Crack Initiation in the Zinc-Mercury Embrittlement Couple

Abstract

Cleavage fracture data from zinc crystals tested in tension in liquid mercury environment at 298˚K and in an inert environment at 77˚K have been used to provide support for the validity of a fracture criterion and to derive reliable values of the energy to initiate cleavage fracture, OI. A value of ~ 55 ergs/cm2 has been derived for OI for zinc in liquid mercury at 298˚K. This value is significantly lower than that of ~ 100 ergs/cm2 derived for O1 for zinc tested in an inert environment at 77˚K. The ratio of the fracture initiation energy for zinc in mercury at 298˚K to that in an inert environment at 77˚K is less than unity (~ 0.55) and is defined as the coefficient of embrittlement for crack initiation, ηI. ηI is considered to be the ratio of the energies to break atomic bonds across the fracture plane in the presence and in the absence of liquid mercury atoms. Also, for zinc-mercury couple, ηI ≃ ηP (where ηP is the coefficient of embrittlement for cleavage crackpropagatpion), probably because in zinc, both crack initiation and propagation occur in the same plane. Since ηI ≃ ηP < 1, the results obtained are considered to provide quantitatie support for the mechanism of liquid metal embrittlement in which adsorption of liquid metal atoms reduces cohesion at the sites of high stress concentrations on the surface of the solid and at the tip of the propagating crack, facilitating thereby crack nucleation and crack propagation in the solid metal.