  The deformation mechanisms of an extruded Mg-5Y-0.08Ca (wt. %) alloy were
analyzed by means of micropillar compression tests on single crystals along
different orientations -- selected to activate specific deformation modes -- as
well as slip trace analysis, transmission electron microscopy and transmission
Kikuchi diffraction. The polycrystalline alloy presented a remarkable ductility
in tension (~32%) and negligible differences in the yield strength between
tension and compression. It was found that the presence of Y and Ca in solid
solution led to a huge increase in the CRSS for <a> basal slip (29 $\pm$ 5
MPa), <c+a> pyramidal slip (203 $\pm$ 7 MPa) and tensile twin nucleation (above
148 MPa), while the CRSS for <a> prismatic slip only increases up to 105 $\pm$
4 MPa. The changes in the CRSS for slip and tensile twinning in Mg-Y-Ca alloys
expectedly modify the dominant deformation mechanisms in polycrystals. In
particular, tensile twinning is replaced by <a> prismatic slip during
compressive deformation along the a-axis. The reduction of twinning (which
generally induces strong anisotropy in the plastic deformation in textured
alloys), and the activation of <a> prismatic slip (which provides an additional
plastic deformation mechanism with limited hardening) were responsible for the
large tensile ductility of the alloy.
