Microstructural development in Al-Si powder during rapid solidification

Powder metallurgy has become an increasingly important form of metal processing because of its ability to produce materials with superior mechanical properties. These properties are due in part to the unique and often desirable microstructures which arise as a result of the extreme levels of undercooling achieved, especially in the finest size powder, and the subsequent rapid solidification which occurs. A better understanding of the fundamental processes of nucleation and growth is required to further exploit the potential of rapid solidification processing. Aluminum-silicon, one alloy of significant industrial importance, was chosen as a model for simple eutectic systems displaying an unfaceted/faceted interface and skewed coupled eutectic growth zone. Al-Si powder produced by high pressure gas atomization was studied to determine the relationship between microstructure and alloy composition as a function of powder size and atomization gas. Critical experimental measurements of hypereutectic (Si-rich) compositions were used to determine undercooling and interface velocity, based on the theoretical models which are available. A revised microstructural map is proposed which allows the prediction of particle morphology based on temperature and composition