In Situ Observations of the Failure Mechanisms of Hydrided Zircaloy-4

Abstract

Notched specimens with high or low constraint were tested in situ with a scanning electron microscope using hydrided Zircaloy-4 to develop an understanding of the mechanism for fracture initiation and propagation. High-resolution electron backscatter diffraction was used to identify the texture and residual stresses near the hydrides prior to testing. A digital stereoimaging technique was used to measure the local strain distribution during testing. Fracture of nonhydrided Zircaloy-4 has been observed to occur by a process of void nucleation, growth, and coalescence that was described using a modified Rice-Tracey dependence for failure strain on triaxiality. For hydrided materials, it was observed that the cracking of select hydrides occurs at local regions of high strain when the macroscopic stresses are elastic. The tendency for hydride fracture is believed to be dependent on local residual stresses and texture. The microcracks nucleated from the hydrides are observed to be blunted by the α grains, and the regions between the hydrides are observed to fracture by a process similar to the nonhydrided material. A micromechanical model is developed to predict the failure strain for hydrided Zircaloy-4 that accounts for the effect of residual stress from hydride formation and stress state.