Defect-mediated structural transformation near the crack tip in a CoCrFeNi alloy

The equiatomic CoCrFeNi alloy is known for its exceptional fracture toughness even at cryogenic temperatures. This study uses in situ straining to provide direct evidence of a combination of structural transformations at nanometer length scales near the crack tip. The study uses specifically designed pre-notched CoCrFeNi films prepared by vapor deposition. The observations reveal micro-void nucleation and coalescence accompanied by slip-band activity, progressive grain thinning, and localized shear. Multiple slip systems become active, forming a crisscross pattern inclined ∼45° to the tensile axis, leading to structural transitions from FCC to faulted, HCP, and amorphous regions near the crack tip. The results reveal a defect-mediated transformation linking slip, phase transformation, and amorphization that governs local stress accommodation and structural evolution during crack-tip propagation in CoCrFeNi alloy.