On the reconciliation of reduced cohesion and enhanced plasticity mechanisms for hydrogen embrittlement

Abstract

The pernicious influence of hydrogen on the crack growth resistance of metals is well established. It is generally believed that the enrichment of hydrogen in the region of hydrostatic tension at the tip of a loaded crack results in embrittlement. As proposed by Westlake, the precipitation and subsequent cracking of brittle hydrides in the region of increased hydrogen content has been shown to be responsible for subcritical cracking in hydride forming metals. The mechanism for reduced resistance to crack propagation in non-hydride forming metals, principally iron and nickel, is much less clear. Hydrogen-assisted cracking in these materials usually occurs transgranularly in the absence of grain boundary segregation. Beachem made careful examinations of fracture surfaces from hydrogen embrittled steels and deduced that the macroscopic embrittlement was actually due to severely localized plasticity. Nakasato and Bernstein demonstrated that hydrogen charging of purified iron induced cleavage on (110) slip planes rather than the preferred cleavage plane, (100). Embrittlement of the iron by the addition of > 0.7% by weight of silicon was required to switch hydrogen induced cracking to the cube planes. Normally, one would not consider a loss of cohesion and an increase in crack tip plasticity to be compatible processes. Therefore,more » a debate has been taking place as to which of these mechanisms is actually responsible for observed macroscopic losses in ductility. The purpose of this note is to offer a model whereby it would be possible to explain this apparent anomaly by demonstrating that, for the special case of hydrogen-assisted slip band cracking, the two processes are complementary.« less