Microstructure and tensile properties of cast Al–15%Mg2Si composite: Effects of phosphorous addition and heat treatment

The effects of solution heat treatment and phosphorous addition on the microstructure and tensile properties of in situ Al–15%Mg2Si composite specimens have been investigated. The Al–15%Mg2Si composite ingot was made by in-situ process and different amounts of phosphorous (0.1, 0.3, 0.5, 0.7 and 1 wt% P) were added to the remelted composite. Then, the specimens were subjected to solutionizing at 500 °C for holding time of 4 h followed by quenching. Optical microscopy (OM) and scanning electron microscopy (SEM) indicated that phosphorous addition not only changes the morphology of primary Mg2Si particles from dendritic to a regular shape, but also it reduces Mg2Si particle size. Solutionizing led to the dissolution of the Mg2Si particles and changed their morphology to round shape. The results obtained from tensile testing revealed that both phosphorous addition and solution heat treatment improve ultimate tensile strength (UTS) and elongation (El.%) values. According to the results, the optimum tensile property was achieved by adding 0.5 wt% P to the Al–Mg2Si composite after solution heat treatment. Fractographic analysis revealed a cellular nature for the fracture surface of the MMC. As a result of P addition the potential sites for stress concentration and crack initiation areas were reduced due to microstructural modification, while increase in the number of fine dimples rendered the nature of fracture from brittle to ductile and also improved tensile properties.