Precipitation of silicon in aluminum-silicon: A calorimetric analysis of liquid-quenched and solid- quenched alloys

The precipitation of silicon in aluminum-silicon was investigated by calorimetric analysis of liquid-quenched (LQ; rapidly solidified) and solid-quenched (SQ: quenched after annealing the solid at elevated temperature) alloys. Nonisothermal annealing experiments (differential scanning calorimetry) were performed using specimens containing 1.3 to 19.1 gross at. pct Si. Initial and resulting microstructures were characterized by X-ray diffraction and transmission electron microscopy. Quantitative analysis showed that heat production in LQ alloys is due to precipitation of dissolved silicon and coarsening of silicon particles already present in the as-liquidquenched state, whereas heat production in SQ alloys can be ascribed solely to precipitation. The kinetics of silicon precipitation are affected by quenched-in excess vacancies in both LQ and SQ alloys. The grain-boundary area as a function of gross silicon content is discussed in terms of its effect on vacancy annihilation and the resulting effective activation energy.