Effect of Zn content on microstructure, mechanical properties and fracture behavior of Mg-Mn alloy

The optical microscope, scanning electron microscope and universal testing machine are used to investigate the effect of Zn content on the microstructure, mechanical properties and fracture behavior of Mg- Mn-Zn alloy. The results indicate that fine （Mg, Mn, AI）-containing phases are distributed uniformly in the Mg-Mn alloy matrix, while small amount of （Mg, Zn）-containing phases are formed in the matrix and the grain boundary becomes coarse when 1wt.% Zn is added. As the Zn content increases, the amount of （Mg, Zn）-containing phases increases, and the grain boundary becomes coarser. When the Zn content is between 3wt.%-5wt.%, slender （Mg, Zn）-containing phases precipitate at the grain boundary. The addition of Zn could reduce the grain size and enhance the mechanical properties of the alloy matrix, and both of the effects can be enhanced by increasing the Zn content further more. When the Zn content is more than 3wt.%, grain size stops decrease, the strength cannot be improved any more and elongation decreases significantly. The fracture behavior of Mg-Mn alloy appears to be cleavage fracture, and transforms into quasi-cleavage fracture as Zn is added. When Zn content exceeded 3wt.%, large amount of （Mg, Zn）-containing phases appear on the fracture face, and act as the crack sources.