  The effect of Ca and Zn in solid solution on the critical resolved shear
stress (CRSS) of <a> basal slip, tensile twinning and <c+a> pyramidal slip in
Mg alloys has been measured through compression tests on single crystal
micropillars with different orientations. The solute atoms increased the CRSS
for basal slip to ~ 13.5 MPa, while the CRSS for pyramidal slip was lower than
85 MPa, reducing significantly the plastic anisotropy in comparison with pure
Mg. Moreover, the CRSSs for twin nucleation and growth were very similar (~ 37
MPa) and the large value of the CRSS for twin growth hindered the growth of
twins during thermo-mechanical processing. Finally, evidence of <a> prismatic
slip and cross-slip between basal and prismatic dislocations was found. It is
concluded that the reduction of plastic anisotropy, the activation of different
slip systems and cross-slip and the weak basal texture promoted by the large
CRSS for twin growth are responsible for the improved ductility and formability
of Mg-Ca-Zn alloys.
