The effect of quenching on the formation of g.p. zones and θ′ in al cu-alloys

Abstract

X-ray methods have been used to estimate the quantities of G.P. zones and θ formed in Al-4% Cu and Al-4% Cu 0·05% In alloys after various heat treatments. The rate of formation of G.P. zones decreases not only on slower quenching but also on lowering the reversion treatment temperature. The addition of a trace of indium (Cd, Sn) is known to decrease the rate of zone formation which is then negligible except after very fast quenching. The rate of formation of the metastable precipitate θ' is dependent on dislocations present in the alloy rather than an excess mobile vacancy concentration. Traces of indium (or cadmium) form even more efficient nuclei for θ' than dislocations but the effect of the indium (or cadmium) is destroyed by fast quenching or cold work. It appears that there is a strong indium-vacancy association which prevents excess vacancies from accelerating the diffusion rate of the copper. When sufficient dislocations are present the indium atoms are trapped and no longer nucleate θ'.