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ng Jia Key Laboratory of Automobile Materials, Ministry of Education, Jilin University, Changchun 130025, China

Jianli Wang and Jie Yang State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS), Changchun 130022, China; and Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

Lidong Wang and Limin Wanga) State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS), Changchun 130022, China (Received 5 December 2007; accepted 28 April 2008)

The microstructures and mechanical properties of Mg–6Zn–5Al–4Gd–1RE (RE ⳱ Ce or Y) alloys were investigated. The addition of Ce or Y obviously refines the grain size for the Mg–6Zn–5Al–4Gd-based alloy, while the Y element has a better refining effect. The Ce and Y show different grain-refining mechanisms: Ce addition mostly promotes the growth of secondary dendrite, while Y addition mainly increases the heterogeneous nucleation sites. The hardness-versus-aging time curves indicate that all the alloys have excellent aging-hardening behavior, but the response to maximum hardness was delayed by the Ce or Y addition. The microstructure observation of the peak-aged alloys indicated a large number of nanocrystalline ␶-Mg32(Al, Zn)49 precipitates in the matrix. The Y addition is beneficial to improve the mechanical properties, and the alloy has optimal values. However, the Ce addition decreases the ultimate tensile strength and elongation of the alloy due to formation of a lot of shrinkage porosities.

I. INTRODUCTION

As the lightest structural metallic materials, magnesium alloys are attractive for using in industry.1,2 Among them the Mg–Al alloy systems, such as AZ91, AM60, etc., are most widely used. However, their application has been limited above 120 °C.3 This is because that low strength and the ␤-Mg17Al12 phase in alloys readily softened and coarsened, which decreases the creep and heat resistance of these alloys. The critical methods in improving the strength and heat resistance for the magnesium alloys can be realized through the following aspects: (i) decreasing the diffusion rate of alloying elements in ␣-Mg matrix; (ii) introducing a fine thermal stable dispersion phase to pin the dislocation; (iii) grainboundary strengthening. In recent studies,3–6 it was rea)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0343 J. Mater. Res., Vol. 23, No. 10, Oct 2008

vealed that addition of rare earth (RE) can effectively improve the heat resistance for the Mg–Al alloy systems. The RE elements have a low diffusion rate in ␣-Mg. Moreover, the Al and RE elements generally form intermetallic compounds in Mg alloys, which possess a high melting point, providing an effective hindrance to grain boundary sliding and dislocation movement at elevated temperature. Among Mg–Al–RE alloys, for example, AE42 (Mg–4Al–2RE) alloy, which is getting commercial application, shows higher ther