PRECIPITATION OF METASTABLE DELTA' IN AL-1-CENTER-DOT-9LI-2-CENTER-DOT-5MG ALLOY

Metastable delta' precipitates formed at 473 K in Al-1.9Li-2.5 Mg allo y have been studied via specific heat measurements, analytical transmi ssion electron microscopy, and atom probe field ion microscopy. The pe ak temperature of heat absorption associated with the resolution of de lta' into the matrix increases by more than 45 K after prolonged aging , i.e. the thermal stability of delta' is enhanced by the aging treatm ent. The coarsening of delta' obeys Ostwald ripening kinetics. However , the increase in the peak temperature is not related to the increase in size of delta' but is instead attributed to an increase of the Mg c ontent in delta' precipitates; presumably, delta' changes from Al3Li t o (Al,Mg)3Li.