Effect of solid solution elements on nanoindentation hardness, rate dependence, and incipient plasticity in fine grained magnesium alloys

The effect of solid solution alloying elements on the plastic deformation of fine grained Mg alloys was studied in five binary systems: Mg-0.3 at.% X (X = Al, Ca, Li, Y or Zn). All the alloys were produced by extrusion and had an average grain size of ∼2-3 μm. Solid solution strengthening is observed for all of the alloys subjected to hardness tests with a scale much larger than the grain size, and the efficacy of the various solutes is in line with expectations based on recent solution strengthening models. The different alloying elements also have an impact on the rate dependence of deformation, with activation volumes ranging between 20 b3 and 80 b3 being consistent with cross-slip as a rate limiting mechanism. For nanoscale indentations the yield point is identified by a pop-in event, and this event is found to have a dramatically different rate dependence and activation volume than does global plasticity. Specifically, the dislocation generation mechanism associated with pop-in has an activation volume of scale ∼0.2-1 b3, and is far less affected by solute content than is global plasticity.