A KINETIC-MODEL FOR DYNAMIC EMBRITTLEMENT

A kinetic model is offered here to describe the intergranular cracking occurring in polycrystalline materials as the result of stress-induced grain boundary penetration of a surface-adsorbed embrittling element. This stress-driven inward diffusion of an embrittling species causes a reduction in the cohesive strength of the grain boundary and leads to what we call dynamic embrittlement. The theoretical analysis shows that in the case of an elastic-plastic stress field, embrittling atoms diffuse into the stressed boundary, and they eventually reach a maximum concentration in the region of the maximum tensile stress ahead of the crack tip. This result is applied to the case of sulfur-induced cracking in a MnMoNiCr steel, coupled with a fracture criterion in which the stress for decohesion is inversely related to the concentration of the embrittling element. The crack growth rate is calculated as a function of the applied stress, the temperature, and the diffusivity of sulfur, and it is shown to be consistent with the experimental data. It is concluded that dynamic embrittlement is the rate-controlling mechanism of the cracking of this steel (i.e., stress-relief cracking) in a range of applied stress intensity which corresponds to brittle intergranular fracture.