Thermodynamics and kinetics of delayed hydride cracking in zirconium alloys: A review

Hydrogen moves in zirconium because of forces associated with gradients in concentration and stress. When solubility limits are reached, stable hydrides form within control volumes that include stabilizing clouds and Cottrell atmospheres of hydrogen in solution, otherwise unstable hydrides form that can continue to grow as observed in delayed hydride cracking and predicted by the Diffusion First Model written in terms of hydrogen partial molar volume, diffusivity, and solvus, matrix yield strength, and sink strength for hydrogen moving under the influence of a hydrostatic stress gradient. This model predicts cracking rates following heating and cooling to temperatures where the hydride that is found on the crack fracture face is a mixture of delta and gamma hydride. A graphical representation is provided to illustrate the temperature history of hydride nucleation and growth. The effects of neutron irradiation and athermal hydrides are discussed. Other models of delayed hydride cracking are critically reviewed.