Precipitation during creep in magnesium-aluminum alloys

We employ a free energy density for Mg-Al alloys that is dependent on concentration, strain, and temperature, and derived from quantum mechanical calculations by Ghosh & Bhattacharya (Acta Mater 193:28-39, 2020), to model the dynamic precipitation of the Mg17Al12 phase during creep experiments in Mg-Al alloys. Our calculations show that the overall volume fraction of the dynamically formed precipitates is influenced by stress, and furthermore, this influence is anisotropic and asymmetric. Specifically, when the stress is volumetric or along the c-axis direction, the volume fraction of the precipitate phase is greater in compression and lower in tension. Surprisingly, stress along the a- or b-axis directions does not alter the volume fraction of the precipitates. The resistance to creep is improved by the presence of finely dispersed precipitates with a small aspect ratio, closer to spherical or ellipsoidal in shape and high number density. A greater volume fraction of these fine particles are produced during compressive creep tests than tensile creep experiments and thereby explaining the higher creep rate observed in tension than in compression in these alloys. Overall, our calculations explain the tension-compression asymmetry of the creep rate observed in creep experiments in Mg-Al alloys (Agnew et al. in Magn Technol 2000:285-290, 2000; Agnew et al. in Magn. Alloys Appl. 685-692, 2000).