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Acta Metall Sin  2020, Vol. 56 Issue (2): 203-211    DOI: 10.11900/0412.1961.2019.00100
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Effect of Cooling Rate on Boride and Room Temperature Tensile Properties of β-Solidifying γ-TiAl Alloys
WANG Xi1,2,LIU Renci1(),CAO Ruxin3,JIA Qing1,CUI Yuyou1,YANG Rui1
1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2. College of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
3. College of Mechanical and Power Engineering, China Three Gorges University, Yichang 443002, China
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Abstract  

β-solidifying γ-TiAl alloys have attracted much attention for their higher specific strength and better mechanical properties at elevated temperature. They usually need some boron addition to refine the lamellar grain size, which is believed to improve their poor room temperature ductility. However, the boron addition may cause some side effects on mechanical properties for the formation of borides with unfavorable morphology and crystal structure, which is severely influenced by the alloy composition and cooling rate during casting. The components of γ-TiAl applied usually have complex structure, such as different thicknesses, which leads to different cooling rates and therefore different microstructures and mechanical properties. To evaluate the influence of cooling rate on the microstructure and mechanical properties of γ-TiAl investment casting, plate with step thicknesses was designed to achieve different cooling rates. Step plates of β-solidifying boron-containing TiAl alloy were fabricated by centrifugal casting in Y2O3 facing coating ceramic moulds. It was found that boride mainly distributed on grain boundary, and its aspect ratio increased with increasing cooling rate, with its morphology varying from short, flat plate to long, curvy ribbon. The short plate and curvy ribbon borides were TiB with B27 and Bf structure, respectively. Both types of boride exhibit anisotropic growth characteristics (especially for Bf structure), with the slowest growth rate along [100] and [010] for B27 structure and Bf structure, respectively. This is attributed to the difficulty of atomic rearrangement along corresponding directions during solidification. The cooling rate increase caused the increase of yield strength but the decrease of room temperature ductility, the former results from the decreasing of grain size and lamellar spacing, while the latter results from the easy cracking nucleation and propagation of the long curvy boride, leaving smooth curvy surfaces on the fracture surface. Samples containing short flat plate boride showed better ductility, and no smooth curvy surface was observed.

Key words:  γ-TiAl alloy      β-solidifying      investment casting      cooling rate      boride      tensile property     
Received:  03 April 2019     
ZTFLH:  TG146.2  
Fund: National Natural Science Foundation of China(51701209);National Key Research and Development Program of China(2016YFB0701304);National Key Research and Development Program of China(2016YFB0701305)
Corresponding Authors:  Renci LIU     E-mail:  rcliu@imr.ac.cn

Cite this article: 

WANG Xi,LIU Renci,CAO Ruxin,JIA Qing,CUI Yuyou,YANG Rui. Effect of Cooling Rate on Boride and Room Temperature Tensile Properties of β-Solidifying γ-TiAl Alloys. Acta Metall Sin, 2020, 56(2): 203-211.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00100     OR     https://www.ams.org.cn/EN/Y2020/V56/I2/203

Fig.1  Schematic of step plate casting sample (unit: mm)
Fig.2  OM (a, d, g), BSE-SEM (b, e, h) and SE-SEM (c, f, i) images of microstructures in step plate with 4 mm (a~c), 8 mm (d~f) and 12 mm (g~i) thicknesses after hot isostatic pressing and heat treatment
Fig.3  SE-SEM images of grain boundary (a, b) and EDS analyses of β (c) and boride (d) after hot isostatic pressing and heat treatment in step plate with 4 mm (a, c) and 12 mm (b, d) thicknesses
PositionPhaseTiAlNbMo
1β53.2538.545.153.06
2Boride62.5827.817.991.62
Table 1  EDS analyses of different positions in Figs.3a and b (atomic fraction / %)
Fig.4  Bright field TEM images of lamellar grain in step plate with 4 mm (a) and 12 mm (b) thicknesses
Fig.5  Bright field TEM images (a, c) and SAED patterns (b, d) of borides in step plate with 4 mm (a, b) and 12 mm (c, d) thicknesses (Inset in Fig.5a shows the enlarged view of boride tip in the rectangle)
Fig.6  Tensile strain-stress curves of samples taken from step plate with different thicknesses at room temperature

Thickness

mm

No.

Yield strength

MPa

Ultimate strength

MPa

Elongation

%

417327440.27
27177330.29
816837430.57
26717200.47
1216667931.17
26667560.79
Table 2  Tensile properties of samples taken from step plate with different thickness at room temperature
Fig.7  Fracture surfaces (a, b), crack nucleation sites (c, d) and corresponding BSE-SEM images (e, f) of samples taken from step plate with 4 mm (a, c, e) and 12 mm (b, d, f) thicknesses (Circles in Figs.7a and b show the crack nucleation sites for high magnification observation. The rectangle in Fig.7e shows the lamellar structure in curvy surface of crack nucleation site)
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