Thermally Activated Nature of Basal and Prismatic Slip in Mg and Its Alloys

Throughout the literature, a large discrepancy exists among the activation volumes reported for Mg and its alloys. The present work surveys the reported values for basal and prismatic slip of pure and alloyed Mg single crystals as well as polycrystals. A focus is placed on recent results obtained for rare earth element solutes, Sc and Y. The measured values are discussed in light of a recently developed predictive model for thermally activated basal-solute interaction in Mg alloys. It is found that if the single crystal activation volumes for basal slip in solid solution alloys are computed using the total stress instead of a presumed “thermal component” of the stress, i.e. admitting that thermal fluctuations can aid in overcoming any obstacles present in those materials, then the experimental results are in much better accordance with the theoretical predictions. Possible implications of the combined activities of different deformation modes on the activation volume of pure and alloyed Mg polycrystals are briefly introduced. Finally, using polycrystal elasto-viscoplasticity modelling, it is shown that under conditions relevant to tests performed on polycrystalline, solute-containing binary Mg alloys, basal slip can be the dominant deformation mode at 0.2% offset strain at which the initial activation volume is often assessed.