Role of grain boundaries on ductility in Mg-Y alloys

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 mu m to over 100 mu m. 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 similar to 20-30 mu m) exhibited good elongation-to-failure of similar to 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 similar to 20-10 mu m) 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.