On hydrogen-induced void nucleation and grain boundary decohesion in nickel-base alloys

Experimental evidence indicates that nickel-base alloys fail in the presence of hydrogen by ductile intergranular fracture. The degradation mechanism involves void nucleation at grain boundary carbides and grain boundary decohesion. In this study, a micromechanical model is suggested to understand the interaction of void nucleation and growth with the failure of the grain boundaries. The analysis is carried out at a unit cell comprising an elastic particle imbedded in a ductile matrix, a grain boundary along a plane of symmetry of the cell, and loaded in plane strain perpendicularly to the grain boundary. A phenomenological model for hydrogen-induced decohesion calibrated at the fast-separation limit of the decohesion theory of Rice [1], Hirth and Rice [2], and Rice and Wang [3] was used to describe the hydrogen effect on the cohesive properties of the particle/matrix interface and grain boundary. The finite element results indicate that hydrogen embrittlement of the alloy 690 is controlled by hydrogen assisted void nucleation at the carbides. The effect of hydrogen on grain boundary cohesion is almost negligible. The grain boundary decohesion, which proceeds almost instantaneously upon initiation, is caused by normal stress elevation due to the interaction of the void with the applied load. Lastly evaluative statements are made on the quantitative effect of hydrogen on the fracture toughness of the alloy 690.