Critical resolved shear stresses for slip and twinning in Mg-Y-Ca alloys and their effect on the ductility

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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 ± 5 MPa), 〈c + a〉 pyramidal slip (203 ± 7 MPa) and tensile twin nucleation (above 148 MPa), while the CRSS for 〈a〉 prismatic slip only increases up to 105 ± 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.