Evaluation of Dynamic Precipitation in Extruded Mg-Zn Alloy

The dynamic precipitation evolution of Mg-3 wt% Zn alloy during extrusion was evaluated in this study. Precipitation evolution during aging has been well investigated for the Mg-Zn alloy, which is known as static precipitation. However, little attention has been given to dynamic precipitation in Mg-Zn alloy. In order to evaluate the dynamic precipitation evolution, Mg-3 wt% Zn alloy was prepared in an electrical resistance melting furnace and extruded at 180 degrees C by direct extrusion. The microstructure of the as-extruded Mg-3 wt% Zn alloy was investigated by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). A bi-modal crystal structure was observed after extrusion, which consisted of dynamically recrystallized (DRX) regions and deformed regions. The 2nd phases that precipitated during the extrusion were distributed in the dynamically recrystallized regions. The precipitates were mainly Mg7Zn3 phase, which is known as a metastable phase, and it was determined that the factor influencing the precipitates' formation and distribution during extrusion was dominantly dislocation. Also, the corresponding mechanical properties were investigated by measuring the Vickers hardness and compared with as-cast Mg-3 wt% Zn alloy after aging at 160 degrees C. The Vickers hardness of the as-extruded Mg-3 wt% Zn alloy was 81.5 HV, which was 36.7% higher than that of the as-cast one after aging at 160 degrees C. This was attributed to grain refinement and dynamic precipitation during severe plastic deformation.