Cr Effects on the Local Plasticity Evolution and Fracture Behaviors of NiCr Alloys Tensile-Tested under Hydrogen Charging

The Cr effects on hydrogen embrittlement behaviors in pure Ni, Ni20Cr, and Ni44Cr alloys with similar grain sizes were investigated using tensile tests under electrochemical hydrogen charging and microstructure observations. The relative elongation (defined as elongation under hydrogen charging divided by elongation in air) in the pure Ni was higher and lower than those of the Ni20Cr and Ni44Cr alloys, respectively. The behaviors of the hydrogen-charged specimens were as follows. The hydrogen embrittlement susceptibility nonmonotonically varied with an increase in the amount of Cr substitution. The fracture surfaces of the pure Ni and Ni20Cr alloy showed intergranular fractures, and that of the Ni44Cr alloy showed a fully ductile feature. Post-mortem electron backscattered diffraction analyses revealed that grain reference orientation deviation (GROD) values, which correspond to local plasticity-induced lattice distortions, were high around grain boundary triple junctions, and the maximum value in the pure Ni at the grain boundary triple junction (18°) was nearly the same as that of the Ni20Cr alloy (17°), although the total elongation of the pure Ni was twice that of the Ni20Cr alloy. This result indicated that Cr addition promoted the plasticity-induced local stress evolution associated with dislocation pile-up. Moreover, the maximum GROD value of the pure Ni near the fracture surface was 56°, which is considered to be the critical level for plasticity-induced cracking in pure Ni. Interestingly, the Ni44Cr alloy showed a similar GROD value (55°) even in the uniformly deformed portion of the fractured specimen, while showing ductile fracture and no cracks in the fractured specimen. This result indicated that the Ni44Cr alloy had higher grain boundary strength than that of the pure Ni even after hydrogen charging.