Ductile-brittle behavior was considered within the framework of the Peierls analysis of dislocation emission from a crack tip. First-principles quantum-mechanical calculations were carried out for the unstable stacking energy along the Shockley partial slip route. The calculations were based upon density functional theory and the local density approximation, and took full account of atomic and volume relaxation. It was found that the value for Al was 0.224J/m2. According to the Peierls prediction, such a value indicated a brittle solid; but Al was ductile. It was suggested that Al was perhaps brittle at absolute zero and became ductile at a finite temperature due to the motion of pre-existing dislocations which relaxed the stress concentration at the crack tip. Alternatively, it was possible that dislocation emission at the crack tip was a thermally activated process. A third possibility was that this metal was ductile at all temperatures, and that the theoretical model which was used would have to be significantly improved in order to resolve the apparent contradiction.

Slip Energy Barriers in Aluminium and Implications for Ductile-Brittle Behaviour. Y.Sun, E.Kaxiras: Philosophical Magazine A, 1997, 75[4], 1117-27