Rare-earth (RE) element addition can remarkably improve the mechanical properties of magnesium alloys through solid solution and age-hardening. Increasing the solubility in the Mg matrix and enhancing the precipitation density are effective measures to improve ageing strengthening of magnesium alloys. In this work, solid solution treatment at 600 similar to 800 degrees C under 4 GPa, and then isothermal ageing at 200 degrees C for Mg96.17Zn3.15Y0.5Zr0.18 alloy was carried out. The microstructures of the high pressure solution treatment Mg96.17Zn3.15Y0.5Zr0.18 alloy before and after ageing were investigated by TEM, HRTEM, SEM and XRD, and age-hardening curves of Mg96.17Zn3.15Y0.5Zr0.18 alloy after solution treatment under the high pressure of 4 GPa have been tested. The results show that, as the rise of the solution treatment temperature, I-Mg3Zn6Y and W-Mg3Zn3Y2 continually dissolved into the Mg matrix, and the solubility of Zn in the Mg matrix drastically improved after solution treatment under the high pressure of 4 GPa. The solubility of Zn in the Mg matrix reached up to 6.60% (mass fraction) after solution treatment at 700 similar to 800 degrees C under the high pressure of 4 GPa than 2.11% after solution treatment at 400 degrees C under the atmosphere, and the supersaturated solid solution alpha-Mg has been attained. After ageing treatment at 200 degrees C, the peak hardness of Mg96.17Zn3.15Y0.5Zr0.18 alloy after solution treatment under the high pressure of 4 GPa could been reached in short ageing time, the peak hardness of the Mg96.17Zn3.15Y0.5Zr0.18 alloy after solution treatment at 800 degrees C under 4 GPa was 105 HV, which was increased by 30% than 81 HV of the alloy after solution treatment at 400 degrees C under the atmosphere. HRTEM analysis results indicated that the high precipitation density was found in the Mg96.17Zn3.15Y0.5Zr0.18 alloy after solution treatment under the high pressure of 4 GPa, and some of precipitation were particle quasicrystal I-Mg3Zn6Y phases.
