Micromechanics of the hydrogen effect on plasticity and interfacial decohesion

Micromechanical models have been developed to investigate the hydrogen effect on plasticity and interfacial decohesion. The hydrogen effect is modeled through hydrogen-induced lattice dilatation, and material softening due to hydrogen-enhanced dislocation mobility. Analytical and numerical studies show that hydrogen can induce plastic shear localization in a power-law hardening von Mises material in a plane-strain tension specimen, and that it can also reduce the macroscopic strain at which necking bifurcation commences. Numerical studies on the effect of specimen geometry on hydrogen embrittlement at a crack tip show that hydrogen-induced lattice dilatation suppresses plasticity and void growth in a deeply notched specimen, while it does the opposite in specimens with shallow cracks. However, hydrogen-induced material softening always increases plasticity and void growth. Thus, hydrogen embrittlement might be more significant in specimens with shallow cracks. Studies on the hydrogen effect on void growth under triaxial stress states indicate that hydrogen does not significantly alter the initial and intermediate stages of void growth. However, it promotes void coalescence by reducing the resistance to plastic flow localization in the inter-void material ligaments.; A traction-separation law that describes the cohesion of an interface in the presence of hydrogen is suggested and implemented through interfacial cohesive finite elements to study interfacial debonding around an elastic particle imbedded in an elastoplastically deforming matrix, while transient hydrogen transport takes place in the matrix, the particle, and the opening interfacial channel. Finite element analyses demonstrate that both hydrogen-induced reduction of interfacial cohesion and matrix softening acting concurrently lead to a reduction of the void nucleation stress at the particle-matrix interface. However, while hydrogen-induced decohesion decreases the void nucleation strain, matrix-softening increases it. Some other issues such as the effect of interfacial diffusivity and strain rate on decohesion are also addressed. Numerical simulations of hydrogen-induced intergranular fracture in nickel-base alloy 690 have been carried out. It is found that hydrogen induced intergranular fracture process in alloy 690 is controlled by the separation process at the interfaces between the grain boundary carbides and the nickel matrix.