Role of grain boundaries on ductility in Mg-Y alloys

Abstract

The impact of the grain size and the chemical composition of an alloying element on mechanical properties in ductility was investigated using wrought processed various Mg-Xat%Y binary alloys (where, X = 0.1, 0.3, 1.0 and 2.0). The average grain sizes of these Mg-Y alloys after a subsequent annealing process varied from around 5 µm to over 100 µm. The elongation-to-failure in tension was influenced by the grain size, irrespective of the yttrium content. The Mg-Y alloys consisting of meso‑grained structures (average grain size of ∼20–30 µm) exhibited good elongation-to-failure of ∼25–30%; however, “further” grain refinement led to a decrease in the properties of ductility. This tendency was confirmed to have no relation to the yttrium content. Deformed microstructural observations revealed that grain boundaries were the source of non-basal dislocation slips, but became crack-propagation sites. Although grain refinement from coarse- to meso‑sizes (up to around ∼20–10 µm) was effective, fine-grained alloys were unlikely to show a large elongation-to-failure due to grain boundary embrittlement. Not only the critical resolved shear stress but also the segregation energy of grain boundary is a considerable characteristic for alloying elements to improve ductility in magnesium alloys. Furthermore, in the case of fine-grained Mg-rare earth alloys, it is necessary to take into consideration that grain boundaries become not only the source of non-basal dislocation slips but also the route for crack-propagation.