1. Key Lab for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110819, China 2. Research Institute, Baoshan Iron & Steel Co Ltd, Shanghai 201900, China 3. High Temperature Materials Research Division, Central Iron & Steel Research Institute, Beijing 100081, China
Cite this article:
Zixing WANG,Shuo HUANG,Beijiang ZHANG,Lei WANG,Guangpu ZHAO. Study on Freckle of a High-Alloyed GH4065 Nickel Base Wrought Superalloy. Acta Metall Sin, 2019, 55(3): 417-426.
GH4065 alloy is a new type of high-alloyed wrought superalloy, in which freckle defect is extremely prone to form in large ingot. In the present research, the freckle of GH4065 alloy bar with the diameter of 280 mm produced by vacuum induction melting (VIM)+electroslag remelting (ESR)+vacuum arc remelting (VAR) triple smelting was studied. The macrostructure, secondary phases and grain structure of the freckle were investigated, the influences of solute elements on the freckle were analyzed, and both the mechanism and control methods were also discussed. The results show that the freckle in GH4065 alloy is caused by channel segregation with the low-density Ti and Nb-rich melt flows. Additionally, lots of lath-like η-phases, block M3B2 borides and MC carbides are formed in the forged condition. It is confirmed by the thermodynamic calculations that the η-phases, M3B2 borides and MC carbides are much easier forming in the freckle than that in matrix. After heat treatment, compared with matrix, the lathy η-phases are still existed in the freckle; the size and quantity of primary γ′ phases increase significantly while the size and morphology of the secondary γ′ phase are basically identical, only with less quantity. It has been found that due to the high content of γ′ phase, the γ′ dissolution temperature in the freckle is higher than that in the matrix. This induces an impeded recrystallization process caused by the coarsened γ′ phases during forging process and the grain size of the freckle region is significantly smaller than that of matrix. Based on this study, the formation of freckle can be effectively controlled by meticulous controlling of the previous smelting process, releasing of electrode residual stress, suitably reducing VAR melting rate, and accelerating VAR cooling.
Fig.2 Backscattered electron (BSE) image (a) and EPMA line scanning (b) of the GH4065 alloy freckle
Fig.3 BSE image and EPMA elemental mappings of the GH4065 alloy freckle
Morphology
Al
Ti
Nb
Cr
Co
Mo
W
B
C
Ni
Phase
Block
0.10
37.20
34.71
0.72
0.79
4.40
6.01
1.47
12.40
2.20
MC
Block
0.10
4.18
3.23
19.74
2.32
38.20
22.11
8.03
1.03
3.06
M3B2
Lath-like
2.42
12.10
2.09
3.62
8.68
1.96
2.65
0.52
0.81
67.97
η
Particle
4.18
11.69
1.16
3.28
7.78
0.95
1.63
0.43
0.92
69.97
γ′
Table 2 EPMA analyses of the secondary phase in the GH4065 alloy freckle (mass fractoin / %)
Fig.4 TEM image of the carbide and boride formed in the GH4065 alloy freckle (a) and electron diffraction pattern of M3B2 (b)
Fig.5 SEM images of normal region (a~d) and freckles (e~h) in the forged (a, b, e, f) and heat-treated (c, d, g, h) GH4065 alloys (Insets in Fig.5d and h show the tertiary γ' morphologies)
Fig.6 EBSD grain boundary reconstruction photos of the forged (a) and heat treated (b) GH4065 alloy freckle (RX—recrystallization)
Fig.7 EBSD misorientation distributions chart of theGH4065 alloy freckle
Fig.8 EBSD results of the forged (a~c) and heat treated (d~f) GH4065 freckle showing banded profile and grain boundary reconstruction (a, d), phase distribution (b, e) and orientation (c, f) (GB—grain boundary, Co——coherency, Inco—incoherency)
Fig.9 Elemental distribution curves of the GH4065 alloy during solidification
Fig.10 Density comparison between matrix and freckle regions of the GH4065 alloy
Fig.11 Macrostructure of GH4065 alloy VAR ingot with the diameter of 508 mm (VAR—vacuum arc remelting)
Fig.12 Dendritic microstructure of GH4065 alloy ingot
Phase
Position
Solidification
Thermodynamic
Precipitated beginning Tem. / ℃
Mass fraction / %
Solidus Tem. / ℃
Mass fraction / %
γ
Freckle
1294.8
Bal.
1294.8
Bal.
Matrix
1350.5
Bal.
1350.5
Bal.
γ′
Freckle
1160.0
39.0
1134.1
40.8
Matrix
1100.9
30.0
1108.6
38.3
η
Freckle
1220.0
8.8
1215.2
17.7
Matrix
1196.2
1.3
-
0
M2B3
Freckle
1150.0
0.26
1112.8
0.62
Matrix
1178.2
0.10
1098.1
0.18
MC
Freckle
1282.1
0.37
1282.3
0.41
Matrix
1265.0
0.074
1272.7
0.081
Table 3 Thermodynamic statistics of the secondary phase in the GH4065 alloy during solidification process and at thermodynamic equilibrium
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