β Phase transformations in the Cu–11mass%Al alloy with Ag additions

Abstract

In the Cu-Al system, due to the sluggishness of the b $ (a ? c1) eutectoid reaction, the b phase can be retained metastably. During quenching, metastable b alloys undergo a martensitic transformation to a b 0 phase at Al low content. The ordering reaction b $ b1 precedes the martensitic transformation. The influence of Ag additions on the reactions containing the b phase in the Cu-11mas- s%Al alloy was studied using differential scanning calo- rimetry and in situ X-ray diffractometry. The results indicated that, on cooling, two reactions are occurring in the same temperature range, the b ? (a ? c1) decompo- sition reaction and the b ? b1 reaction, with different reaction mechanisms (diffusive for the former and ordering for the latter) and, consequently, with different reaction rates. For lower cooling rates, the dominant is the decomposition reaction and for higher cooling rates the ordering reaction prevails. On heating, the (a ? c1) ? b reverse eutectoid reaction occurs with a resulting b phase saturated with a. The increase of Ag concentration retards the b ? (a ? c1) decomposition reaction and the b ? b1 ordering reaction, which occurs in the same temperature range, becomes the predominant process.