Heat-treatment parameters for a 359/Al 2O 3/10p composite modified with 0.07 wt% strontium

Abstract

In melt-produced composites, the properties of particle-reinforced aluminum metal-matrix composites (MMCs) can be compromised by the interfacial reactions that may occur between the molten matrix and the reinforcement depending on the melt conditions. The reinforcement can also considerably influence the response of the matrix alloy to solution heat treatment and age-hardening. A study of the melting and heat-treatment parameters and their effect on the tensile properties and fracture behaviour of 359/Al 2O 3/10p composite modified with 0.07 wt% (700 ppm) strontium (Sr) was conducted, with the aim of assessing the properties of particulate alumina (Al 2O 3) as reinforcement compared to silicon carbide (SiC), as well as the effect of a high Sr content. It was found that the latter significantly improved the fluidity/castability of the composite alloy. Although overmodification occurred, no associated Sr-containing intermetallics were observed, owing to the high silicon content of the matrix. During solidification, mechanical trapping of the alumina particles was observed for particle sizes much larger than that of the α-aluminum dendrites, as well as nucleation of eutectic silicon on their surfaces. Spinel particles were also observed to form at the alumina-particle/matrix interface, their formation progressing during solution heat treatment. Prolonged solution treatment at high temperatures resulted in a significant lowering of the strength upon subsequent ageing. Isothermal/isochronal ageing of the composite showed that the alumina particles only accelerate the kinetics but do not alter the precipitation behaviour of the matrix. Composite fracture can occur within the aluminum matrix, along the matrix/reinforcement interface, as well as by shearing of the alumina particles.