Effect of Pre-Strain Induced Microstructure Evolution on Hydrogen Embrittlement Resistance of a CoCrNi Medium-Entropy Alloy

The effect of pre-strain-induced microstructural evolution on the hydrogen embrittlement (HE) resistance of an equiatomic CoCrNi medium-entropy alloy was systematically investigated by mechanical property testing, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) characterization. Three pre-strain levels (0%, 30%, and 50%) were applied to produce distinct microstructures: dislocation-free and twin-free (P0), high dislocation density with few deformation twins (P30), and high densities of both dislocations and deformation twins (P50). Mechanical tests combined with hydrogen charging revealed that the P50 specimen exhibited the highest yield strength (1163.88 MPa) and the lowest HE-induced elongation loss (2.74%), indicating an improvement in HE resistance. By using SEM, detailed observations of the fracture morphology and crack propagation paths revealed that deformation twins can effectively reduce stress concentration, delay the nucleation and propagation rates of cracks, and suppress brittle intergranular fracture, thereby improving mechanical properties and resistance to hydrogen embrittlement. A detailed analysis was conducted of the HE resistance mechanism associated with the influence of deformation twins on hydrogen transport and distribution.