Some fractographic aspects of hydrogen-induced delayed cracking in Zr-2.5 wt% Nb alloys

The fractographic features after hydrogen-induced delayed cracking (HIDC) in Zr-2.5 wt% Nb pressure tube material have been studied as a function of stress intensity factor (K), temperature and hydride dispersion. The important observations were of regularly spaced ductile striations parallel to the main crack front, and brittle cleavage markings between the striations. The striations were cusped due to dimple formation, and individual cleavage regions were separated by ductile ridges or shear cliffs. The size and morphology of the cleavage regions was consistent with the size and preferred orientation of hydrides near the crack tip. The inter-striation spacing increased exponentially with temperature, but within the range of study, was essentially independent of K. Neither hydride content nor dispersion had a significant effect on the fracture morphology. The results are consistent with a discontinuous mechanism of crack propagation involving the accumulation of hydride at the crack tip, followed by fracture through this embrittled region and subsequent crack arrest. The observations support recent theoretical models for HIDC which interpret the inter-striation spacing as a critical hydride length for crack advance. (Auth.)