DOI: 10.1002/adem.201200054 Production of High Purity Magnesium Alloys by Melt Purification with Zr** By Arvind Prasad, Peter J. Uggowitzer, Zhiming Shi and Andrej Atrens* Magnesium (Mg) alloys have a low density and a good strength to weight ratio, so are interesting for such transport applications as cars, aircraft, and personal electronic devices. However, Mg is inherently reactive so that Mg alloys have poor resistance to corrosion. [1–7] There is also the perception that Mg burns easily. [8,9] Recent evaluations of WE43 and AZ31 [10,11] used tests that simulated an impact-survivable aircraft accident in which spilled fuel ignited and entered the cabin. This research [10,11] indicated that the Mg alloy WE43 did not increase the hazard level inside the passenger cabin when compared with base line tests using typical aluminum seat-frame components. The poor corrosion resistance of Mg alloys [1–7] results from (i) the high intrinsic corrosion tendency of Mg, which is only weakly inhibited by corrosion product films, and (ii) the presence of second phases acting as local cathodes and thus causing micro-galvanic acceleration of corrosion, so that Mg alloys have corrosion rates greater than that of high purity (HP) Mg. Regarding corrosion, HP Mg means that the impurity elements, Fe, Ni, Cu, and Co are each below their tolerance limit, [1–7,12,13] which depend on the alloy composition. [13] Mg alloys have high corrosion rates [1–7,12,13] if the Mg alloy contains any of these impurity elements at a concentration greater than the tolerance limit for that element. The impurity content has dominated corrosion behavior in some stu- dies [13–15] even though completely different phenomena were the subject of investigation. The impurity element Fe is of particular importance because Mg alloys are typically melted using steel crucibles, and Fe pickup by the molten Mg is a significant possibility unless appropriate precautions are taken. The tolerance limit of Fe was explained by Liu et al. [13] in terms of the calculated MgFe phase diagram, as given in Figure 1. The MgFe phase diagram is an eutectic phase- diagram with an eutectic temperature of 650 8C. The alpha-Mg (with a hexagonal close packed (HCP) crystal structure and labelled ‘‘HCP’’ in Figure 1) has a maximum Fe solubility of 10 ppm Fe. Cooling from the melt (i.e., from the area labeled ‘liquid’ in Figure 1) of a MgFe alloy containing more than 180 ppm, causes precipitation, from the melt, of an Fe-rich body centred cubic (BCC) phase (labelled ‘‘BCC’’ in Figure 1). For a Fe content of 180 ppm, the calculated phase diagram predicts that on cooling, the liquid Mg alloy undergoes eutectic solidification at 650 8C to form a-Mg containing about 10 ppm iron in solid solution plus the BCC phase. For hypo-eutectic Fe contents also, the solidification products in equilibrium would also be the HCP phase (a-Mg) and the HCP-BCC eutectic. However, the two-phase region (liquid Mg þ a-Mg) is extremely narrow, so that it would be expected that the pre-eutectic and eutectic reactions would be suppressed during normal (non-equilibrium) cooling of a Mg ingot or casting, so that a Mg alloy containing less than 180 ppm Fe would tend to solidify to a single a-Mg phase with Fe in supersaturated solid solution in the Mg lattice. The implication is that castings normally are single-phase up to a COMMUNICATION [*] Dr. A. Prasad, Dr. Z. Shi, Prof. A. Atrens The University of Queensland, Materials Division, Brisbane, Qld 4072, Australia E-mail: andrejs.atrens@uq.edu.au Prof. P. J. Uggowitzer ETH Zurich, Department of Materials, CH-8093 Zurich, Switzerland [**] The research on Mg corrosion, flammability, and stress cor- rosion cracking is supported by Boeing Research and Techno- logy, CAST CRC, the Australian Research Council Centre of Excellence Design of Light Alloys, and GM Global Research and Development. CAST CRC was established under, and is funded in part by the Australian Federal Government’s Cooperative Research Centre scheme. Support is gratefully acknowledged of Donald S Shih and of Boeing Research & Technology, St Louis, Missouri, USA. The efficacy of Zr in removing Fe from molten Mg-X binary alloys was studied experimentally and using calculated phase diagrams. Zr is effective in reducing the Fe content from the Mg melt for Mg-X binary alloys, X ¼ Y, Si, Sn, Ca, Sr, Ce, Gd, Nd, Li, La, Mn, and Zn. Purification occurs by the precipitation from the melt of Fe rich precipitates, and the settling of the precipitates to the bottom of the melt. Any desired Fe content down to 1 ppm can be achieved by appropriate melt treatment. ADVANCED ENGINEERING MATERIALS 2012, 14, No. 7 ß 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 477