Superior hydrogen embrittlement resistance of CoCrNi-based medium-entropy alloy via coherent precipitation and grain boundary strengthening

The strength and HE resistance of CoCrNi-based medium-entropy alloys were simultaneously improved via adding 264 at.ppm boron and the precipitation of nanoscale coherent L1₂ (γ'-type) particles. After aging treatment, the proportion of intergranular cracking decreased from 58.2 % in the solution-treated alloy to 27.6 %. Meanwhile, the yield strength increased by 102 %, and the ductility decreased by only 35.9 %. Notably, the elongation loss is only 9.4 %. On one hand, the added boron segregates to the grain boundaries (GBs), enhance the GB cohesive strength. On the other hand, precipitated L1₂ captures H, thereby reducing the concentration of diffusible H in the matrix and decelerating the diffusion rate of H. During plastic deformation, the L1₂ particles impedes the movement of dislocations and reduces stress concentration at GBs. This is associated with the presence of a completely coherent interface between the precipitated phase and face centered cubic matrix. This research provides insights into the beneficial effects of L1₂ phase precipitation and GB boron segregation on the HE resistance of M/HEAs.