Kinetics of precipitation and of relaxation of precipitation-induced stresses in aluminium-silicon alloys

Abstract

Liquid quenching as well as solid quenching of aluminium-silicon alloys yields an aluminiumrich matrix supersaturated with silicon. Ageing of quenched alloys implies silicon precipitation (cf. phase diagram in Ref. i). Since the atomic volume of silicon as precipitated (diamond structure) is much larger than as dissolved in the Al-rich matrix, large stresses will develop in the Al-rich phase on ageing. This is clearly demonstrated by the behaviour of the Al-rich phase lattice parameter, aAl, as a function of ageing time, t, cf. Fig. 1 (2, 3). Silicon precipitation from the Al-rich phase causes an overall increase of the Al-rich phase lattice parameter, but precipitation-induced stresses and their subsequent relaxation cause a hump, AaXl(tmax), (3). This hump occurs at much shorter times for liquid-quenched (LQ) than for solid-quenched (SQ) alloys (compare Fig. la to Fig. ib). For the LQ alloys the determination and interpretation of ~Xl(tmax) is hardly possible because of the interwoven effects On the Al-rich phase lattice parameter of silicon precipitation, precipitation-induced stresses and relaxation stresses. In this letter the silicon precipitation and subsequent relaxation of precipitation-induced stresses in solid-quenched AISi alloys are investigated by measurement of the Al-rich phase lattice parameter as a function of temperature and time of ageing.