1 Key Lab for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang 110819 2 High Temperature Material Research Institute, Central Iron and Steel Research Institute, Beijing 100081
Cite this article:
Jinlan AN,Lei WANG,Yang LIU,Guohua XU,Guangpu ZHAO. INFLUENCES OF LONG-TERM AGING ON MICRO- STRUCTURE EVOLUTION AND LOW CYCLE FATIGUE BEHAVIOR OF GH4169 ALLOY. Acta Metall Sin, 2015, 51(7): 835-843.
GH4169 superalloy is one kind of important metallic materials used for manufacturing turbine discs in aero-engine. In order to meet the demand of higher strength, high ratio alloying elements have to be added, resulting in the complex microstructure evolution during the long-term service at elevated temperature. Furthermore, the turbine disc usually bears overloading which will lead to the low cycle fatigue (LCF) damage in real working and result in fatal security problem. Besides, it is meaningful to decide the relationship between the microstructure evolution and performance degradation. In the present work, microstructure evolution and LCF behavior of GH4169 alloy during long-term aging were investigated. The microstructure evolutions of GH4169 alloy during long-term aging at 750 ℃ for 500, 1000, 1500 and 2000 h and the influences of long-term aging on the LCF behavior were investigated. The results show that the size of g″ phases increases and the volume fraction decreases with the increase of aging time, compared with the increase of both size and volume fraction of d phases. Both the fatigue strength and fatigue life of the alloy decrease with the increase of aging time. For the specimen aged for the same time, the cyclic stress firstly contributes to cyclic hardening, then cyclic stability, and finally cyclic softening with the increase of cyclic numbers. It is found that the decrease of cyclic stress contribution is slightly effected by the size of g″ phases increase and volume fraction decrease after long-term aging. Therefore, the LCF life of the alloy decreases since the crack easily propagates along with the long needle-like d phases and the g″ phases precipitate free zones.
Fund: Supported by High Technology Research and Development Program of China (No.2012AA03-A513) and Ministry of Education Technical Foundation (No.625010337)
Fig.1 OM images of GH4169 alloy after standard heat treatment (SHT) (a) and aged at 750 ℃ for 500 h (b), 1000 h (c), 1500 h (d) and 2000 h (e)
Fig.2 Morphologies of g’ and g” phases in GH4169 alloy after SHT (a) and aged at 750 ℃ for 500 h (b), 1000 h (c), 1500 h (d) and 2000 h (e)
Fig.3 Morphologies of d phase in GH4169 alloy aged at 750 ℃ for 500 h (a), 1000 h (b), 1500 h (c) and 2000 h (d) (PFZ—precipitate free zone)
Aging time
Size of g′ phase
Size of g" phase
Volume fraction of g" phase
Volume fraction of d phase
h
nm
nm
%
%
SHT
12.4[28]
19.9
15.9[25-28]
0
500
69.9
425.9
18.1
3.5
1000
86.9
512.4
15.9
8.2
1500
101.8
713.0
6.4
17.1
2000
127.9
779.3
4.0
19.4
Table 1 Size variation and volume fraction of precipitate phases in GH4169 alloy aged at 750 ℃ for different times
Area
Al
Ti
Cr
Mn
Fe
Co
Ni
Nb
Mo
PFZ
1.43
1.22
20.67
0.29
20.39
1.97
49.96
2.20
1.86
d phase
0.57
2.60
3.72
0.15
4.71
1.31
67.87
17.50
1.57
Table 2 Chemical compositions of d phases and PFZ in GH4169 alloy aged at 750 ℃ for 500 h
Fig.4 Cyclic stress amplitude curves of GH4169 alloy aged at 750 ℃ for different times
Aging time / h
Max cyclic stress / MPa
Low cycle fatigue life / cyc
SHT
927
8504
500
850
7247
1000
763
6848
1500
705
5816
2000
627
5533
Table 3 Max cyclic stress and low cycle fatigue life of GH4169 alloy aged at 750 ℃ for different times
Fig.5 Dislocation configurations in GH4169 alloy aged at 750 ℃ for 1500 h after 0 cyc (a) and 50 cyc (b) (The inset in Fig.5b shows the SAED pattern)
Fig.6 Steady cyclic stress-strain curves of GH4169 alloy aged at 750 ℃ for different times
Fig.7 Morphologies of fracture crack initiation zone in GH4169 alloy after low cycle fatigue (LCF) after SHT (a) and aged at 750 ℃ for 1500 h (b) and 2000 h (c) (The arrows point to the fracture crack initiation areas)
Fig.8 Morphologies of fracture crack steady propagation zone in GH4169 alloy after LCF aged at 750 ℃ for 500 h (a) and 1500 h (b)
Fig.9 Morphologies of cross-section near LCF fracture surface in GH4169 alloy aged at 750 ℃ for 2000 h (The arrows indicate that the cracks propagate along the needle-like d phases)
Fig.10 Morphologies of fracture crack fast propagation zone in GH4169 alloy after LCF aged at 750 ℃ for 500 h (a), 1500 h (b) and 2000 h (c) (The arrows indicate the d phases)
Fig.11 Cross-sectional (a) and longitudinal (b) morphologies of fracture in GH4169 alloy after LCF aged at 750 ℃ for 2000 h (The arrows indicate that needle-like d phases are broken)
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