Effect of solutes on strength and ductility of Mg alloys

Abstract

This work investigates the origin of the simultaneous increase in strength and ductility that takes place in Mg polycrystals alloyed with Al and Zn solutes. With that purpose, twelve polycrystalline binary Mg–Zn and Mg–Al alloys, with up to 2 wt % of alloying additions and average grain sizes comprised between 3 and 42 μm, were prepared by casting, hot rolling and annealing and were tested at room temperature and quasi-static strain rates. Electron backscattered diffraction-assisted slip trace analysis was then utilized to characterize slip activity, and the latter was related to the grain size, to the texture, and to the topology of the grain boundary network. Basal slip was found to be the dominant deformation mechanism in all the binary alloys, irrespective of composition and grain size. Alloying additions were observed to have little influence on texture development but acted as strong modifiers of the topology of the grain boundary network developed during processing. In particular, they reduced the connectivity of grains that are well oriented for basal slip, preventing intergranular slip localization and, in turn, leading to considerable strengthening of basal slip. Solutes act also as enhancers of diffuse slip within individual grains. It is proposed that the simultaneous increase in strength and ductility of Mg alloys by the addition of solutes must be understood as a multiscale phenomenon resulting from the coupling of solute-dislocation interactions at the atomic scale with alterations of the topology of the grain boundary network at the mesoscale.