The thermodynamics of decohesion

We present a thermodynamic description of decohesion that provides a link between first principles studies of decohesion and cohesive zone models used in continuum simulations of crack growth. The properties of a cohesive zone are described by thermodynamic excess variables extracted from first-principles calculations. Applied to decohesion of fcc aluminum, we find that the excess energy for decohesion along adjacent (1 1 1) planes is well described by the universal binding relation of Rose et al. [Phys. Rev. Lett. 47 (1981) 675; Phys. Rev. B 28 (1983) 1835]. We also present a first principles model to investigate the effect of impurity atoms on slow decohesion when the impurity chemical potential can remain constant by modifying the impurity concentration in the decohering zone. In studying the effect of hydrogen or oxygen impurities on decohesion of aluminum along a pair of (1 1 1) planes, the model predicts a Van der Waals transition above a critical impurity chemical potential. This transition involves the saturation with impurity atoms of the region between the decohering planes and leads to a dramatically reduced maximum stress for decohesion.