Bimodally grained microstructure development during hot extrusion of Mg–2.4 Zn–0.1 Ag–0.1 Ca–0.16 Zr (at.%) alloys

We have investigated the microstructure development of Mg–2.4 Zn–0.1 Ag–0.1 Ca(–0.16 Zr) alloys during hot extrusion using electron backscattered diffraction (EBSD), transmission electron microscopy and three-dimensional atom probe (3DAP) analysis. The Zr-free alloy showed a uniformly recrystallized grain structure, while the Zr-containing alloy showed a bimodal grain structure composed of fine grains of several microns and unrecrystallized coarse grains containing fine precipitates. The precipitates were identified as Mg(Zn,Zr), which are considered to suppress dynamic recrystallization, resulting in a partially recrystallized grain structure with a bimodal grain distribution. EBSD analysis showed that both Zr-free and Zr-containing alloys had a fiber texture of a (0 0 0 1) basal plane aligned with the extrusion direction. Three-dimensional atom probe (3DAP) analysis revealed that the Mg(Zn,Zr) precipitates were eliminated by solution heat treatment and β1′-MgZn particles precipitated by subsequent aging, resulting in an enhanced yield strength of the extruded alloy.