Influence of undissolved second-phase particles on dynamic recrystallization behavior of Mg–7Sn–1Al–1Zn alloy during low- and high-temperature extrusions

Abstract

This study investigates the effects of fine and coarse undissolved particles in a billet of the Mg–7Sn–1Al–1Zn (TAZ711) alloy on the dynamic recrystallization (DRX) behavior during hot extrusion at low and high temperatures and the resultant microstructure and mechanical properties of the alloy. To this end, partially homogenized (PH) and fully homogenized (FH) billets are extruded at temperatures of 250 and 450 °C. The PH billet contains fine and coarse undissolved Mg2Sn particles in the interdendritic region and along the grain boundaries, respectively. The fine particles (<1 μm in size) retard DRX during extrusion at 250 °C via the Zener pinning effect, and this retardation causes a decrease in the area fraction of dynamically recrystallized (DRXed) grains of the extruded alloy. In addition, the inhomogeneous distribution of fine particles in the PH billet leads to the formation of a bimodal DRXed grain structure with excessively grown grains in particle-scarce regions. In contrast, in the FH billet, numerous nanosized Mg2Sn precipitates are formed throughout the material during extrusion at 250 °C, which, in turn, leads to the formation of small, uniform DRXed grains by the grain-boundary pinning effect of the precipitates. When the PH billet is extruded at the high temperature of 450 °C, the retardation effect of the fine particles on DRX is weakened by their dissolution in the α-Mg matrix and the increased extent of thermally activated grain-boundary migration. In contrast, the coarse Mg2Sn particles in the billet promote DRX during extrusion through the particle-stimulated nucleation phenomenon, which results in an increase in the area fraction of DRXed grains. At both low and high extrusion temperatures, the extruded material fabricated using the PH billet, which contains both fine and coarse undissolved particles, has a lower tensile strength than that fabricated using the FH billet, which is virtually devoid of second-phase particles. This lower strength of the former is attributed mainly to the larger grains and/or absence of nanosized M2Sn precipitates in it.