Chemical equilibrium fracture mechanics − hydrogen embrittlement of two-phase hydride forming alloys

Abstract

Chemical Equilibrium Fracture Mechanics (CEFM) is a multidisciplinary approach of solid mechanics, material science, thermodynamics and mathematics, for the study of crack-tip fields and structural integrity, based on the assumption of material deterioration under chemical equilibrium. A major application has been the development of crack-tip fields in hydride and non-hydride forming alloys, subjected to mechanical loads in a hydrogen environment. According to earlier studies, in single phase alloys, the crack-tip fields, in the case of hydride precipitation, deviate significantly from the well-known fields in linear elastic and elastic–plastic materials, thus necessitating the modification / extension of linear elastic, elastic–plastic and constraint-based fracture mechanics. In the present study, CEFM is applied to two-phase hydride forming alloys, by taking into account hydride precipitation as well as hydrogen residing in both interstitial lattice sites and dislocation traps. The distributions of stress and hydrogen concentration near the tip of a plane strain mode I crack are derived and applied to widely used α/β titanium alloys. The deviations from the crack-tip fields of hydrogen-free metals are confirmed in the case of two-phase alloys as well. It is shown that the partitioning of hydrogen in solid solution in the two phases, near a crack-tip in the hydride precipitation zone, is controlled by the constant hydrostatic stress and therefore varies, depending on alloy yield stress and average hydrogen content. Alloy yield stress has also a strong effect on average hydrogen content, at which hydride precipitation and therefore embrittlement initiates.