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Acta Metall Sin  2018, Vol. 54 Issue (6): 911-917    DOI: 10.11900/0412.1961.2017.00400
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Effect of Zr Addition on the Grain Refinement Mechanism of Mg-Gd-Er Alloys
Shubo LI1, Wenbo DU1(), Xudong WANG2,3, Ke LIU1, Zhaohui WANG1
1 College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
2 Beijing Institute of Graphene Technology, Beijing 100095, China
3 AVIC Beijing Institute of Aeronautical Materials, Beijing 100095, China
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Shubo LI, Wenbo DU, Xudong WANG, Ke LIU, Zhaohui WANG. Effect of Zr Addition on the Grain Refinement Mechanism of Mg-Gd-Er Alloys. Acta Metall Sin, 2018, 54(6): 911-917.

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Abstract  

In recent years, Zr is widely used as an important additive element in magnesium alloys containing rare earth (RE), to improve the mechanical properties of Mg-RE alloys such as strength, ductility, creep resistance and corrosion resistance property. Heterogeneous nucleation mechanism and peritectic reaction mechanism are recognized as the main grain refining mechanisms. Whereas, during the solidification process, the melt wetting angle and nucleation energy are important factors which influence the nucleation. In this work, the effect of Zr on the solidification microstructure of the Mg-Gd-Er alloy was analyzed by using OM and EBSD; the undercooling of alloy melts was tested by using DSC; and the Mg/Zr interface relationship and interfacial energy were investigated by using HRTEM. Moreover, the effects of Zr on the wetting angle and nucleation activation energy of the Mg-11Gd-2Er and Mg-11Gd-2Er-0.4Zr alloys were investigated; the refinement mechanism of Zr on the alloys was discussed. The results indicates that the addition of Zr element can significantly refine the grain, and the grain size decreased from 1000 μm to 50 μm. Compared with the Zr-free alloy, the nucleation wetting angle of the present alloy melt decreased from 18.3° to 11.1°, and the activation energy of nucleation decreased by 44.4%. The (1010) plane of Mg was completely coherent with the (1100) plane of Zr, reducing the interfacial energy between the (1010)Mg and the (1100)Zr. The grain refinement of Mg-Gd-Er alloy was ascribed to the decrease of melt wetting angle and the fully coherent interface relationship between Mg and Zr.

Key words:  Mg-Gd-Er-Zr alloy      wetting angle      nucleation activation energy      interfacial energy      grain refinement mechanism     
Received:  22 September 2017     
ZTFLH:  TG146  
Fund: Supported by National Key Research and Development Program of China (No.2016YFB0301001) and Beijing Natural Science Foundation (Nos.2172013 and 2162003)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00400     OR     https://www.ams.org.cn/EN/Y2018/V54/I6/911

Alloy Mg Gd Er Zr
Mg-11Gd-2Er 87.65 10.62 1.73 -
Mg-11Gd-2Er-0.4Zr 87.39 10.54 1.69 0.38
Table1  Chemical compositions of the tested alloy (mass fraction / %)
Fig.1  The effect of Zr on the grain size of Mg-11Gd-2Er alloy (a, b) and Mg-11Gd-2Er-0.4Zr alloy (c, d)
(a, c) bright field OM images (b, d) dark field OM images
Fig.2  EBSD analyses (a, b) and grain size distributions (c, d) of Mg-11Gd-2Er alloy (a, c) and Mg-11Gd-2Er-0.4Zr alloy (b, d)
Fig.3  SEM image (a) and EDS map of Zr (b) of Mg-11Gd-2Er-0.4Zr alloy
Fig.4  TEM image (a) and EDS (b) of Zr core
Fig.5  DSC curves of Mg-11Gd-2Er and Mg-11Gd-2Er-0.4Zr alloys
(a) melting curves (b) solidification curves
Alloy TL TN ΔT
Mg-11Gd-2Er 904.3 889.2 15.1
Mg-11Gd-2Er-0.4Zr 905.7 894.4 11.3
Table 2  Phase transition temperatures of Mg-11Gd-2Er and Mg-11Gd-2Er-0.4Zr alloys (K)
Fig.6  HRTEM image of interface between the Mg and Zr core
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