical role. This study investigates the deformation mechanisms and strain-hardening behavior of CoCrNi alloys (45Ni, 33Ni, 24Ni) with different stacking fault energies (SFEs). The 45Ni and 33Ni exhibit a dislocation-dominated deformation mechanism. In contrast, stacking faults (SFs) dominate in the 24Ni alloy, which is closely related to the very low SFE. SFs not only strengthen the FCC matrix but also promote the hexagonal close-packed (HCP) phase nucleation. The overall effect of the nanoscale thickness and the significant volume fraction of the HCP phase leads to a sustained high strain-hardening rate. In addition, the product of ductility and strength of 24Ni is significantly higher than that of equiatomic CoCrNi alloy and 316 L stainless steel at the critical grain size of ∼0.7 μm. These findings provide new insights for further improving the mechanical properties of FCC alloys.