Multiscale study about the implication of plasticity and grain boundary character in hydrogen-assisted brittle fracture mechanisms in pure nickel

Overview & Synopsis

Hydrogen, increasingly being used as an alternative energy source, can lead to significant material degradation, particularly in nickel, which is widely used in superalloys. This phenomenon, known as hydrogen embrittlement, arises primarily from the interaction of hydrogen as a solute with metallurgical heterogeneities, which tend to accumulate at grain boundaries. These interactions negatively impact the mechanical properties of the material, potentially causing premature fractures with an intergranular nature, combined with plasticity mechanisms, resulting from a reduction in the cohesive energy of grain boundaries under the influence of hydrogen. For deeper understanding of hydrogen interaction with microstructural heterogeneities, a multi-scale study is being conducted on various types of nickel microstructures. The research is focused on the plasticity mechanisms that contribute to the degradation of nickel, specifically exploring the hardening and softening effects induced by hydrogen consequence on intergranular fracture. The study will investigate three types of nickel microstructures: single crystals, bicrystals, and polycrystals with experimental approach. The first part of this study will concentrate on nickel single crystals to avoid any influence of grain boundaries, allowing for a focused analysis of hydrogen effect on different crystallographic orientations. Both elastic and plastic properties will be assessed using nanoindentation (at the microscale) and tensile testing (at the macroscale). This phase aims to provide insights into the anisotropic nature of hydrogen diffusion and its impact on damage, by evaluating the elastic modulus of nickel in each orientation and analyzing dislocation activity during plastic deformation. The second part will focus on nickel bicrystals, examining four specific types of grain boundaries. Tensile tests will be employed to study the contribution of grain boundaries to the damage mechanisms by evaluating defects caused by hydrogen near these boundaries in relation with a potential transmission of slip across grain boundary. Additionally, work hardening and dislocation activity will be discussed to further elucidate the effects of hydrogen. The final part will address polycrystals, where the combined effects of different grain boundary characteristics and grain orientations will be studied. Various hydrogen concentrations will be explored to understand their impact on the degree of embrittlement in relation with the fraction of Coincident Site Lattice grain boundaries. This part of the study will examine work hardening, damage modes, and dislocation patterns, with the aim of providing elements of understanding into intergranular crack initiation propagation and fracture induced by hydrogen.