Vickers hardness measurements of unidirectionally solidified Al-Si eutectic alloy grown at different rates

Abstract

An important commercial material is the Al-Si piston alloy. It consists mainly of the Al-Si eutectic modified with small additions of Cu and Mg to improve the strength by metastable precipitation, and Ni to improve the high temperature strength. A disadvantage of these precipitation hardening alloys is the significant decrease in strength in the 150 to 250/sup 0/C temperature range due to dissolution of the precipitates. Other means to improve high temperature properties of aluminum alloys, such as addition of oxide dispersions in the Al matrix, have been successful, but require expensive sintering processes. It is of interest to determine if directional solidification can improve the mechanical properties of the Al-Si eutectic alloy. The Al-Si eutectic is of the faceted-nonfaceted type, according to the classification of Hunt and Jackson. In conventional casting at high freezing rates, it is found that the Si phase grows independently into the melt ahead of the Al phase with no noticeable coupling to it. This results in a microstructure in which the eutectic Si occurs as more or less randomly oriented needles or irregular plates in the Al matrix. Directional solidification at low rates produces ''fiber'' alignment, but the fibers have a complicated cross sectionmore » first described by Day and Hellawell. In this work studies are reported of the microstructures and hardness of the unmodified Al-Si eutectic which has been directionally solidified at different growth rates.« less