Enhanced Resistance to Hydrogen Embrittlement in a CrCoNi-Based Medium-Entropy Alloy via Grain Boundary Engineering: A Case Study of Boron Segregation

The face-centered cubic (fcc) structured CrCoNi medium-entropy alloy exhibits outstanding mechanical properties, but is extremely susceptible to hydrogen-induced grain boundary (GB) embrittlement. In this work, we found boron doping in the ppm level can substantially reduce the ductility loss after gas-hydrogen charging from ~71% to ~45.8%, while the fracture mode also transfers from predominantly intergranular to ductile transgranular dominated. Electron backscatter diffraction measurements revealed no difference between boron -undoped and -doped CrCoNi in phase structure (single-fcc), average grain sizes (18 μm), and GB characters. The main difference was revealed by atom probe tomography for local compositional analysis, where boron-doped CrCoNi contains apparent GB decoration of boron up to 1.5 at.% and nanometer-scaled scattered borides. Such local chemical difference leads to enhanced GB cohesion and reduced hydrogen diffusivity along GBs, resulting in greatly improved immunity against hydrogen-induced boundary cracking and suppressed mechanical degradation in the newly developed boron-doped CrCoNi alloy.