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Acta Metall Sin  2016, Vol. 52 Issue (4): 445-454    DOI: 10.11900/0412.1961.2015.00389
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EFFECTS OF TOPOLOGICALLY CLOSE PACKED μ PHASE ON MICROSTRUCTURE AND PROPERTIES IN POWDER METALLURGY Ni-BASED SUPERALLOY WITH Hf
Yiwen ZHANG1,2(),Benfu HU3
1 High Temperature Material Institute, Central Iron and Steel Research Institute, Beijing 100081, China
2 Beijing Key Laboratory of Advanced High Temperature Materials, Central Iron and Steel Research Institute, Beijing 100081, China
3 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Cite this article: 

Yiwen ZHANG,Benfu HU. EFFECTS OF TOPOLOGICALLY CLOSE PACKED μ PHASE ON MICROSTRUCTURE AND PROPERTIES IN POWDER METALLURGY Ni-BASED SUPERALLOY WITH Hf. Acta Metall Sin, 2016, 52(4): 445-454.

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Abstract  

It is widely acknowledged that topologically close packed (TCP) phases are detrimental to comprehensive properties of superalloys, as TCP phases deplete strengthening elements from matrix and easily become crack initiations. In this work, the precipitation kinetics and morphology of topologically close packed μ phase in FGH4097 powder metallurgy (PM) superalloy with (0~0.89%)Hf and the effect of μ phase on the mechanical properties of FGH4097 PM superalloy billet with 0.30%Hf has been investigated. The results showed that μ phase precipitated obviously in the alloys with 0.30%Hf and 0.89%Hf after long-term ageing at 750~900 ℃, the amount and size of μ phase increased as the ageing temperature, ageing time and Hf content increasing. μ phase mainly precipitated in grains with strip and flake shapes. After long-term ageing at 550~650 ℃, no μ phase precipitated in FGH4097 PM superalloy billet with 0.30%Hf and the tensile properties and stress-rupture properties at high temperature were not decreased, which showed excellent microstructure stability. After long term ageing at 750 ℃, precipitated μ phase had little effect on tensile strength at high temperature, however, the tensile ductility increased and high temperature stress rupture life reduced, and the stress rupture ductility increased by about 20%. In this work, the precipitation behavior of μ phase, the redistribution of elements in γ solid solution and the FGH4097 PM superalloy fracture morphology characteristics have been discussed in detail. The mechanism of the brittle and ductile dual effect of μ phase on the mechanical properties has been explained. The methods of controlling and avoiding excessive μ phase precipitation which leaded to performance deterioration have been proposed.

Key words:  powder metallurgy superalloy      FGH4097      Hf      μ      phase      mechanical property     
Received:  16 July 2015     
Fund: Supported by International Science & Technology Cooperation Program of China (No.2014DFR50330)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00389     OR     https://www.ams.org.cn/EN/Y2016/V52/I4/445

Fig.1  γ′ precipitates and carbides in heat treated FGH4097 alloys with Hf contents of 0 (a), 0.16% (b), 0.30% (c), 0.58% (d) and 0.89% (e) (Insets show morphologies of γ′ precipitates)
Fig.2  SEM images of FGH4097 with 0.89%Hf after ageing at 850 ℃ (a) and 900 ℃ (b) for 1000 h
Fig.3  TEM image and indexed SAED pattern (inset) of μ phase (a) and its EDS result (b) in FGH4097 alloy with 0.89%Hf after ageing at 900 ℃ for 1000 h
Element Mass fraction / % Atomic fraction / %
Al 0.26 0.78
Ti 0.33 0.56
Cr 10.65 16.55
Co 16.88 23.13
Ni 20.57 28.30
Nb 1.55 1.35
Mo 18.58 15.64
W 31.18 13.70
Table 1  Compositions of μ phase measured by EDS
Fig.4  TEM image and indexed SAED pattern (inset) (a) and EDS result (b) of M6C in FGH4097 alloy with 0.89%Hf after ageing at 850 ℃ for 1000 h
Element Mass fraction / % Atomic fraction / %
C 26.13 71.51
Cr 17.11 10.81
Co 1.67 0.93
Ni 2.96 1.65
Nb 1.99 0.70
Mo 33.14 11.35
W 17.00 3.04
Table 2  Compositions of M6C measured by EDS
Fig.5  TEM image and indexed SAED pattern (inset) (a) and its EDS result (b) of M23C6 in FGH4097 alloy with 0.89%Hf after ageing at 850 ℃ for 1000 h
Element Mass fraction / % Atomic fraction / %
C 1.82 8.21
Cr 73.99 76.93
Co 3.20 2.93
Ni 5.19 4.77
Mo 9.33 5.26
W 6.47 1.90
Table 3  Compositions of M23C6 measured by EDS
Fig.6  SEM images of FGH4097 alloy without (a~c) and with 0.30%Hf (d~f) and 0.89%Hf (g~i) heat treated (a, d, g), and after ageing at 900 ℃ for 200 h (b, e, h) and 1000 h (c, f, i)
Status γ′ MC M3B2+M6C+μ M23C6
Heat treated 62.78 0.296 0.133 (no μ phase) -
Aged at 650 ℃ for 5000 h 64.04 0.281 0.186 -
Aged at 700 ℃ for 5000 h 64.12 0.254 0.206 0.045
Aged at 750 ℃ for 5000 h 64.21 0.214 1.234 0.158
Table 4  Compositions of precipitations in FGH4097 alloy disk billet before and after long term ageing treatment (mass fraction / %)
Fig.7  SEM images of FGH4097 alloy disk billet after ageing at 750 ℃ for 1000 h (a), 2000 h (b) and 5000 h (c)
Fig.8  Complete morphology (a) and image of fracture source (b) of rupture specimen tested at 650 ℃ for FGH4097 alloy disk billet after ageing at 650 ℃ for 5000 h
Ageing temperature / ℃ Ageing time / h σb / MPa σ0.2 / MPa δ / % Ψ / % τ / h
550 2000 1190 975 17.5 19.5 264
5000 1190 980 22.0 21.0 281
650 2000 1190 980 23.0 24.0 285
5000 1240 1040 15.0 14.5 260
700 2000 1150 930 31.5 31.5 245
5000 1180 960 22.5 23.5 269
750 2000 1140 975 23.0 24.5 248
5000 1160 985 11.0 15.0 253
Heat treated - 1220 1020 12.5 15.5 243
Table 5  Tensile property at 750 ℃ and stress rupture life at 650 ℃ of FGH4097 alloy disk billet before and after ageing treatment
Fig.9  Complete morphology (a) and image of fracture source (b) of rupture specimen tested at 750 ℃ and 637 MPa for FGH4097 alloy disk billet after ageing at 750 ℃ for 5000 h
Ageing temperature / ℃ τ / h δ / %
550 757 9.0
650 962 8.0
700 263 12.0
750 430 12.0
Heat treated 754 9.0
Table 6  Stress rupture life (τ) and elongation (δ) at 750 ℃ and 637 MPa of FGH4097 alloy disk billet before and after ageing at different temperatures for 5000 h (smooth specimen)
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