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e-hardening response of certain magnesium alloys has been shown to be particularly sensitive to microalloying addition. Bettles et al.[1] showed that the number density of particles in Mg-4Zn could be significantly increased by the addition of a small microalloying addition of Ca. This was commensurate with a refinement in the particle size, and a significant improvement in the age-hardening response. It has also been shown that there are a number of other elements, such as V[2] and Li[3], which refine the particle size and increase the number density of particles in the Mg-Zn system. Hono and co-workers have explored this phenomenon quite extensively and showed that it can be exploited in other systems (e.g.[4]). Of interest for the current study is that a strong acceleration of precipitate hardening has been reported for Mg-Sn alloys microalloyed with Na.[5–7] Here we report the finding that these microalloying additions can also serve to stabilize the precipitating species at elevated temperatures and that this can be exploited for grain refinement during extrusion, presumably due to Zener pinning. Following the study of Mendis et al.,[5] we chose to study the ternary alloy Mg-2Sn-1Zn (wt pct) with and without a microalloying addition of 0.3 wt pct Na. The base alloy was cast as a 400 g heat in an induction furnace, using the pure elements as starting materials. NICOLE STANFORD, Senior Research Academic, MOHAN SETTY, Research Engineer, and MATTHEW R. BARNETT, Associate Professor, are with the Centre of Excellence for Design in Light Metals, Deakin University, Geelong, VIC 3217, Australia. JESSICA R. TERBUSH, Research Fellow, is with the Centre of Excellence for Design in Light Metals, Monash University, Clayton, VIC 3800, Australia. Contact e-mail: [email protected] Manuscript submitted March 3, 2013. Article published online April 3, 2013 2466—VOLUME 44A, JUNE 2013

To add Na, a Sn-Na master alloy was first produced by adding Na to molten Sn. After casting, the ingot was solution treated under argon by holding for 2 hours at 593 K (320 C), and then ramped to 773 K (500 C) at a rate of 2 C/min. The sample was held at this higher temperature for 20 hours and then water quenched. Cylindrical billets of 29-mm diameter by 20-mm height were extruded using an MTS 385 kN press at a speed of 0.1 mm/s and an extrusion ratio of 30:1. Temperatures for extrusion were varied from 623 K to 773 K (350 C to 500 C). Following extrusion, the samples were air cooled. Characterization was performed using optical microscopy, transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM), using an FEI CM20 and JEOL 2100F, respectively. Both microscopes were operated at 200 kV. In addition, energy dispersive spectroscopy (EDS) maps were collected in HAADF-STEM mode using the JEOL 2100F. Samples for (S)TEM were prepared using mechanical grinding followed by ion milling to electron transparency with the Gatan PIPS. The extruded microstructures showed a strong sensitivity to the 0.3 wt pct Na-addition,