Structure and Thermal Expansion of High-alloy Alloys of the Al-Si-Cu System for Special Purposes
The influence of copper on the microstructure and temperature coefficient of linear expansion (TCLЕ) of Аl-30÷40% Si-Cu alloys was studied. The results of dilatometric studies have shown that the introduction of alloying elements in equal quantities leads to a decrease in the thermal expansion of the Аl-30%Si-30%Cu alloy to values of 6,0÷7,0·10-6 deg-1. Increasing the copper content to 40% not only does not lead to a further decrease in the TCLЕ values, but its increase is observed in the high-temperature test range. It was found that an alloy with an equal content of alloying elements Al-40%Si-40%Cu has the lowest TCLЕ values in the entire test interval (α50-150 = 4,5·10-6 deg-1) than triple alloys with the same total content of alloying elements, but with a different ratio. Metallographic analysis has shown that the introduction of copper into alloys with a non-eutectic composition of Al-30%Si-Cu, in which microstructure, in addition to eutectic, initially contains particles of the siliceous phase, contributes to an increase in the size of these particles. The structure of Аl-40%Si-Cu alloys is characterized by large plate crystals of the siliceous phase. Moreover, with an increase in the copper content to 50%, the volume fraction of eutectic decreases. It is shown that the limit of solubility of silicon in aluminum decreases with high copper content.