Faulted dislocation loops in quenched aluminium

Abstract

Dislocation loops containing stacking faults have been observed in quenched aluminium using the electron microscope. It is found that lowering the quenching rate relaxes the stringent purity conditions governing the retention of faulted loops, such that a high proportion of the loops in oil-quenched aluminium (99·97 % purity) contain faults. The results are discussed in terms of the influence of quenching stresses on the stability of the faulted loop. A low quenching rate also favours double-faulting, in which a Frank sessile dislocation loop is nucleated on an existing fault. The stability of this defect is attributed to the lower stacking-fault energy of a double fault compared with that of two intrinsic stacking faults. Under the action of stress the outer fault of the double defect can sometimes be removed to produce a faulted loop within a prismatic loop. The annealing behaviour of the four different types of dislocation loop, i.e. prismatic, single-faulted, double-faulted and faulted prismatic loop, has been studied using the heating stage. The kinetic data have been analysed to deduce a value for the intrinsic and extrinsic stacking-fault energy of aluminium.