In presence of hydrogen, crack-induced hydride formation can occur in specific metallic structures, reduce their mechanical properties and facilitate failure. Phase boundaries have been observed to be preferential site for hydride formation. The inevitable presence of both cracks and phase boundaries is consequently a threat for these metals. This paper presents a phase field approach describing hydride formation induced by a crack lying near a grain boundary, by using Allen-Cahn's formulation associated with linear elastic fracture mechanics. The analysis of the results reveals an enhancement of crack-induced hydride development in the proximity of a grain boundary.
Hydrogen-rich environments are commonly seen as threats to metallic structures in operation. The interaction hydrogen/matter can lead to the reduction of the mechanical performance or even the derogation of the function of a component (ASM International, 2002). In specific metals such as titanium and zirconium alloys, this interaction can result in the formation of brittle phases within the bulk affecting the material properties. One hydrogen-induced degradation process involving hydride formation is the so-called delayed hydride cracking (DHC), for which crack propagation is delayed and occurs stepwisely as brittle hydride phases form in the crack-tip vicinity (Puls, 2012). In fact, the stress is known to be a vector for hydrogen diffusion in metallic materials. Hydrogen migrates towards high hydrostatic stress regions, where the concentration can exceed the solubility limit, which can itself be reduced by the presence of tensile stress. Thus, hydrides possibly form in tensile stress regions such as areas in proximity of a crack-tip as observed in (Shih, et al., 1988). Additionally, grain boundaries have been found to be preferential sites for hydride formation in polycrystals (Coleman, 2003; Liu, et al., 2018). When a crack lies near a grain boundary, hydride formation might be promoted by the combination of the two aforementioned situations. The phase-field theory based on a smooth interface formulation is suited, in term of simplicity and computational efficiency, to study microstructure evolution and second phase precipitation (Provatas & Elder, 2010). This approach has been used in numerous applications (Moelans, et al., 2008; Chen, 2002) and has been successfully employed to study defect-induced hydride formation (Nigro, et al., 2018; Bjerkén & Massih, 2014; Massih, 2011). In this paper, a phase field method is used to investigate hydride precipitation, within lifetime predictions of industrial materials, in presence of an opening crack lying near a grain boundary. To this end and to minimize the computational cost, linear elastic fracture mechanics (LEFM) is utilized to model the implicit presence of the crack near and perpendicular to the interface between two similar phases. The solid solution is assumed to be isotropic and elastic. Possible plastic effects are disregarded. During phase transformation, a volume change connected to the lattice misfit between hydride and solid solution has been observed to occur (Banerjee & Arunachalam, 1981; Carpenter, 1973). This type of dilatational effects is taken into account in the model. Moreover, the bulk energy density, also known as the Landau potential, is written such that the energy barrier level in the grain boundary can be controlled. In this paper, the evaluation of the model potentialities is made by focusing on the effects of the crack-tip/grain boundary distance and the magnitude of the energy barrier in the grain boundary on hydride formation. The results are presented for a Ti-6Al-4V alloy for which we consider a α/α crystal interface.
