Interdiffusion in the Mg-Al System and Intrinsic Diffusion in β-Mg₂Al₃

Solid-to-solid diffusion couples were assembled and annealed to examine the diffusion between pure Mg (99.96 pct) and Al (99.999 pct). Diffusion anneals were carried out at 573 K, 623 K and 673 K (300 °C, 350 °C and 400 °C) for 720, 360, and 240 hours, respectively. Optical and scanning electron microscopes were used to identify the formation of the intermetallic phases, γ-Mg₁₇Al₁₂, and β-Mg₂Al₃, as well as the absence of the ε-Mg₂₃Al₃₀ in the diffusion couples. The thicknesses of the γ-Mg₁₇Al₁₂ and β-Mg₂Al₃ phases were measured and the parabolic growth constants were calculated to determine the activation energies for growth. Concentration profiles were determined with electron microprobe analysis using pure elemental standards. Composition-dependent interdiffusion coefficients in Mg-solid solution, γ-Mg₁₇Al₁₂, β-Mg₂Al₃, and Al-solid solutions were calculated based on the Boltzmann-Matano analysis. Integrated and average effective interdiffusion coefficients for each phase were also calculated, and the magnitude was the highest for the β-Mg₂Al₃ phase, followed by γ-Mg₁₇Al₁₂, Al-solid solution, and Mg-solid solution. Intrinsic diffusion coefficients based on Huemann’s analysis (e.g., marker plane) were determined for the ~ Mg-62 at. pct Al in the β-Mg₂Al₃ phase. Activation energies and the pre-exponential factors for the interdiffusion and intrinsic diffusion coefficients were calculated for the temperature range examined. The β-Mg₂Al₃ phase was found to have the lowest activation energies for growth and interdiffusion among all four phases studied. At the marker location in the β-Mg₂Al₃ phase, the intrinsic diffusion of Al was found to be faster than that of Mg. Extrapolations of the impurity diffusion coefficients in the terminal solid solutions were made and compared with the available self-diffusion and impurity diffusion data from the literature. Thermodynamic factor, tracer diffusion coefficients, and atomic mobilities at the marker plane composition were approximated using the available literature values of Mg activity in the β-Mg₂Al₃ phase.