PRECIPITATION IN AN ALUMINUM-14 AT.PERCENT LITHIUM ALLOY

The early stages of low-temperature ageing of an Al-14 at.% Li alloy h ave been studied by differential scanning calorimetry, X-ray diffracti on and transmission electron microscopy. Results show precursor struct ures form before delta' (Al3Li) precipitation and these have been attr ibuted to congruent ordering and spinodal decomposition. The developme nt of the precursor structures are shown to be controlled by excess qu enched-in vacancies and therefore very sensitive to quench rate, hold time at room temperature and small amounts of cold work applied betwee n quenching and ageing. Large amounts of cold work (95%) do not comple tely destroy the ordered structure of the as-quenched alloy but the hi gh-diffusivity channels arising from the presence of dislocations are found to influence both the precursor events and the delta' precipitat ion.