Effect of Second-Phase Precipitates on Local Elongation in Extruded Magnesium Alloys

The effect of precipitates on room-temperature tensile ductility, specifically post-uniform elongation (local elongation), was investigated in three magnesium alloys. The alloys were prepared by extrusion and had different types of precipitate morphology. Tensile tests revealed an almost total absence of local elongation in an alloy with intergranular precipitate particles (a few hundred nanometers in diameter), in spite of a relatively high total elongation to fracture of 17%. On the other hand, an alloy with densely dispersed fine intragranular precipitates (up to a few tens of nanometers in diameter) showed the highest local elongation at 17% and an alloy with a sparse dispersion showed moderate local elongation. The fracture surfaces of the latter two alloys had a primarily ductile appearance with dimples. For the specimen that showed the highest local elongation, twinning was observed in the necked portion, whereas very few twins were observed in the uniformly elongated portion. It was suggested that fine precipitates in the grain interior inhibit the formation of twins, a feature that can be exploited to enhance local elongation in magnesium alloys.