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Acta Metall Sin  2018, Vol. 54 Issue (4): 566-574    DOI: 10.11900/0412.1961.2017.00240
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Oxidation and Microstructure Evolution of CoAl Coating on Directionally Solidified Ni-Based Superalloys DZ466
Weipeng REN1(), Qing LI1, Qiang HUANG1, Chengbo XIAO1, Limin HE2
1 Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China
2 Aviation Key Laboratory of Science and Technology on Advanced Corrosion and Protection for Aviation Materials, Beijing Institute of Aeronautical Materials, Beijing 100095, China
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Abstract  

Aluminide coatings are widely employed to protect internal cooling channels of high grades blades and buckets in gas turbines have always been in severe conditions including high temperature oxidation and hot corrosion. There is a major concern for the application of aluminide coatings that refer to the inter-diffusion between aluminide coating and superalloy substrate at high temperatures. Diffusion of Al from the coating to the underlying substrate usually leads to depletion of Al in the coating, resulting in inferior oxidation resistance of the coating. Accordingly, Ni declines to diffuse counter currently from the substrate into the coating, as well as other refractory elements, such as Cr, Mo and W etc.. The inter-diffusion between aluminide coating and superalloy substrate results in degradation or various evolution behaviors of aluminide coatings, in other words, substrate composition significantly affects the properties of aluminide coatings. CoAl coating was prepared on directionally solidified superalloy DZ466 by low pressure chemical vapour deposition (LP-CVD). Oxidation behavior and microstructure evolution of CoAl coating was investigated during long term (about 5000 h) exposure at 900 ℃. Results suggested that, high concentration of aluminum did help to form Al2O3 on the surface of coating, improving oxidation resistance of DZ466 at 900 ℃. Evolution of matrix phase and precipitates in the CoAl coating during exposure was displayed, β-NiAl/CoAl phase in the coating transformed gradually to γ'-Ni3Al phase, higher transformation rate for the γ' phase closed to the substrate due to the diffusion between the coating and the sub strate superalloy. During exposure, α-Cr phase precipitated in the middle layer, which inclined to form close to carbides and grow by consuming them. Needle like TCP phase (μ phase) grew in the inner layer that arranged in order, which was due to the cubic microstructure of γ/γ'. Heredity-effect was in company with the precipitates evolution.

Key words:  Ni-based superalloy      DZ466      CoAl      coating      oxidation     
Received:  16 June 2017     
ZTFLH:  TG174.4  
Fund: Supported by National Science and Technology Major Project of China (No.2012ZX04007-021-03)

Cite this article: 

Weipeng REN, Qing LI, Qiang HUANG, Chengbo XIAO, Limin HE. Oxidation and Microstructure Evolution of CoAl Coating on Directionally Solidified Ni-Based Superalloys DZ466. Acta Metall Sin, 2018, 54(4): 566-574.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00240     OR     https://www.ams.org.cn/EN/Y2018/V54/I4/566

Fig.1  Oxidation kinetics curves of DZ466 and DZ466-CoAl for exposure at 900 ℃

(a) mass gain vs time t

(b) curves of mass gain per aera Δm/A by t1/2 (c) mass change vs t

Fig.2  Surface morphologies of DZ466-CoAl as-deposited (a) and exposure at 900 ℃ for 500 h (b), 1000 h (c) and 5029 h (d)
Fig.3  XRD spectra of DZ466-CoAl as-deposited (a) and exposure at 900 ℃ for 500 h (b)
Fig.4  Surface SEM images of DZ466-CoAl as-deposited (a), and exposured at 900 ℃ for 500 h (b~d) and 5029 h (e), and EDS of the square aera in Fig.4d (f) (Figs.4c and d show the enlarged view of zones I and II in Fig.4b, respecticvely)
Fig.5  Cross-sectional SEM images of DZ466-CoAl as-deposited (a) and exposure at 900 ℃ for 5029 h (b) (Inset is the high magnification of the corresponding area tied up by the rectangle)
Phase Al Co Ni Ti Cr Mo W Hf Ta
β-NiAl/CoAl 27.39 7.98 61.69 0.56 0.78 - - - -
β 19.21 7.56 63.26 0.88 5.99 - - - -
MC - 2.80 5.37 2.93 1.09 - - 48.99 37.26
M6C - 7.55 10.27 0.68 13.56 7.80 46.37 - 12.88
M23C6 2.69 12.56 23.46 3.78 32.91 6.99 16.34 - -
μ 1.57 6.99 25.37 0.87 10.35 8.21 35.25 - 10.17
Table 1  Composition of phases in Fig.5(mass fraction / %)
Fig.6  Kinetics curve of γ' in the aluminide coating at 900 ℃
Fig.7  Schematic for evolution of CoAl coating[18]

(a) as deposited (b) exposure for short term (c) exposure for long term

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