Improved formability with theoretical critical shear strength transforming in Mg alloys with Sn addition

First-principles calculation of the theoretical critical shear strength (τmax) (reflecting the value of critical resolved shear stress) was performed on pure Mg and Mg-Sn models for basal and pyramidal slip systems. The effects of Sn content on the formability of Mg-Sn monophasic solid solution (MSS) alloys and the mechanical behavior of the as-extruded Mg-Sn sheets were also investigated. The calculated results showed that after doping Sn atoms to pure Mg, the τmax in basal slip increased from 114 to 149 MPa, while that in pyramidal slip decreased from 1571 to 446 MPa. The decreased τmax ratio of the pyramidal slip to the basal slip might make the pyramidal slip systems more likely to be activated, leading to an improvement in the formability of Mg-Sn alloys. This result was further confirmed by experimental observations: with increasing Sn content, the formability of Mg-Sn MSS alloys and the ductility of as-extruded Mg-Sn sheets were all enhanced, accompanied by weakened basal texture.