Thermal stability of nanocrystalline in surface layer of magnesium alloy AZ91D

Isothermal and isochronal annealing was conducted to study the thermal stability of the nanocrystalline in the surface layer of Mg alloy AZ91D induced by high-energy shot peening (HESP). Field emission scanning electron microscope (FESEM) and X-ray diffractometer were used to characterize the microstructure. Results showed that nanocrystalline produced by HESP on the surface layer of the magnesium alloy AZ91D was 60-70 nm on average. The nanocrystalline could remain stable at about 100 °C, and grew up slowly between 100 °C and 200 °C. When the annealing temperature reached 300 °C, the growth rate of the nanocrystalline increased significantly. The kinetic coefficient n of the nanocrystalline growth was calculated to be 2-3 and the grain growth activation energy Q=39.7 kJ/mol, far less than the self-diffusion activation energy of magnesium atoms in the coarse polycrystalline material.