EFFECT OF HEAT-TREATMENT ON MICROSTRUCTURE AND STRENGTH OF QUASICRYSTAL- CONTAINING EXTRUDED Mg-Zn-Y ALLOYS

摘要

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 Mg_(95)Zn_(4.2)Y_(0.8) and Mg_(92.5)Zn_(6.5)Y. The quasicrystal particles, facetted and showing a definite orientation relationship with the matrix, become rounded on heating to 400℃. In the matrix, fine rods of MgZn_2 precursor phase occur parallel to the hexagonal axis. A ternary phase, τ_1, related to Mg_4Zn_7, precipitates in the matrix upon annealing of alloy Mg_(95)Zn_(4.2)Y_(0.8), and is be replaced by precipitates of quasicrystal phase on further annealing. In the alloy Mg_(92.5)Zn_(6.5)Y, the quasicrystal phase transforms to a hexagonal Mg_(25)Zn_(58)Y_(17) phase at 400℃. The quasicrystal then reprecipitates on its interface, forming a nano-composite. Effects of microstructural changes on the deformation behavior are described.