On the Plasticity and Deformation Mechanisms in Magnesium Crystals

This work presents an overview of the mechanical response and microstructure evolution of specifically oriented pure magnesium single crystals under plane strain compression at room temperature. Crystals of 'hard' orientations compressed along the c-axis exhibited limited room temperature ductility, although pyramidal < c + a > slip was readily activated, fracturing along crystal-lographic {1124} planes as a result of highly localized shear. Profuse {10 (1) over bar2} extension twinning was the primary mode of incipient deformation in the case of orientations favorably aligned for c-axis extension. In both cases of compression along < 11 (2) over bar0 > and < 10 (1) over bar0 > directions, {10 (1) over bar2 > extension twins completely converted the starting orientations into twin orientations; the subsequent deformation behavior of the differently oriented crystals, however, was remarkably different. The formation of {< 10 (1) over bar2 >} extension twins could not be prevented by the channel-die constraints when c-axis extension was confined. The presence of high angle grain boundaries and, in particular, {10 (1) over bar2} twin bound-aries was found to be a prerequisite for the activation of {10 (1) over bar1} contraction twinning by providing nucleation sites for the latter. Prismatic slip was not found to operate at room temperature in the case of starting orientations most favorably aligned for prismatic slip; instead, cooperative {10 (1) over bar2} extension and {10 (1) over bar1} contraction twinning was activated. A two-stage work hardening behavior was observed in 'soft' Mg crystals aligned for single or coplanar basal slip. The higher work hardening in the second stage was attributed to changes in the microstructure rather than the interaction of primary dislocations with forest dislocations.