Creep Deformation in Mg-Al-Ca-based Alloys.

Abstract

Magnesium alloys, with a lower density than steel or aluminum, have the potential to reduce the mass of automotive components. However, new alloys with improved creep resistance must be developed before Mg can be used for high temperature powertrain applications. Limiting the development of these alloys is the lack of fundamental knowledge of creep deformation in Mg alloys. This dissertation investigates the dependence of creep resistance on elemental partitioning during solidification, using thermodynamic modeling and experimental composition mapping. The effect of Sn additions on Mg-Al-Ca alloys is examined with respect to solute and precipitation strengthening, and the results are used to develop strategies for future alloy development.

Viscous glide of dislocations in the α-Mg phase was determined to be the operative creep deformation mechanism through observation of dislocation substructures in crept samples. Thus, compositional and microstructural changes offer a pathway to improved creep resistance through solute and precipitation strengthening of the α-Mg phase.

Addition of 0.75-1wt% Sn to Mg-5Al-3Ca increased the Ca partitioning to the α-Mg phase during solidification and led to a higher average Ca concentration in the α-Mg by changing the phase free energy. Additions of greater than 1wt% Sn changed the solidification path, leading to lower Ca partitioning to the α-Mg. Increasing the local Ca concentration led to a greater decrease in minimum creep rate than a corresponding change in local Al concentration, indicating that the Ca concentration in the α-Mg phase contributes more to solute strengthening than the Al concentration. The lowest minimum creep rate was observed in Mg-5Al-3Ca-0.75Sn, which combined a high concentration of Ca in the α-Mg phase and increased Ca partitioning during solidification.

Precipitation strengthening accounts for a significant portion (20-50%) of the creep strength of Mg-Al-Ca alloys. Basal precipitates of Al2Ca in α-Mg cells led to modest increases in creep resistance. The Mg-5Al-3Ca-0.75Sn alloy had the smallest precipitate spacing and exhibited a minimum creep rate approximately two times lower than other Sn-containing alloys. Alloying additions should therefore be targeted that can increase both the local Ca concentration in the α-Mg and the volume fraction of Al2Ca precipitates for improved creep resistance.