Room temperature grain boundary sliding behavior of fine-grained Mg-Mn alloys

The impact of chemical composition, i.e., the manganese element, on room-temperature mechanical properties and its deformation behavior was examined using a series of Mg-Xat% (X = 0.05, 0.1, 0.3, 0.45, 0.6 and 0.8) Mn binary alloys. The average grain size of these alloys was controlled to be 2-3 µm by extrusion temperature. The manganese segregates at grain boundaries, and the fraction of dispersed particles increases with increasing manganese addition. The results obtained from tensile tests reveal that the chemical composition is unlikely to affect the strain rate dependence, which is the major deformation mechanism. The strain rate sensitivity (m-value) is calculated to be ~ 0.1 and ~ 0.2-0.3 in the quasi-static and in the low strain rate regimes, respectively, for all the alloys. This indicates that the contribution of grain boundary sliding on deformation increases at lower strain rates. On the other hand, the elongation-to-failure in tension is clearly influenced by manganese concentration. The chemical composition of 0.3 at% manganese brings about a good ductility of nearly 95% and 145% in the strain rates of 1 × 10−3 and 1 × 10−5 /s, respectively; however, a further manganese addition leads to a decrease in the elongation-to-failure. Deformed microstructural observations show that cavities nucleate at the interface between matrix and particles without any deformation twin formation, and they are the origin of fracture. A high amount of manganese causes formation of the dispersed particles; as a result, it is ineffective to improve the ductility.