Enhancement of room temperature stretch formability via grain boundary sliding in magnesium alloy

The enhancement method of room temperature formability was investigated by controlling the rate-controlling mechanism. Wrought processed pure magnesium and its alloys (Mg-Al-Zn and Mg-Mn alloys), which have a basal texture, were used in this study. Pure magnesium had two kinds of average grain sizes (~7 µm and ~30 µm), and the other alloys were 20-30 µm in average grain size. The forming load vs. displacement curves in Erichsen tests were not influenced by the forming rate in the Mg-Al-Zn alloy; however, the fine-grained pure magnesium and Mg-Mn alloy showed a high forming rate dependence. The limited dome heights were 4.7, 2.0 and 5.2 for the fine-grained pure magnesium, Mg-Al-Zn and Mg-Mn alloys, respectively. Deformed microstructural observations revealed that, in the meso-grained pure magnesium and the Mg-Al-Zn alloy, deformation twins formed and were closely related to crack propagation. On the other hand, the formation of such deformed structures was difficult to identify in the fine-grained pure magnesium and meso-grained Mg-Mn alloy. The good stretch formability and high rate dependence of the fine-grained pure magnesium and Mg-Mn alloy resulted from the major contribution of grain boundary sliding to deformation during Erichsen testing. The addition of an alloying element, which plays a role in the enhancement for grain boundary sliding, is quite effective to improve the room temperature stretch formability.