Effect of heat-treatment on microstructure and strength of quasicrystal-containing extruded Mg-Zn-Y alloys

Quasicrystal-containing dilute Mg-Zn-Y alloys exhibit high ductility with moderate strength and a stable microstructure at elevated temperatures. We report here the effect of heat treatment on two extruded alloys Mg95Zn4.2Y0.8 and Mg92.5Zn6.5Y. The quasicrystal particles, facetted and showing a definite orientation relationship with the matrix, become rounded on heating to 400 degrees C. In the matrix, fine rods of MgZn2 precursor phase occur parallel to the hexagonal axis. A ternary phase, tau(1), related to Mg4Zn7, precipitates in the matrix upon annealing of alloy Mg95Zn4.2Y0.8, and is be replaced by precipitates of quasicrystal phase on further annealing. In the alloy Mg92.5Zn6.5Y, the quasicrystal phase transforms to a hexagonal Mg25Zn58Y17 phase at 400 degrees C. The quasicrystal then reprecipitates on its interface, forming a nano-composite. Effects of microstructural changes on the deformation behavior are described.