An HVEM study of hydrogen effects on the deformation and fracture of nickel

Abstract

The mechanisms of hydrogen “embrittlement” of nickel have been investigated by performing in situ straining experiments in a high-voltage electron microscope equipped with an environmental cell. The generation rate and velocity of dislocations and the crack propagation rate were markedly increased by the presence of hydrogen. The advance of transgranular cracks, in both vacuum and hydrogen, occurs by either the direct emission of dislocations from the crack tip or by a complex process of hole nucleation and growth ahead of the crack. Intergranular cracks also propagate by these mechanisms; these cracks advance along the deformation zone that follows the contour of the boundary rather than along the boundary interface. The effect of hydrogen is to decrease the stress required for crack advance and to localize the deformation. When viewed macroscopically, this confinement of the plastic deformation to a narrow zone gives the impression of a brittle type fracture. Hydrogen “embrittlement” of nickel therefore occurs by a mechanism whereby hydrogen locally enhances plastic processes rather than by a decohesion mechanism.