A quantum-mechanically informed continuum model of hydrogen embrittlement

摘要

We present a model of hydrogen embrittlement based upon: (ⅰ) a cohesive law dependent on impurity coverage that is calculated from first principles; (ⅱ) a stress-assisted diffusion equation with appropriate boundary conditions accounting for the environment; (ⅲ) a static continuum analysis of crack growth including plasticity; and (ⅳ) the Langmuir relation determining the impurity coverage from its bulk concentration. We consider the effect of the following parameters: yield strength, stress intensity factor, hydrogen concentration in the environment, and temperature. The calculations reproduce the following experimental trends: (ⅰ) time to initiation and its dependence on yield strength and stress intensity factor; (ⅱ) finite crack jump at initiation; (ⅲ) intermittent crack growth; (ⅳ) stages Ⅰ and Ⅱ of crack growth and their dependence on yield strength; (Ⅴ) the effect of the environmental impurity concentration on the threshold stress intensity factor; and (ⅵ) the effect of temperature on stage Ⅱ crack velocity in the low-temperature range. In addition, the theoretically and experimentally observed intermittent cracking may be understood as being due to a time lag in the diffusion of hydrogen towards the cohesive zone, since a buildup of hydrogen is necessary in order for the crack to advance. The predictions of the model are in good quantitative agreement with available measurements, suggesting that hydrogen-induced degradation of cohesion is a likely mechanism for hydrogen-assisted cracking.