Revealing the role of hydrogen content on the mechanical behavior and cracking mechanisms of Zircaloy-4 under strain

Abstract

The mechanical property of zirconium-alloy reactor cladding materials inevitably degrades because of hydrogen absorption and hydride precipitation. In this study, mechanical behavior and delayed hydrogen cracking (DHC) mechanisms of Zircaloy-4 alloys with a broad range of hydrogen contents were investigated. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and a universal testing machine were jointly employed for this purpose. Results showed that the mechanical property of the hydrogenated specimen not only depended on the size and orientation of hydride, but also was affected by the continuity degree of the load-bearing matrix. The relevant microstructural observation indicated that the lower continuity degree of the load-bearing matrix, the less ductility of Zircaloy-4. More importantly, hydride phase transformation from δ-ZrH 1.5 to γ-ZrH occurred around the crack tip and reduced the local strain, while twinning just appeared at the crack bridging sites in the higher hydrogen-content specimens for changing the crystal orientation and mitigating the local stress concentration. Both those effects are expected to be potential mechanisms of the reappearing yield phenomenon because they are closely linked to dislocation movement. Additionally, based on the combination effect of crack blunting of γ-ZrH and the strain accommodation of twinning, the trend of crack propagation is delayed locally as expected. • The hydride phase transformation and twinning phenomenon around cracks were analyzed using EBSD. • The continuity of the load-bearing matrix profoundly influenced the mechanical properties of hydrided zircaloy-4. • The appearance and disappearance of yield-suppression phenomenon in hydrided zircaloy-4 were investigated. • Both γ-ZrH and twins are beneficial to delay the localized crack propagation.