Interdiffusion kinetics of the intermetallic coatings on AZ91D magnesium alloy formed in molten salts at lower temperatures

A continuous Mg-Al diffusion coating can be formed on the AZ91D Mg alloy by the diffusion coating treatment in molten salts in the lower temperature range from 280 to 400 °C. The microstructure and composition of the diffusion coatings were investigated by scanning electron microscopy and energy dispersive X-ray analysis. The results showed that the diffusion coating consists of continuous γ-Mg17Al12 phase and β-Mg2Al3 phase. The β-Mg2Al3 phase layer grows faster than the γ-Mg17Al12 phase layer. The interdiffusion coefficients for each phase were investigated by Heumann's method. As the temperature increases from 320 to 400 °C, the interdiffusion coefficient in γ-Mg17Al12 phase (D̃γ) increases from 2.2 × 10-12 to 9.6 × 10-11 cm2/s. When the temperature increases from 360 to 400 °C, the interdiffusion coefficient in β-Mg2Al3 phase (D̃β) increases from 3.5 × 10-10 to 7.4 × 10-10 cm2/s. The activation energies for the interdiffusion in γ-Mg17Al12 and β-Mg2Al3 phases are 155.9 and 66.3 kJ/mol, respectively.