The effect of quenching history, quenching temperature and trace impurities on vacancy clusters in aluminium and gold

Abstract

It has been shown that in zone-refined aluminium, under specific quenching conditions, regions of stacking fault bounded by dislocation loops with a 1/3 [111] Burgers vector can be formed. The same type of defect can be produced by multiply quenching 99•995% aluminium. Repeated quenching therefore produces effects similar to those produced by increased purity. The observation of large, faulted loops in aluminium, a material assumed to have a high stacking-fault energy, is explained in terms of the difficulty of nucleating a Shockley partial dislocation to unfault the loop. The occurrence of such loops in the purest material only, is explained by postulating that in the purer material loop nucleation is more difficult than in a material of moderate purity. This results in some loops forming at low temperature, a condition which favours the formation of faulted loops. The purification produced by multiple quenching is discussed in terms of the sweeping out of impurities by the vacancies as they anneal out to the surface (the predominant vacancy sink in the present experiments). This [quench refining] effect has been confirmed in a parallel study of the effect of purity and multiple quenching on the type of clustered-vacancy defect in gold. In particular, it has been shown that the quenching temperature corresponding to the transition from the formation of stacking-fault tetrahedra to the formation of black-spot defects can be lowered by multiple quenching.