Abstract
Aging behavior of Al-base alloys containing 0.7∼2.7wt% Li is studied by measuring specific heat vs temperature (S-T) curves during heating of the alloys which have been quenched from 540°C and then aged at various temperatures ranging from room temperature to 300°C for various times. The S-T curves are interpreted by comparing the shapes of the curves of other age-hardening alloys, and an ordering alloy. It is proposed that the precipitation in Al–Li alloys takes place in a five-stage process; solid solution→disordered G. P. zones→ordered G. P. zones→δ″ (short range ordered Al3Li)→δ′ (long range ordered Al3Li)→δ(AlLi), during isothermal aging at temperatures below ∼90°C, where, δ″ is a fine particle assumed to be a one- or two-dimensional superlattice of Al3Li in which only the short range order is developed. The symbol δ′ is restricted to describe only the large particle of a three-dimensional superlattice of Al3Li. Experimental determinations are made for the stable and metastable solid solubility curves of the δ phase and G. P. zones. A “pseudo-critical temperature” (Tc′) of δ″ and the critical temperature (Tc) of δ′ are obtained and plotted against the Li content of the alloys. Here, the “pseudo-critical temperature” is the temperature above which the δ″ is considerably destroyed during heating of the alloys.
The δ″ and δ′ phases are already present in the as-quenched state from 540°C, in the case of alloys containing more than 1.7wt% Li.
