Microstructure Evolution of GH2909 Low Expansion Superalloy During Heat Treatment
LI Zhao1, JIANG He2(), WANG Tao1, FU Shuhong1, ZHANG Yong1
1.Science and Technology on Advanced High Temperature Structural Materials Laboratory, AECC Beijing Institute of Aeronautical Materials, Beijing 100095, China 2.School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
LI Zhao, JIANG He, WANG Tao, FU Shuhong, ZHANG Yong. Microstructure Evolution of GH2909 Low Expansion Superalloy During Heat Treatment. Acta Metall Sin, 2022, 58(9): 1179-1188.
GH2909 alloy is a low expansion superalloy developed on the base of GH2907 alloy. The mass fraction of Si is increased to accelerate the precipitation of ε phase, which improves resistance to stress-induced oxidative brittleness at grain boundaries. Increasing the mass fraction of Si also complicates the types of precipitates, and there is a long-time argument for determining precipitates in GH2909 alloy. The mechanical property is closely related to microstructure and precipitate. This work investigated the microstructure evolution of GH2909 low expansion superalloy during standard heat treatment by SEM, TEM, EPMA, and micro-chemical phase analysis. The Laves phase is the predominant phase in the wrought GH2909 alloy, according to the study. In the GH2909 alloy, the Si-rich Laves phase has a blocky form and a short rod shape. In solution treatment, the Laves phase dissolves gradually. After two-stage solution treatment, the short rod-shaped Laves phase almost completely dissolves. Slow cooling is needed to avoid re-precipitation of short rod shape Laves phase during solution treatment because Laves phase is sensitive to the cooling rate. Discontinuous G phase particles decorate grain boundaries after normal heat treatment, and a sizable discal phase precipitates in the matrix. There is also a fine phase rich in Ni and Ti in the matrix with the chemical formula Ni2.26Fe0.16Co0.50Nb0.62Ti0.43Al0.02. In the GH2909 alloy, the Laves phase, G phase, and ε phase are high in Si and Nb. During precipitation, these phases compete for Si and Nb elements. Furthermore, the micro-chemical phase analysis results demonstrate that 30% of the Si in the GH2909 alloy is finally precipitated. As a result, Si should be given special consideration in the microstructure control of the GH2909 alloy.
Fund: National Natural Science Foundation of China(51701011);Fundamental Research Funds for the Central Universities(FRF-TP-19-038A2);Key Laboratory Foundation(6142903180205)
980oC, 15 min, AC + 980oC, 1 h, water cooling (WQ)
T4
980oC, 15 min, AC + 980oC, 1 h, AC + 720oC, 8 h, furnace cooling (0.9oC·min-1) to 620oC, 8 h, AC
Table 1 Heat treatment processes of GH2909 in present work
Fig.1 Calculation results of GH2909 alloy by JMatPro software (a) property diagram (Inset is the local magnified diagram) (b) solid fraction variation of phases during solidification
Fig.2 Microstructure characterizations of as-received GH2909 alloy (a) and partial magnification (b)
Fig.3 TEM image and selected area electron diffraction (SAED) patterns of Laves phase in as-received GH2909 alloy (a, b) bright field TEM image of blocky Laves phase and related SAED pattern (c, d) bright field TEM image of short rod-like Laves phase and related SAED pattern
Fig.4 EPMA results of element distribution of Laves phase in wrought GH2909 alloy (SEM—secondary electron morphology, BSE—back-scaterred electron image)
Fig.5 SEM images of GH2909 alloy after solution treatments (a) T1 (b) T2 (c) T3
Fig.6 SEM images of microstructures of GH2909 alloy after standard heat treatment (T4) (a) and partial magnification (b)
Fig.7 TEM images and SAED patterns of G phase on grain boundary in GH2909 alloy after standard heat treatment (T4) (a) TEM images of G phase on grain boundary in GH2909 alloy after standard heat treatment (T4) (b, d) partial magnifications of the areas in Fig.7a, respectively (c, e) SAED patterns of Figs.7b and d, respectively
Fig.8 EPMA results of element distribution in GH2909 alloy after standard heat treatment (T4)
Fig.9 Bright field TEM image of γ′ phase in GH2909 alloy after standard heat treatment (T4)
Fig.10 TEM images of ε phase in GH2909 alloy after standard heat treatment (T4) (a) bright field image (b) dark field image and SAED pattem (inset) (c) high angle annular dark field image and EDS result
Fig.11 Uneven distribution of ε phase in GH2909 alloy after standard heat treatment (T4)
Fig.12 Micro-chemical phase analysis results for GH2909 alloy after different heat treatments (a) mass fraction of intermetallic compound phases (wi) (b) mass fraction of Si element in intermetallic compound phases account for the total mass of alloy (wi-Si)
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