Effects of dilute additions of Zn and Ca on ductility of magnesium alloy sheet

The present study seeks to clarify the mechanism by which dilute additions of zinc (Zn) and calcium (Ca) improve ductility of Mg alloy sheet. Herein, the ductility and microstructure of fully annealed pure Mg and Mg-0.1Ca, Mg-0.4Ca, Mg-0.4Zn, and Mg-0.3Zn-0.1Ca (at.%) alloy sheet were systematically investigated and compared. It is found that the ternary alloy displays better ductility than either pure Mg or binary Mg-alloys, when fully recrystallized and possessing similar grain size. In the deformed grains of the ternary alloy, traces of basal and pyramidal slip and {101̅2} twins are observed, whilst only basal slip traces and {101̅2} twins are observed in the pure Mg and binary alloys. Grain boundary cracks are observed in all the tensile-tested alloys. However, significantly less grain boundary cracks are observed in the ternary alloy, posited to be due to enhanced grain boundary cohesion. These observations suggest that the combination of enhanced pyramidal slip and suppressed grain boundary cracking leads to the appreciably improved ductility of the Mg-0.3Zn-0.1Ca alloy sheet.