(V) Effects of Sn Addition on Mechanical Properties in a Binary Mg-1Ca Alloy

Microstructural study of Mg-Ca alloy system with addition of Sn is a new opportunity for hardening in Mg alloys. During thermo-mechanical treatment of this alloy system, the mechanism for deformation is yet to establish. One window for such observation would be study of grains and microstructure. Therefore, grain-refining by addition of Ca and promoting precipitation by Sn addition is expected to contribute to very effective strengthening in Mg alloys. This work focuses on understanding the effects of Sn in a binary Mg-Ca system. Sn was added in variable quantity (1%, 2% and 4%). The alloys were homogenized prior to hot-rolling. Tensile specimens were prepared from the flat-rolled sheets and tested in uniaxial tension at four different temperatures (300, 350, 400 and 450˚C) and at two different strain rates (5x10^-5 s^-1 and 10^-5 s^-1). Microstructures were observed by using optical microscope and SEM. The experimental results showed that Sn refined the grains and increased the formation of primary intermetallic particles. With the addition of Sn, tensile strength and per cent elongation values were typically increased. Furthermore, with temperature, tensile strength values were decreased and per cent elongation values were increased. For slower strain rate (10^-5 s^-1), tensile strength values were generally lesser, contrary to elongation values, compared to higher strain rate (5x10^-5 s^-1). Maximum tensile strength was 92 MPa at temperature 300˚C and strain rate 5x10^-5 s^-1 for 1% Sn addition, while maximum elongation was 231% at temperature 400˚C and strain rate 10^-5 s^-1 for 1% Sn addition. Optimum results were found for Mg-1Ca-1Sn alloy. Such finding is mostly attributed to the second phase particles formed by the addition of Sn and also the hindering of grain growth by the pinning effect of particles at elevated temperature.