Micromechanics of hydride formation and cracking in zirconium alloys

Transient hydrogen diffusion and hydride formation coupled with material deformation are studied in Zr-2.5Nb alloys used in the pressure tubes of CANDU nuclear generating stations. The energetics of the hydride formation is revisited and the terminal solid solubility of hydrogen in solution is defined on the basis of the total elastoplastic work done on the system by the forming hydride and the external loads. Probabilistic precipitation of hydride is modeled in the neighborhood of a crack tip under mode I plane strain loading and a uniform initial hydrogen concentration below the stress free terminal solid solubility. Finite element analysis is used to monitor the local distribution and time evolution of hydrogen concentration, hydride volume fraction, stress, and strain as the externally applied loads increase. The mechanistic effects of the solute hydrogen and hydride formation on the stresses at the crack tip are analyzed and their consequence on the fracture toughness resistance of the material is determined.