SLIP ENERGY BARRIERS IN ALUMINUM AND IMPLICATIONS FOR DUCTILE-BRITTLEBEHAVIOR

We consider the ductile-brittle behaviour of aluminium in the framewor k of the Peierls-model analysis of dislocation emission from a crack t ip. To this end, we perform first-principles quantum mechanical calcul ations for the unstable stacking energy gamma(us) of aluminium along t he 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 pr edict a brittle solid which poses an interesting problem since alumini um 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 concentrat ion at the crack tip; secondly dislocation emission at the crack tip i s itself a thermally activated process; thirdly aluminium is actually ductile at all temperatures and the theoretical model employed needs t o be significantly improved in order to resolve the apparent contradic tion.