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Acta Metall Sin  2020, Vol. 56 Issue (3): 340-350    DOI: 10.11900/0412.1961.2019.00229
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Microstructures and Tensile Deformation Behavior of Directionally Solidified Mg-xGd-0.5Y Alloys
SUN Heng,LIN Xiaoping(),ZHOU Bing,ZHAO Shengshi,TANG Qin,DONG Yun
School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China
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Abstract  

The poor plastic deformation ability of magnesium alloy, resulted from its close-packed hexagonal structure and only two independent basal <a> slip systems at room temperature that cannot meet the von Mises criterion, has extremely restricted its application. As the α-Mg dendrites grow along with the heat flow in directional solidification, the uniform columnar crystal structures obtained in Mg can effectively improve its mechanical properties. And the mechanical properties of the anisotropic magnesium alloys were heavily affected by the orientation controlled by the directional solidification parameters. In this work, the effects of Gd content (3.0%, 4.5%, 6.0%, mass fraction) on the microstructure and mechanical properties of directionally solidified Mg-xGd-0.5Y alloy were investigated. The tensile deformation behavior at room temperature was analyzed by EBSD technique. The results showed that the Mg-xGd-0.5Y alloys have a longitudinal grain boundary parallel to the heat flow direction and a preferential growth along the normal direction of the (112?0) plane at a withdrawn rate of 3 mm/min. The cross section of the columnar crystal was triangle or crisscross petal in shape, and the secondary branch gradually changed from three branches of 3.0%Gd to four branches of 6.0%Gd. The Mg-6.0Gd-0.5Y alloy with more columnar crystal growing along with <224?3> direction had higher tensile strength (107 MPa) and post-break elongation (32.56%) at room temperature, and its deformation mechanism was basal <a> slipping and {101?2} extension twinning. When the crystal growth directions dispersed (concentrated on the <1?21?0> and <224?3>) in the Mg-3.0Gd-0.5Y alloy, it had low post-break elongation (14.88%) because of poor synergistic deformation ability, which have {101?2} extension twins and {101?1} contraction twins to accommodate strain.

Key words:  Mg-Gd-Y alloy      crystal orientation      twinning mechanism      post-break elongation     
Received:  12 July 2019     
ZTFLH:  TG146.22  
Fund: National Natural Science Foundation of China(51775099);National Natural Science Foundation of China(51675092);Natural Science Foundation of Hebei Province(E2018501033)
Corresponding Authors:  Xiaoping LIN     E-mail:  1000629@neuq.edu.cn

Cite this article: 

SUN Heng,LIN Xiaoping,ZHOU Bing,ZHAO Shengshi,TANG Qin,DONG Yun. Microstructures and Tensile Deformation Behavior of Directionally Solidified Mg-xGd-0.5Y Alloys. Acta Metall Sin, 2020, 56(3): 340-350.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00229     OR     https://www.ams.org.cn/EN/Y2020/V56/I3/340

Fig.1  Dimension diagram of tensile specimen (unit: mm)
Fig.2  OM images of longitudinal (a, c, e) and transverse (b, d, f) sections of directionally solidified Mg-xGd-0.5Y alloy(a, b) x=3.0 (c, d) x=4.5 (e, f) x=6.0
Fig.3  Pseudo binary phase diagram of Mg-Gd-Y ternary alloy at 0.5Y section
Fig.4  XRD spectra of the Mg-xGd-0.5Y alloys and standard Mg powder
Fig.5  Stress-strain curves (a) and feature distribution map (b) of the Mg-xGd-0.5Y alloys tensioned at room temperature
Fig.6  Microstructures of directionally solidified Mg-6.0Gd-0.5Y alloy stretched to 25% at room temperature(a) macromorphology of the alloy (b~g) magnifications of the boxes of D (b), E (c), F (d, e), H (f) and I (g) in Fig.6a, respectively
Fig.7  EBSD analyses of the tensile deformation microstructures of directionally solidified Mg-6.0Gd-0.5Y alloy, including Euler map (a), large-angle grain boundary map (b), small-angle grain boundary and twin type diagram (c), strain contouring map (d), misorientation distribution map (e), basal <a> slip Schmid factor map (f) and X0 inverse pole figure (g)Color online
Fig.8  Microstructures of directionally solidified Mg-3.0Gd-0.5Y alloy stretched to 10% at room temperature(a) macromorphology (b~e) magnifications of the boxes of D (b), E (c) and F (d, e) in Fig.8a, respectively
Fig.9  EBSD analyses of the tensile deformation microstructure of directionally solidified Mg-3.0Gd-0.5Y alloy, including Euler map (a), large-angle grain boundary map (b), small-angle grain boundary and twin type diagram (c), strain contouring map (d), misorientation distribution map (e), basal <a> slip Schmid factor map (f) and X0 inverse pole figure (g)Color online
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