DELTA' NUCLEATION AND GROWTH ON LOW-ANGLE DISLOCATION BOUNDARIES IN AL-LI

A transmission electron microscopy (TEM) investigation indicates that, at small undercooling, delta' (Al3Li) precipitates are restricted alm ost entirely within the strain field of dislocations in the early stag es of aging and preferentially nucleate on edge dislocations and low-a ngle dislocation boundaries. The study is conducted on binary Al-Li al loys which are cold rolled to a 50% reduction in area. Two types of di slocation boundaries, i.e. simple tilt boundaries and a planar network of dislocations, are characterized by two-beam analysis. The role of the simple tilt boundaries on competitive growth kinetics of delta' pr ecipitates is quantitatively examined. The experimental results suppor t the related theoretical prediction on the effect of the simple tilt boundary stress field on solute diffusion. Concerning the dislocation network, delta' nucleates on dislocation node configurations where the reacted dislocation has a large edge component and at the site of the network plane where the dilatation stress is compressive. This is con firmed by calculation of the dilatation stress field of the dislocatio n nodes which also further clarifies and is consistent with the experi mental observations.