Microstructure and Creep Behavior of High-Pressure Die-Cast Magnesium Alloy AE44
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I.
INTRODUCTION
MG-AL alloys, e.g., AZ91 (Mg-9Al-0.7Zn-0.2Mn, all compositions in wt pct throughout this article unless specified otherwise) and AM60 (Mg-6Al-0.3Mn), are the most commonly used magnesium alloys in the automotive industry because of good mechanical properties at room temperature, excellent die castability, and adequate corrosion resistance. However, these alloys are prone to excessive creep deformation when exposed to moderate levels of load at temperatures above 398 K (125 C); thus, their use has been restricted to roomtemperature applications where creep is not a concern. The poor creep resistance of Mg-Al alloys is generally considered to be related to the formation of Mg17Al12 phase that has poor thermal stability at elevated temperatures.[1] There have been considerable efforts in improving the creep resistance of Mg-Al alloys by further alloying with rare earth (RE) or alkali earth metals to suppress the formation of Mg17Al12.[2–8] Among the alloys developed, AE42 (Mg-4Al-2RE) has been a benchmark creepresistant Mg die casting alloy, which exhibits superior creep resistance to AZ91 at temperatures up to 423 K (150 C).[2–4] This alloy contains Al as the dominant alloying element to maintain adequate castability, whereas the addition of Ce-based misch metal improves the mechanical properties at elevated temperatures S.M. ZHU, Research Fellow, J.F. NIE, Professor, and M.A. EASTON, Senior Research Fellow, are with CAST CRC, Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia. Contact e-mail: [email protected] M.A. GIBSON, Principle Scientist, is with CAST CRC, CSIRO Process Science and Engineering, Clayton, Victoria 3169, Australia. P. BAKKE, Research Scientist, is with New Materials & Processes, Hydro Aluminium Metal, N-3908 Porsgrunn, Norway. Manuscript submitted October 5, 2011. Article published online June 6, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
because of the preferential formation of Al-RE intermetallic phases, including A111RE3 and A12RE, over Mg17Al12. However, the creep resistance of AE42 deteriorates rapidly at temperatures above 423 K (150 C).[9] Powell et al.[10] proposed a mechanism to explain the deterioration in creep resistance of AE42 at temperatures above 423 K (150 C). According to this mechanism, the A111RE3 phase is unstable and decomposes to the Al2RE phase at elevated temperatures, releasing some Al atoms that subsequently react with Mg atoms to form the Mg17Al12 phase, thus leading to the deterioration in creep resistance. Recently, Zhu et al.[11] investigated the microstructure and microstructural stability of die-cast AE42 in more detail. According to this work, both A111RE3 and Al2RE phase are stable at temperatures up to 473 K (200 C), with no distinct decomposition observed. They also reported the continuous precipitation of Mg17Al12 in the a-Mg matrix after aging at 473 K (200 C) and related the precipitation to the supersaturation of Al solute in the a-Mg matrix. Based on these experimental results, they propo
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