Abstract
Solid-to-solid diffusion couples were assembled and annealed to examine the diffusion
between pure Mg (99.96%) and Al (99.999%). Diffusion anneals were carried out at 300°, 350°,
and 400°C for 720, 360, and 240 hours, respectively. Optical and scanning electron microscopes
were utilized to identify the formation of the intermetallic phases, γ-Al12Mg17 and β-Al3Mg2 and
absence of the ε-phase in the diffusion couples. Thicknesses of the γ-Al12Mg17 and β-Al3Mg2
phases were measured and the parabolic growth constants were calculated to determine the
activation energies for the growth, 165 and 86 KJ/mole, respectively. Concentration profiles
were determined with electron microprobe analysis using pure elemental standards.
Composition-dependent interdiffusion coefficients in Mg-solid solution, γ-Al12Mg17 and β-
Al3Mg2 and Al-solid solutions were calculated based on the Boltzmann-Matano analysis.
Average effective interdiffusion coefficients for each phase were also calculated, and the
magnitude was the highest for the β-Al3Mg2 phase, followed by γ-Al12Mg17, Al-solid solution
and Mg-solid solution. Intrinsic diffusion coefficients based on Huemann’s analysis (e.g., marker
plane) were determined for the ~38 at.% Mg in the β-Al3Mg2 phase. Activation energies and the
pre-exponential factors for the inter- and intrinsic diffusion coefficients were calculated for the
temperature range examined. The β-Al3Mg2 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-Al3Mg2 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 to the available self- and impurity diffusion data from literature. Thermodynamic
factor, tracer diffusion coefficients and atomic mobilities at the marker plane composition were
approximated using available literature values of Mg activity in the β-Al3Mg2 phase.