P70] Diffusion study on Mg-based alloys: an experimental and modeling approach

Abstract

Magnesium alloys are receiving global attention in transportation sectors thanks to their superior light weight capacity which allows the design of fuel efficient vehicles with lower CO2 emissions. To develop commercial Mg alloys and optimize their manufacturing processes, fundamental knowledge in thermodynamics and diffusion kinetics is indispensable. In these thermally activated processes, diffusion can play a key role in the optimization of process parameters. A good knowledge of diffusion is thus imperative to understand the relationships between the microstructure and the high temperature process parameters. In the present study, diffusion kinetics of Mg alloys was investigated by both experiments and simulations.Pure magnesium has an hcp crystal structure (c/a ratio = 1.6236) and as a result, self-diffusion of Mg shows anisotropic behavior. Anisotropic diffusion is also observed for various alloying elements such as Ag, Cd, In, Sb, and Sn in hcp Mg. Apart from Ag, the diffusion of solute elements and Mg itself along the a-axis of Mg is faster than along the c-axis. However, the anisotropic behavior of the most common alloying elements like Al and Zn has not been investigated so far. Moreover, no data are available for rare earth elements (REE) despite the fact that their importance is increasing in Mg alloys.In the present study, the anisotropic impurity diffusion coefficients of Al, Zn, Gd, and Y were found to follow the same trend as Mg-self diffusion. The diffusion coefficients along the a-axis is maximum and is about 1.3 times faster than along the c-axis in the conventional material processing temperature. In addition, the effect of the basal plane orientation of Mg on Al and Zn diffusion was investigated and it was found that the diffusion coefficients of Al and Zn in hcp Mg decrease linearly with an increase of the tilting angle of the basal plane. Interdiffusion coefficients and growth constants of all binary intermetallics in Mg-Al, -Zn, -Gd, and –Y systems were also obtained from diffusion couple experiments.The grain boundary diffusion of Al at high angle grain boundaries in polycrystalline Mg was also investigated with diffusion couple experiments. To obtain Al concentration profiles at the grain boundary, high resolution cold field emission-secondary electron microscope (CFE-SEM) energy dispersive spectroscopy (EDS) was used. Al diffusion at grain boundaries close to 75° misorientation is about two orders of magnitude higher than the one through for the bulk.In the present study, a multiphase diffusion couple simulation model was developed which can successfully explain the experimental results. A homogenization and dissolution model was also developed to explain the solution treatment process of Mg-Al and Mg-Zn alloys. The model was validated with the help of annealing experiments for the binary Mg-Al (3, 6 and 9 wt. % Al) and Mg-Zn (1.5, 4.0 and 5.5 wt. % Zn) alloys. The model was then extended to the ternary Mg-Al-Zn system to predict the homogenization and dissolution phenomenon for the AZ…