Hydrogen embrittlement of the equi-molar FeNiCoCr alloy

Abstract

The influence of hydrogen on the mechanical response of an FCC equi-molar solid-solution alloy, FeNiCoCr, was investigated. Hydrogen caused a reduction in ductility in terms of total elongation of only a few percent in the FeNiCoCr alloy, but the local reduction in area after necking was reduced, and the fracture mode transitioned from transgranular ductile microvoid coalescence to predominantly intergranular. This result demonstrates unequivocally that this equi-molar alloy is susceptible to hydrogen embrittlement. Specifically, hydrogen reduced the elongation of the grains in the loading direction, increased the out-of-plane distortion of grains on the specimen surface, increased the orientation deviations with respect to the mean of individual grains, and induced a more advanced microstructural state in the form of dislocation tangles and dislocation cells. Hydrogen transport by dislocations caused a redistribution of the hydrogen increasing the hydrogen concentration on grain boundaries. Hydrogen accommodated in dislocation cell walls and on dislocations, effectively locked the microstructure in a specific configuration that inhibited strain transfer across grain boundaries. Consequently, an incompatibility constraint across grain boundaries developed. Together these effects explain why the hydrogen-charged alloy did not exhibit necking and transitioned to a grain boundary failure mode.