Effects of Sn on microstructure and dynamic grain growth in a binary Mg-Ca alloy

Abstract

The evolution of microstructure and dynamic grain growth of a binary Mg-1Ca alloy with different tin addition (1, 2 and 4 wt.%) was investigated under various conditions of temperatures and strain rates. Mg-1Ca, Mg-1Ca-1Sn, Mg-1Ca-2Sn and Mg-1Ca-4Sn samples were cast and homogenized. The homogenized samples were rolled at 4000C. Optical microstructures of both cast and rolled samples were taken for microstructural analysis. Scanning electron microscopy of four rolled samples was carried out for identifying phases present. Tensile test of rolled alloys was carried out at temperatures 300, 350, 400 and 450°C, and at two different strain rates 5x〖10〗^(-4) s^(-1) and 1x〖10〗^(-4) s^(-1). Optical microstructures of tensile tested samples were acquired for observing dynamic grain growth. Samples were taken from six different location of failed (tensile loaded) samples. For understanding deformation mechanisms during grain growth and total deformation, activation energy, strain hardening exponent and strain rate sensitivity index were calculated. It was found that tin addition increased dendrite formation in as-cast alloys and continuously reduced grain sizes of rolled alloys. Addition of small amount of Sn increased both strength and elongation to failure values. However, higher amount of Sn addition lowered the properties to some extent due to formation of excessive primary intermetallic particles. At lower temperatures, work hardening rate is higher leading to higher strength and lower elongation to failure values. Conversely, at elevated temperatures, strength values were reduced due to inevitable softening and greater grain growth was observed as well. At slower strain rate (1x〖10〗^(-4) s^(-1)), higher dynamic grain growth was observed than higher strain rate (5x〖10〗^(-4) s^(-1)). At slower strain rate, value of strain rate sensitivities became higher (m>0.33), which indicated superplastic behavior, in addition, deformation mechanisms of grain boundary sliding and diffusional creep.