Microscale deformation in CoCrNi based medium-entropy alloys: high strain-rate nanoindentation test

CoCrNi-based medium entropy alloys are promising structural materials for crashworthiness and ballistic applications due to their exceptional dynamic properties. These superior mechanical properties arise from their multiple deformation mechanisms: dislocations, stacking faults (SFs), and deformation twins (DTs). However, conventional macro-scale impacting tests induce complex microstructures over extensive deformation volumes due to high contact stress. This complexity obstructs our understanding of how these deformation mechanisms sequentially evolve. Furthermore, the accommodation of sharp-indenter-induced strain gradients through stacking fault-mediated processes remains scarcely studied. High strain rate nanoindentation tests have been conducted on the (CoCrNi)_1-xAl_x (x = 0.3 and 0.75) alloys. The microscale deformed volume reveals a unique rate- and strain-dependent hierarchical microstructure. The surface zone subjected to high strain rate (>10³ s⁻¹) develops dense SFs/DTs bands. The SFs density progressively decreases while DTs nearly vanish in the plastic zone under intermediate strain rate regime (10² - 10³ s⁻¹). In far-field zones dominated by low strain rate (<10² s⁻¹), elongated dislocations emerge. The coordinated strain-gradient accommodation arises through intersection of primary SFs/DTs bands and crystallographic bending induced by secondary lateral SFs. Additionally, the Al addition increases the impact resistance of CoCrNi alloys through combined solid solution and B2-phase precipitation strengthening.