9 - Hydrogen and Crystal Defects Interactions: Effects on Plasticity and Fracture

Abstract

Many mechanisms have been proposed to describe the different hydrogen embrittlement processes. The materials' variability and their properties, their conditions of use and the nature of the surrounding environment make it difficult to establish a single fundamental theory or approach to describe hydrogen embrittlement (HE). Experimental validations of these mechanisms are often based on a set of conditions (metallurgy, mechanical loading, surface hydrogen activity, volume concentration, etc.) which are favorable to the emergence of a particular mechanism, making it difficult to get an overview of the various interaction modes between the adsorbed hydrogen or hydrogen in solid solution and the crystal defects. Kirchheim points out that from a thermodynamic point of view, all the approaches proposed in the literature are based on a decrease of the defect formation or emission energy (dislocations, vacancies, microcavities), of the cohesion and surface energies, or the energy associated with the displacement of defects such as dislocations, in the presence of adsorbed or absorbed hydrogen. The brittle fracture of deformable crystal materials subjected to quasi-static monotonic loading arises from a competition between, on the one hand, the accumulation rate of elastic energy at the crack tip until a critical value intrinsic to the material is reached (the critical elastic energy release rate, related to the fracture toughness and to Young's modulus) and, on the other hand, the dynamics of crystal defect creation/multiplication that blunt the existing cracks or their nuclei, or screen the loading applied to them. Understanding the hydrogen-induced damage and fracture requires taking into account the effects of hydrogen on the crystal cohesion as well as the plasticity or the phase transformations at the crack tip. In the presence of intrinsically brittle interfaces, heterogeneities or phases, hydrogen can affect the material's cohesion through (co-)segregation effects, through the accumulation of deformation incompatibilities or induced precipitation. If, from a formal point of view, these mechanisms belong to the two preceding categories (intrinsic embrittlement or plasticity modification), from a practical point of view, their study and modeling have led to specific developments. We thus find four major classes of hydrogen-assisted damage mechanisms in the literature: brittle fracture through the reduction of the material's cohesion, damage due to the formation of vacancies and their condensation, fracture due to the local increase of plasticity (“direct”, through localized shear; or “indirect”, through the accumulation of internal stresses near the interfaces) and fracture through the formation of a brittle phase (hydride). The first model is based on the idea of the decrease in the interfaces' cohesion energy (lattice, grain boundaries, inter-phase, etc.) caused by the segregation of hydrogen, which promotes the formation and propagation of cracks. The second model is based on the formation of new defects such as vacancies followed by microcavities in the presence of hydrogen. The multiplication and localization of these defects can lead to the initiation and propagation of cracks. The third approach of HE is based on the emission of dislocations favored by the presence of hydrogen (decrease of elastic interactions and line tension) which can induce the initiation of a crack and the localization of the plastic deformation in front of the crack tip. Finally, the fourth model is based on the formation of a brittle hydrogen-rich hydride phase. In order to better understand the different aspects of the hydrogen-material interaction as well as its harmful effects, we will describe in more detail the four HE models and the various associated mechanisms.