Void nucleation and cracking at grain boundaries

Abstract

Dimpled grain boundary fracture occurs in creep tests, stress relief cracking and hydrogen attack of steels. While the growth of these voids by grain boundary diffusion is well established, the mode of void nucleation is uncertain. It is shown that the reduction of surface energy by solute adsorption plays an essential role in giving easy void nucleation. A calculation is given for the stress needed for this mode of nucleation using data for phosphorous in steel. Numerous examples exist of strongly adsorbing solute inducing elevated temperature grain boundary cracking. A row of voids growing by stress driven boundary diffusion is shown to develop a tensile stress maximum which aids void nucleation, giving rise to dimpled grain boundary fracture. Our cracking model involves repeated void nucleation and is thus fundamentally different from the steady-state Hull–Rimmer model. At times and temperatures too low for void formation, adsorption can lead to smooth grain boundary cracking at a rate controlled by solute diffusion (adsorption).