Dilatometry revealing Si precipitation in Al–Si-alloys

Abstract

Abstract The precipitation kinetics of Si after different thermal treatments in AlSi-alloys have been studied by means of isothermal calorimetry, differential scanning calorimetry and dilatometry tests. The measured exothermic and endothermic effects produced by the precipitation and dissolution of Si are correlated with the increase and reduction in the coefficient of thermal expansion CTE(T), respectively. The deviation of the CTE(T) of the studied AlSi1.1 – 1.7 alloys with respect to pure Al correspond to the volume fraction of precipitating Si according to thermodynamic solubility limits. Two different kinetics in the growth of Si precipitates are distinguished: while Si precipitates in the range of 200 – 300 °C during heating of solution quenched samples supersaturated with vacancies, retardation into the temperature range of 300 – 420 °C is observed in samples slowly cooled after solution treatment. The presence of the eutectic Si increases the quench sensitivity of the precipitation kinetics of Si as observed in CTE(T) curves. The ripening of the eutectic Si during prolonged solution treatment retards the Si precipitation slightly.