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rk Easton and Suming Zhu Department of Materials Engineering, CAST Cooperative Research Centre, Monash University, Victoria 3800, Australia

Guohua Wua) and Wenjiang Ding School of Materials Science and Engineering, National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, Shanghai 200240, China; and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China (Received 25 May 2012; accepted 24 August 2012)

The addition of Al to a Mg–10Gd alloy was found to lead to substantial grain size reduction during casting at concentrations between 0.8% and 1.3%. At these concentrations, Al2Gd particles were found at the center of grains, and the orientation relationship ½112Al2 Gd k ½2110aMg ; ð110ÞAl2 Gd k ð0110ÞaMg was found reproducibly between Al2Gd and a-Mg, indicating that these are the heterogeneous nucleant particles that form in situ at these Al contents. Most of these nuclei were between 2 and 7 lm in size. Furthermore, little grain coarsening was observed during solution treatment, particularly compared with an alloy grain refined by Zr particles where substantial coarsening occurred. This appears to be because Al2Gd particles restrict grain boundary motion during solution treatment. I. INTRODUCTION

As the lightest structural metal, magnesium alloys have high specific strength and good machinability and are attractive for aerospace, automobile, and electronic applications. Mg alloys can be classified into two broad groups: Al free and Al bearing. Although Mg–Al alloys have been widely used due to their good room temperature mechanical properties and castability, their poor creep resistance and significant reduction in tensile stress at elevated temperatures (.150 °C) limit their industrial application.1 In contrast, Mg alloys containing rare-earth (RE) elements (Mg–RE alloys) have attracted a lot of interest because of their better mechanical properties at elevated temperatures.1 Among various Mg–RE alloys, the Mg–Gd system is particularly promising owing to the dramatic precipitation strengthening response and thermally stable main strengthening phase.2,3 Grain refinement often leads to a refinement of secondary phases and a reduction in microsegregation and improves the ductility, strength, machinability, and castability of alloys.4,5 Mg alloys have a high Hall–Petch coefficient,6 thus grain refinement increases their strength considerably. It has also been found that refining the grain size can reduce the steady-state creep rate in cast Mg–RE alloys.7 There has been a lot of research on the grain a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2012.313 2790

J. Mater. Res., Vol. 27, No. 21, Nov 14, 2012

http://journals.cambridge.org

Downloaded: 07 Apr 2015

refinement of Mg alloys.4 Zr is a very powerful grain refiner for Al-free alloys,4 and it is often introduced through adding commercial binary Mg–Zr master alloys containing Zr particles ranging from submicrometer to about 50 lm in size.8 D