Precipitation hardening in Mg-Zn-Sn alloys with minor additions of Ca and Si

Al-free Mg-Zn alloys with alternative alloying elements were investigated in order to form Mg-Zn-based alloys with improved thermal stability. Based on thermodynamic calculations, Zn and Sn additives are expected to form stable intermetallic phases with Mg and cause precipitation hardening. An Mg alloy containing 4.5 wt.% Zn and 3.8 wt.% Sn was examined in detail and subsequently alloyed with Ca and Si, in order to promote additional precipitation. The as-cast microstructure and its stability during exposure to elevated temperatures were investigated. Precipitation hardening was investigated in the temperature range of 175-250°C by Vickers hardness measurements and analysis of the microstructure evolution, using X-ray diffraction and electron microscopy techniques. During aging at 175°C for 1 to 96 h, two hardness maxima were observed: the first after 2 h, and the second after 16 h. The occurrence of two hardness peaks can be related to a precipitation sequence. Firstly, MgZn₂ precipitates form, and secondly Mg₂Sn. The results indicate that the precipitation mechanism is most likely diffusion-controlled. The precipitates are uniformly distributed in the Mg matrix with two morphologies: needle- and plate-like shapes. The addition of Si and Ca did not result in the expected effect on the precipitation behavior.