Influence of MC Carbides on the Formation of γ/γ′ Eutectics in Single Crystal Superalloy CM247LC
Dexin MA1, Fu WANG2, Xuhui WEN3, Dejian SUN4, Lin LIU4()
1 Wedge Central South Research Institute, Shenzhen 518045, China 2 Foundry Institute, RWTH Aachen University, Aachen 52072, Germany 3 Energy Conversion R&D Center, Central Academy, Dongfang Electric Co., Chengdu 611731, China 4 State Key Laborotory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
Cite this article:
Dexin MA, Fu WANG, Xuhui WEN, Dejian SUN, Lin LIU. Influence of MC Carbides on the Formation of γ/γ′ Eutectics in Single Crystal Superalloy CM247LC. Acta Metall Sin, 2017, 53(12): 1603-1610.
The structure formation of superalloys is very complicated because of their multicomponent composition and multiphase transition processing. Duo to the limitation of some pre-conditions, the structure formation can not be accurately determined by thermodynamic calculation method. Knowledge about the structure is critical for the design of the following heat treatment process. In this work, a single crystal (SC) sample of superalloy CM247LC was directional solidified in a labor Bridgman furnace with a pulling rate of 0.2 mm/min and then water quenched, to investigate the solidification sequence including MC carbide and γ/γ′-eutectic. It was observed that the γ-phase is firstly formed in the form of dendrites; it is then followed by the homogeneously precipitation of MC carbides from the liquid behind dendrite tips. Near the end of solidification the interdendritic residual liquid transits into γ/γ′-eutectics. It is interesting to found that the γ/γ′ eutectics do not nucleate on the existing γ -phase, but preferably on the MC carbides which have completely different chemical composition and crystal structure. The result of EBSD examination indicates that the γ/γ′ eutectics formed on the MC substrates have random crystal orientations compared to the SC γ -matrix, exhibiting the misoriented multi-crystal microstructure in the so called "single crystal" superalloy casting.
Fund: Supported by National Natural Science Foundation of China (Nos.51331005, 51690163 and 51631008), National Key Research and Development Program (No.2016YFB0701405) and Science and Technology Innovation Commission of Shenzhen Municipality (No.JSGG20150731142227736)
Fig.1 OM images of longitudinal section in the quenched sample (a), and transverse sections in depths of 3.25 mm (b) and 20.00 mm (c) in the mushy zone
Fig.2 Transverse section morphologies of MC carbides in depths of 1.15 mm (a), 1.75 mm (b) and 19.00 mm (c) in the mushy zone
Fig.3 Transverse sections of the water quenched mushy zone showing the nucleation (a, b) and growth process (c, d) of γ /γ' eutectic islands based on the existing MC carbides
Element
Average
ki =
MC element
C
3.38
0.09
37.56
Yes
Al
0.12
5.49
0.02
No
Ti
7.17
0.74
9.69
Yes
Cr
0.33
8.03
0.04
No
Mo
0.63
0.50
1.26
N.D.
Co
0.28
9.41
0.03
No
W
6.43
9.87
0.65
N.D.
Ta
58.60
2.90
20.20
Yes
Hf
21.10
1.36
15.52
Yes
Ni
1.95
61.61
0.03
No
Table 1 The mean compositions of element i in MC carbide () and in alloy () and ki
Fig.4 OM image (a) and EBSD orientation map (b) showing solidification structure consisting of MC carbide, γ /γ' eutectic and γ dendrite
Fig.5 SEM image (a) and EBSD orientation map (b) of another section showing the structure and crystal orientations of γ-dendrite, MC carbide and γ /γ' eutectic
Fig.6 Schematic of solidification sequence in the investigated mushy zone
Fig.7 As-cast structure of a CM247LC casing, showing the γ /γ'-eutectics nucleated on the small MC carbides
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