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Acta Metall Sin  2015, Vol. 51 Issue (1): 67-76    DOI: 10.11900/0412.1961.2014.00281
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SUN Wen1, 2, QIN Xuezhi2, GUO Jianting2, LOU Langhong2, ZHOU Lanzhang2
1 University of Science and Technology of China, Hefei 230022; 2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016
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Abstract  The Ni-based superalloys are widely used as microstructural components of modern turbine engines due to its good high temperature strength, good fatigue and creep property and excellent hot-corrosion resistance. In order to increase their high temperature strength, more and more refractory elements, such as W and Mo, are added into these alloys while Cr content gradually decreases. During long-term aging, these alloys generally experience various microstructural changes, including coarsening of g' phase coarsening, formation of a continuous grain boundary (GB) carbide network, precipitation topologically close-packed (TCP) phase, and degeneration of MC carbide. However, there is limited available data about the effect of (W+Mo)/Cr ratios on the microstructural evolution of Ni-based superalloys. In this work, the cast Ni-based superalloys with different (W+Mo)/Cr ratios (mass ratios) are fabricated by vacuum induction furnace. After standard heat treated (1110 ℃, 4.5 h, air cooling+750 ℃, 10.5 h, air cooling), they are thermally exposed at 850 ℃ for different times. The stress-rupture tests are operated under the condition of 800 ℃, 294 MPa. Effects of (W+Mo)/Cr ratios on the microstructure evolutions and mechanical properties are investigated by the combination of OM, SEM, TEM and stress-rupture tests. The experiment results show that the (W+Mo)/Cr ratio has no obvious influence on the standard heat treated microstructure, which is mainly composed of g matrix, g' phase, MC carbide and secondary carbides distributing at grain boundaries. During long-term thermal exposure, the microstructure evolutions occur by g' phase coarsening, TCP phases formation, MC degeneration and grain boundary coarsening. The g' phase coarsening behavior is not affected obviously by the (W+Mo)/Cr ratio. However, the amount of TCP phases decreases significantly with decreasing of (W+Mo)/Cr ratio and the type of TCP phases transforms from m phase to coexist of m and s phases when (W+Mo)/Cr ratio decreases from 0.55 to 0.37. There are no TCP phases observed in the sample with (W+Mo)/Cr ratio of 0.22. The thermal stability of MC carbide is reduced obviously and the grain boundaries coarsen more severely by the decrease of (W+Mo)/Cr ratio. The degradation of stress-rupture property is attributed to the coarsening of g' phase and grain boundaries and the formation of TCP phases. Combined with the effect of (W+Mo)/Cr ratio on the solid solution strengthening, microstructure evolution and stress-rupture property, it can be concluded that the optimum stress-rupture property can be obtained when the (W+Mo)/Cr ratio is about 0.37.
Key words:  Ni-based superalloy      (W+Mo)/Cr ratio      long-term aging      microstructural evolution      mechanical property     
ZTFLH:  TA211.8  
Fund: ; Supported by National Natural Science Foundation of China (No.51001101) and High Technology Research and Development Program of China (No.2012AA03A501)
Corresponding Authors:  Correspondent: QIN Xuezhi, associate professor, Tel: (024)83978469, E-mail:     E-mail:

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Typical microstructures of sample A1 after heat treatment

(a) OM image
(b) SEM image of g' phase
(c) SEM image of grain boundary


SEM images of g' phase in different alloy samples after aged at 850 ℃ for different times


Microstructures of A1 (a), A2 (b), A3 (c) and A4 (d) after aged at 850 ℃ for 0.5×103 h (The insets corresponds to the TEM images and SAED patterns)


Microstructures of A1 (a), A2 (b), A3 (c) and A4 (d) after aged at 850 ℃ for 1×104 h


Morphologies of MC degeneration after thermal exposure at 850 ℃ for 1×103 h in A1 (a), A2 (b), A3 (c) and A4 (d)


Morphologies of MC degeneration after thermal exposure at 850 ℃ for 1×104 h in A1 (a), A2 (b), A3 (c) and A4 (d)


Degeneration degree (D) of primary MC changes with aging time t


Morphologies of grain boundaries after thermal exposure at 850 ℃ for 1×104 h in A1 (a), A2 (b), A3 (c) and A4 (d)


Effect of (W+Mo)/Cr ratio on stress-rupture life of alloys after different exposure time


Cross section microstructures of samples A1 (a), A2 (b), A3 (c) and A4 (d) after thermal exposure at 850 ℃ for 1×104 h under stress-rupture test at 800 ℃, 294 MPa (GB—grain boundary)

Table 1  

Chemical compositions of Ni-based superalloys

Table 2  

Compositions and thermal stability parameters of MC in alloys

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