The interactions of deformation twins, zirconium hydrides, and microcracks

One of the main degradation mechanisms of the zirconium alloys used in nuclear reactors is hydrogen embrittlement and the formation of zirconium hydrides. This study focuses on understanding the interactions among deformation twins, hydrides, and the microcracks that form within hydrides. For this purpose, in-situ scanning electron microscopy and interrupted ex-situ tensile experiments were conducted on hydrided zirconium specimens with favorable initial textures for the formation of extension twins. Electron backscatter diffraction (EBSD) was used to measure the orientations of the grains located in the specimens' gauges and map them into a crystal plasticity finite element model to study hydrides and twins interactions. High spatial resolution EBSD and high-resolution imaging were used to follow the formation of microcracks, and twins live. Although the specimens were deformed to a moderate level of applied strain (similar to 7 %), it was observed that two types of twins nucleate, {1012 } and {1121 }. While the former nucleates either before or after the nucleation of microcracks within hydrides, the latter nucleates after the formation of microcracks and grows with them. It is shown that the formation of twins may contribute to crack nucleation, yet the shear energy density on a given slip system within hydrides is the main driving force for crack nucleation. Regardless of hydride interactions with twins, a significant slip activity is recorded within hydrides prior to cracking.