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Micromechanics and numerical modelling of the hydrogen–particle–matrix interactions in nickel-base alloys

The mechanics of hydrogen-induced decohesion and subsequent void formation at the interface of an elastic inclusion embedded in a ductile matrix is studied in an effort to understand the micromechanics of hydrogen embrittlement in nickel-base alloys that fail by ductile intergranular fracture initiating at grain boundary carbides. A phenomenological decohesion model calibrated with the use of the thermodynamic theory of Rice (1976), Hirth and Rice (1980), and Rice and Wang (1989) at its `fast-separation limit' is employed to describe the cohesive properties of the inclusion/matrix interface in the presence of hydrogen. Finite element solution to the transient hydrogen transport through the plastically deforming matrix, the elastic inclusion, and the decohering interface coupled with interfacial debonding and large-strain deformation in the surrounding matrix is obtained incrementally at a unit cell through an updated Lagrangian formulation scheme. The numerical results are used to analyse: (a) the interaction of hydrogen-induced decohesion with hydrogen-induced matrix softening; (b) the relationship between energy expenditures on bulk deformation and interfacial decohesion; and (c) the importance of parameters such as strain rate and relative magnitude between interfacial and bulk diffusivities on void nucleation at the particle/matrix interface. For material data pertaining to alloy 690, it was found that hydrogen by weakening the interface promotes plastic flow localization in the surrounding matrix. In general, hydrogen was found to decrease both the macroscopic stress and strain at which void initiation commences and reduce the energies expended on bulk deformation and interfacial separation.