Thermodynamics and kinetics of interfacial decohesion

Thermodynamic and kinetic aspects of interfacial decohesion are analyzed for a uniform separation along an interface subjected to a uniform tensile stress normal to the interface. During the separation process, an embrittling impurity can penetrate into the interface and reduce its cohesive strength. Rice (1976) and Hirth and Rice (1980) analyzed this process in great detail, and in particular considered two limiting cases: slow separation (constant chemical potential of the impurity) and fast separation (constant impurity concentration). Our work extends their analysis to intermediate, time-dependent situations, which correspond to the problem at hand. A phenomenological model is introduced to describe the free energy of the interfacial 'solid solution', stress-separation curves as functions of the impurity concentration, and kinetics of separation. The observed separation kinetics, as well as the work of decohesion, cohesive strength and other interfacial properties, depend on the interrelation between the strain rate and the impurity diffusion rate. An equation of stress-driven diffusion along the interface is derived, and the origin of the stress effect on diffusion is analyzed.