Hydride-Induced Embrittlement in Metals — Stress and Temperature Effects

Abstract

A robust mathematical model for the hydrogen embrittlement of hydride forming metals has been developed. The model takes into account the coupling of the operating physical processes, namely: (i) hydrogen diffusion, (ii) hydride precipitation, (iii) non-mechanical energy flow, and (iv) hydride/solid-solution deformation. Crack growth is simulated by using a new version of de-cohesion model with time-dependent energy of de-cohesion due to the gradual process of hydride formation. Zircaloy-2 hydrogen embrittlement and fracture initiation have been studied by using a finite element implementation of the model. Delayed hydride cracking has been considered in two configurations: (i) a semi-infinite crack, under mode-I K-field dominance and constant temperature, and (ii) a cracked plate, under tensile stress and temperature gradient. The initial and boundary conditions, in case (ii), are those encountered in the fuel cladding of boiling water reactors, during operation, and lead to loss of K-field dominance soon after the application of loading. The numerical simulation predicts hydride precipitation at some distance from the crack tip. The near-tip hydride platelets fracture, when the remote loading is sufficiently strong, and leave behind ligaments, which are stretched plastically, in agreement with experimental observations. The numerical results on hydride size, incubation period and crack growth velocity are compared with experimental data.Further development of the model should be combined with accurate experimental determination of the mechanical and thermal properties of the hydrides.