Effects of yttrium on recrystallization and grain growth of Mg-4.9Zn-0.7Zr alloy

Recrystallization and grain growth in Mg-4.9Zn-0.7Zr and Mg-4.9Zn-0.9Y-0.7Zr alloys as a function of temperature on deformation were investigated with regards to hot rolling and annealing. The influence of yttrium addition on the microstructure was examined by X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results indicated that yttrium addition promoted nucleation of recrystallization during hot rolling process. The grain size of Mg-4.9Zn-0.7Zr alloy samples grew significantly with annealing temperature (300-400℃) and holding time (0-120 min), while the microstructure of the alloy with yttrium addition remained unchanged and fine. The activation energy of grain boundary migration for Mg-4.9Zn-0.9Y-0.7Zr alloy samples (56.34 kJ/mol) was higher than that for Mg-4.9Zn-0.7Zr (42.66 kJ/mol) owing to the pinning effect of Y-containing particles. The proposed growth models of recrystallized grains for the two studied alloys conformed well to E. Robert's grain-growth equation. Besides, the ultimate strength and yield strength of the alloys with yttrium addition were improved with good plasticity.