The effect of Zn/Ca ratio on the microstructure, texture and mechanical properties of dilute Mg–Zn–Ca–Mn alloys that exhibit superior strength

Abstract

The microstructure, texture and mechanical properties of a series of Mg–Zn–Ca–Mn alloys with three Zn/Ca ratios (2.63, 1.22 and 0.53, by weight ratio) were investigated. The dominant second phase changed from MgZn to Ca2Mg6Zn3 as the Zn/Ca ratio decreased from 2.63 to 1.22. With decreasing Zn/Ca ratio, the grain size of the as-cast alloys was significantly decreased, accompanied by an increase in the volume fraction of second phase. For as-extruded Mg–1.4Zn–2.6Ca–0.5Mn, the finest (~ 0.36 μm) recrystallized grain structures, containing both fine MgZn2 precipitates and α-Mn particles, were obtained at an extrusion speed of 0.01 mm/s. The texture of the deformed structure was more intense (~ 30.39 mud) relative to the recrystallized region (~ 8.33 mud). As the Zn/Ca ratio decreased, basal plane texture was weakened deriving from grain refinement following recrystallization. Superior mechanical properties with a yield strength of ~ 387.8 MPa and ultimate tensile strength of ~ 409.2 MPa were achieved in the Mg–1.4Zn–2.6Ca–0.5Mn alloy extruded at 270 °C at an extrusion speed of 0.01 mm/s. A number of factors were determined to contribute to strengthening including Hall–Petch effects from the fine recrystallized grains (contribution ~ 58.7%), dislocation strengthening of the deformed region (contribution ~ 29.3%) and Orowan strengthening (contribution ~ 12%).