Viewpoint Paper The relationship between microstructure and creep resistance in die-cast magnesium–rare earth alloys S.M. Zhu, a, * M.A. Gibson, b M.A. Easton a and J.F. Nie a a CAST CRC, Department of Materials Engineering, Monash University, Victoria 3800, Australia b CAST CRC, CSIRO Process Science and Engineering, Clayton, Victoria 3169, Australia Received 19 November 2009; revised 28 January 2010; accepted 2 February 2010 Available online 6 February 2010 Abstract—Die-cast Mg–La, Mg–Ce and Mg–Nd binary alloys varying in composition have been used to investigate creep resistance and its relation to microstructure. The remarkable differences in creep resistance observed in these alloys are shown to be related to different levels of rare earth (RE) solute supersaturated in the a-Mg matrix. The results seem to suggest that strengthening of the a- Mg matrix by solid solution and/or precipitation is more important than grain boundary reinforcement by intermetallic phases for the creep resistance of Mg–RE alloys. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium alloys; Creep; Rare earth; Precipitation; Solid solution 1. Introduction Despite its natural abundance and light weight, mag- nesium and its alloys have not found widespread appli- cation. One major hurdle that restricts the widescale use of Mg alloys is the excess creep these materials undergo at elevated temperatures. A widely accepted view is that grain boundary sliding makes a major contribution to creep deformation of Mg alloys [1–4]. For example, Bell and Langdon claimed that the contribution of grain boundary sliding to the creep deformation can be as high as 80% for a Mg–0.78Al (composition in wt.% hereafter unless specified) alloy [3]. Based on this view, the development of creep-resistant Mg alloys has so far focused on the reinforcement of grain boundaries with thermally stable intermetallic particles to inhibit grain boundary sliding. Typical creep-resistant die- casting alloys developed up to date include AE42 (Mg–4Al–2RE) [5], AJ52 (Mg–5Al–2Sr) [6] and AX53 (Mg–5Al–3Ca) [7], MRI153 (Mg–9Al–0.7Zn–1Ca– 0.1Sr) [8] and AE44 (Mg–4Al–4RE) (RE = rare earth) [9]. In recent years, however, increasing numbers of studies have shown that strengthening of the a-Mg ma- trix by solid solution and/or precipitation also has a great influence on the creep resistance of Mg alloys. Maruyama et al. [10] reported that Y has a more signif- icant effect in improving the creep resistance of Mg al- loys than Al, which is largely due to a higher solid- solution strengthening effect. Blum et al. [11] showed that annealing (6 h at 413 °C plus 2 h at 353 °C and 10 h at 413 °C) of die-cast AZ91 (Mg–9Al–1Zn) reduced the secondary creep rate as a result of the precipitation of Mg 17 Al 12 , though the primary creep strain was in- creased. However, as they stated, the precipitation hard- ening is of limited value for the creep resistance of AZ91 because the Mg 17 Al 12 precipitate has a relatively high coarsening rate. Improved creep resistance in die-cast AZ91 after ageing (100 h at 150 °C) was also reported by Dargusch et al. [12]. Suzuki et al. [13] studied the pre- cipitation strengthening effect of Al 2 Ca in a die-cast Mg–5Al–3Ca–0.15Sr alloy and reported that the creep resistance was improved by a factor of 1.5–2 by a peak ageing treatment (1 h at 250 °C). The limited improve- ment in creep resistance was attributed to the low vol- ume fraction of the precipitate. Recently, Zhu et al. [14] reported that the different primary creep strains exhibited by two die-cast Mg–2.5RE–0.6Zn alloys were associated with the different levels of RE solute satu- rated in the matrix, which led to dynamic precipitation during creep. In this work, binary Mg–La, Mg–Ce and Mg–Nd alloys of various compositions (0.5–8% RE) were pro- duced by high-pressure die casting (HPDC) to investi- gate the relationship between microstructure and creep 1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.02.005 * Corresponding author. Tel.: +61 3 99059297; fax: +61 3 99054940; e- mail: suming.zhu@eng.monash.edu.au Available online at www.sciencedirect.com Scripta Materialia 63 (2010) 698–703 www.elsevier.com/locate/scriptamat