FRACTURE FEATURES OF A SILICON-DISPERSED ALUMINUM-ALLOY EXTRUDED FROM RAPIDLY SOLIDIFIED POWDER

The tensile fractography of an AI-20Si-3Cu-1 Mg alloy consolidated from rapidly solidified powder by extrusion has been investigated using optical and electron microscopy, and related to the processing conditions as well as the tensile behaviour of the alloy at room and elevated temperatures. The alloy studied shows distinct fracture features owing to the presence of dispersed silicon crystal particles with a bimodal distribution in size and of prior powder particle boundaries in the extrudate. It has been found that at room temperature cracks initiate by cracking the primary silicon crystal particles. Crack propagation occurs along the interfaces between the eutectic silicon crystal particles and the matrix and also between the prior powder particles, where microvoids are formed by the interfacial decohesion. At 300 °C, the fracture of the alloy involves microvoid nucleation, growth and coalescence at the interfaces between the silicon crystal particles and the aluminium matrix and between the prior powder particles. It has also been observed that the fractographic features of the alloy correspond well to the processing conditions including extrusion temperatures and subsequent heat treatment. The importance of minimizing the coarsening of the silicon crystals in processing in order to use the full strength potential of the alloy investigated is emphasized. © 1990 Chapman and Hall Ltd.