Deformation mechanisms of Mg-Ca-Zn alloys studied by means of micropillar compression tests

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

“…And several authors reported a large improvement in the ductility of binary Mg-Zn and Mg-Al alloys through the addition of small amount of Ca (Hofstetter et al, 2015;Sandlöbes et al, 2017;Wang et al, 2021b). This behavior was supported by our recent micropillar compression tests that showed that the addition of Ca to Mg-Zn alloys reduced the pyramidal-to-basal CRSS ratio values, that were similar to those found in Mg-Y alloys (Wang et al, 2021a).…”

“…Micropillars of 5 × 5 μm 2 square cross and an aspect ratio 2:1 were milled from the selected grains using a FEI Helios G4 UX Focused Ion Beam (FIB)/SEM dual beam microscope operated at 30 kV. These dimensions are known to minimize size effects during mechanical deformation while the time and effort to mill each micropillar are reasonable (Wang et al, 2021a). An initial ion current of 9.3 nA was used to remove the surrounding material and it was reduced to 2.5 nA when the beam was getting closer to the actual dimensions of the micropillar.…”

“…To this end, the elastic modulus of each grain was determined via the nanoindentation method with a Berkovich tip in the same grain where the micropillar was milled. More details about micropillar manufacturing and the compression set-up can be found in (Sneddon, 1965;Wang et al, 2021a).…”

“…8b The activation of the <a> prismatic slip during compression along the a-axis has been reported recently in Mg-Zn-Ca alloy (Wang et al, 2021b) in combination with tensile twinning. However, the activation of <a> prismatic slip and the suppression of tensile twinning during compression along the a-axis has not been found at ambient temperature in pure Mg (Y. or any Mg alloys (Li et al, 2021b(Li et al, , 2021aWang et al, 2021aWang et al, , 2020Wu et al, 2020). This result is very surprising because compression of Mg and its alloys along the a-axis (or equivalent extension along the caxis) easily leads to the nucleation and growth of {101 $ 2} tensile twins, because the associated CRSS to promote tensile twin is much lower than that necessary to activate <c+a> pyramidal slip or <a> prismatic slip.…”

“…Regarding the effect of solid solution on tensile twinning, several investigations reported an increase in the CRSS for twin nucleation and growth with the addition of Al (Wang et al, 2020), Zn (Li et al, 2021a), Y (Li et al, 2021b) as well as Ca to Mg-Zn alloys (Wang et al, 2021a). However, the CRSS for twin nucleation and growth were lower than that for <c+a> pyramidal slip in the corresponding alloy, thus, tensile twinning was still preferred over pyramidal slip to accommodate plastic deformation in grains suitable oriented for twinning.…”