Hydrogen Embrittlement Behavior of Pure Ni and Ni-20Cr Alloy with Different Grain Sizes

The effects of grain size on the hydrogen embrittlement susceptibility of pure Ni and Ni-20Cr alloy were investigated. The hydrogen embrittlement susceptibility was evaluated through tensile testing under electrochemical hydrogen charging. Relative elongation, defined as the elongation under hydrogen charging divided by elongation in air, increased with increasing grain size in pure Ni (the grain size was in the range of 11-22 µm). In contrast, the relative elongation of the Ni-20Cr alloy increased with decreasing grain size from 13 to 1.8 µm. Correspondingly, the intergranular fracture was suppressed by grain coarsening in pure Ni and grain refinement in the Ni-20Cr alloy. In addition, the intergranular fracture surface in pure Ni exhibited curved slip lines and that in the Ni-20Cr alloy exhibited straight line marks. These fractographic features imply that the mechanisms of the hydrogen-assisted intergranular crack growth in pure Ni and Ni-20Cr alloy were different, which could be attributed to the difference in their stacking fault energies.