Slip energy barriers in aluminium and implications for ductile-brittle behaviour

We consider the ductile-brittle behaviour of aluminium in the framework of the Peierls-model analysis of dislocation emission from a crack tip. To this end, we perform first-principles quantum mechanical calculations for the unstable stacking energy gamma(us) of aluminium along the Shockley partial slip route. Our calculations are based on density functional theory and the local density approximation and include full atomic and volume relaxation. We find that, in aluminium, gamma(us) = 0.224 J m(-2). Within the Peierls-model analysis, this value would predict a brittle solid which poses an interesting problem since aluminium is typically considered to be ductile. The resolution may be given by one of three possibilities: firstly, aluminium is indeed brittle at zero temperature and becomes ductile at a finite temperature owing to motion of pre-existing dislocations which relax the stress concentration at the crack tip; secondly dislocation emission at the crack tip is itself a thermally activated process; thirdly aluminium is actually ductile at all temperatures and the theoretical model employed needs to be significantly improved in order to resolve the apparent contradiction.