Effect of Ca additions on microstructure and microhardness of an as-cast Mg–5.0 wt.% Al alloy Lihong Han ⁎ , Henry Hu, Derek O. Northwood Department of Mechanical, Automotive and Materials Engineering, University of Windsor, Windsor, Ontario, Canada N9B 3P4 Received 16 March 2007; accepted 17 May 2007 Available online 25 May 2007 Abstract The effects of Ca additions (0.5–2.0 wt.%) on the microstructure and the microhardness of an as-cast Mg–5.0 wt.% Al alloy have been investigated. The coarse microstructure of the base alloy can be refined through adding Ca. DSC and TEM results showed that, as Ca additions increased up to 1.5 wt.% Ca, the β-Mg 17 Al 12 phase was completely replaced by a (Al, Mg) 2 Ca phase. The Vickers microhardness of the as-cast Mg–Al–Ca alloys increased with increasing Ca content. Tests on the Mg–5.0Al–2.0Ca (wt.%) alloy showed an indentation size effect, which was well described by Meyer's Law. © 2007 Elsevier B.V. All rights reserved. Keywords: Mg–Al–Ca alloy; Hardness; Indentation size effect; Microstructure 1. Introduction The low specific gravity of magnesium has stimulated the development of Mg alloys for applications in the automotive and aerospace industries [1]. Conventional magnesium casting alloys have been based essentially on the Mg–Al system, with additions of Zn, Mn, and Si in some cases. For example, the commercial AZ91alloys (Mg–9Al–1Zn, wt.%), which have good castability, mechanical properties and corrosion resis- tance, are widely used in the automotive industry. However, due to the rapid degradation of the mechanical properties at elevated temperatures, especially the creep resistance, application of these alloys is limited to specific components that operate at temperatures below 150 °C [1,2]. Although the high temper- ature mechanical properties can be improved by adding Si to the Mg–Al base alloys, e.g. alloy AS21, the castability of the alloy deteriorates [3]. Previous work has shown that an improvement in micro- structure and mechanical properties, including creep resistance, can be achieved by the addition of rare earth elements to Mg alloys [4,5]. However, the cost of these alloys is somewhat higher than for conventional magnesium alloys. Ca is a relatively inexpensive alloying element, and the addition of Ca to Mg–Al alloys can significantly refine the as-cast microstructure of the alloys [6], reduce the oxidation during melting, and significantly improve the mechanical properties at room or high temperatures [7,8]. However, a detailed study of the microstructural dependence on the level of Ca addition, or the effect of the resulting microstructure on the microhardness of Mg–Al–Ca alloys, has not been reported. The eutectic compounds formed in Mg–Al–Ca alloys are variously reported to be Al 2 Ca [6], Mg 2 Ca [9], (Al, Mg) 2 Ca [7,8] and mixtures of these three phases [10] because of the similarity of their crystal structures. Given the discrepancy in the published results for the microstructure and phase composition of as-cast Mg–Al–Ca alloys, a more detailed examination seemed to be warranted. Also, some materials show an indentation size effect (ISE) in a decrease in hardness with increase in load. It was not known whether the Mg–Al–Ca alloys would show such an ISE effect. In the present study, Mg–5.0 wt.% Al alloys containing 0.5 to 2.0 wt.% Ca were cast using a permanent mold process. The resulting microstructures were characterized by SEM, TEM and optical microscopy. The Vickers hardnesses were determined Available online at www.sciencedirect.com Materials Letters 62 (2008) 381 – 384 www.elsevier.com/locate/matlet ⁎ Corresponding author. Tel.: +1 519 2533000x4155. E-mail address: hanb@uwindsor.ca (L. Han). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.05.047