Twinning and texture randomization in Mg and Mg-Ce alloys

The deformation behavior of extruded pure Mg and Mg 0.5%Ce alloys is studied using in-situ electron backscatter diffraction (EBSD) under compressive loading. The microstructure after compression shows profuse twinning in pure Mg samples and reduced twinning in Mg-Ce samples. A significant number of twins do not follow Schmidt's law, contrary to the commonly assumed pseudoslip mechanism employed in most crystal plasticity simulations studies of twinning in Mg. A careful examination of twinning activity and Schmidt factors for dislocations in Mg shows that the non-Schmidt twin formation is favored in grains that also have low Schmidt factor for dislocation. Crystal plasticity simulation using a modified threshold energy criterion for twinning is shown to predict the observed twin formation. The growth rate of the twins is shown to depend on the coherency of twin interface both experimentally and computationally. The result suggest that twinning behavior of Mg and Mg-Ce are quite similar and any observed differences are due to texture differences. Texture modification is shown to be a consequence of higher c+a activity in Mg-Ce solid solution and reduced grain growth due to grain boundary pinning by Ce-Mg precipitates during recrystallization. Grains of random orientation nucleate during recrystallization in both Mg and Mg-Ce but highly mobile boundaries of basal orientation grains swiftly consume grains of other orientations in pure Mg while grain boundaries in Mg-Ce alloys get pinned by Ce-rich particles, allowing the final microstructure to retain the “randomized texture” in the later.