Modeling hydride growth and strain-field evolution at a stress concentrator in zirconium alloys

Abstract

The redistribution of hydrogen in zirconium-based components of nuclear reactors is strongly influenced by local stress states. Such a stress-state can be influenced by the stress concentration associated with crack tips or blunt flaws as well as the volumetric expansion resulting from the precipitation and growth of hydrides themselves. To date, the way the matrix material accommodates hydride transformation strain is not fully understood. In this study, two approaches were employed to accommodate the 17.2% volumetric hydride induced strain accompanying the phase transformation. In the first approach, this strain is assumed to accommodate along both a and c-axis of the hcp α-Zr crystal based on known crystallography, while the second approach assumed the entire strain was in the alloy material’s circumferential direction, with zero expansion in either the axial or radial directions. A finite element model fully coupling diffusion with stress analysis is used to gain insight into the progression of hydride precipitation and the associated effect on the stress/strain at the tip of a crack. The shapes and distributions of hydrided material when the entire transformation strain is confined to the circumferential direction of the model were found to be close to experimental observations. However, the impact of the hydride induced strain and matrix plasticity was found to be insufficient to drive the experimentally observed relaxation.