Strength and toughness of semi-solid processed hypereutectic [formula omitted] alloys

Abstract

Hypereutectic Al/Si alloys are used in the automotive industry and other applications where good wear behavior is necessary. Important improvements in properties can be obtained with increases in Si content but the AUSI phase diagram shows that high temperatures are required for melting high Si content alloys and, in consequence, a coarse network of primary Si particles leads to adverse ductility and toughness when conventional casting techniques are used. To avoid these problems, novel metal matrix composites (MMCS) based on the AUSI system and containing up to 30% Si have been prepared by a combination of spray-forming and thixoforming. Conventional mechanical properties and fracture toughness at room temperature of some MMCs obtained by combining spray-forming, extrusion and thixoforming, this last being a near-net shape technique, have been measured and have been correlated with microstructure. Microstructure and, mainly, local clustering of Si particles play a very important role in MMC mechanical behavior. Fracture initiates at particle clusters growing rapidly through CuAl{sub 2} phase and the matrix. Microstructural optimization, i.e., decreasing Si particle connectivity, would result in better mechanical behavior.