Effect of hydrogen trapping on void growth and coalescence in metals and alloys

The hydrogen effect on void growth and coalescence is investigated by studying the deformation of a unit cell containing a spherical void in the presence of hydrogen. Hydrogen affects the mechanical response of the matrix material by softening the plastic response and by dilating the lattice. The intensity of these effects varies within the matrix material and depends on the amount of the local hydrogen concentration. The hydrogen concentration in the matrix is determined by assuming that hydrogen is in equilibrium with (a) local hydrostatic stress which dictates the amount of hydrogen accumulation in the stressed lattice relatively to the stress-free lattice and (b) local plastic strain which dictates the amount of hydrogen solute atoms trapped at dislocations. The coupled boundary value problem is solved by the finite element method. Numerical results for a niobium system indicate that hydrogen has no significant effect on void growth at all triaxialities when trapping is characterized by one hydrogen atom per atomic plane threaded by a dislocation. In contrast, when hydrogen solutes can be trapped by dislocations at larger amounts (e.g., 10 hydrogen atoms per atomic plane threaded by a dislocation), hydrogen was found to have a strong effect on the coalescence stage at small triaxialities (e.g., 1/3) and cause the link-up of voids. The acceleration of void coalescence serves as a mechanistic model to characterize the hydrogen-induced ductile rupture processes within the framework of the hydrogen-enhanced localized plasticity (HELP) mechanism for hydrogen embrittlement.