Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 °C

doi:10.1007/s40195-020-01056-6

Abstract

Abstract:

Low-cycle fatigue (LCF) behaviors of a second-generation nickel-based single-crystal superalloys with [001] orientation at 760 °C have been investigated. Different strain amplitudes were introduced to investigate the creep-fatigue effects. The LCF life of none tensile holding (NTH) was higher than that of the 60-s tensile hold (TH) at any strain amplitude. As the strain amplitude was 0.7%, the stacking and cross-slip dislocations appeared together at the γ/γ’ coherent microstructure in both TH and NTH specimens. At the strain amplitude of 0.9%, plenty of the cross-slip dislocations appeared in γ channel and other dislocations were stacking at γ/γ’ interfaces. However, the SFs still appeared in γ’ phase with 60-s TH which caused cyclic softening. As the strain amplitude increased up to 1.2%, the dislocations are piling up at the γ/γ’ interfaces and cutting through the γ’ phase in both TH and NTH tests, which caused cyclic hardening. The influences of strain amplitude and holding time were complicated. Different stress response behaviors occurred in different loading conditions. The surface characteristic and fracture mechanism were observed by scanning electron microscopy. This result is helpful for building the relationship of various blade fatigue failure modes, cyclic stress response and microstructure deformation under different strain amplitudes.

Key words: Low-cycle fatigue, Cyclic stress response behavior, Dislocation morphologies

Jian Zhang, Yuan-Yuan Guo, Mai Zhang, Zhen-Yu Yang, Yu-Shi Luo. Low-Cycle Fatigue and Creep-Fatigue Behaviors of a Second-Generation Nickel-Based Single-Crystal Superalloy at 760 °C[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(10): 1423-1432.

Figures/Tables 8

Table 1 Chemical composition (wt%) of the alloy

Cr	Co	W	Mo	Ta	Re	Al	Hf	B	C
4	8	7	2	7	3	6	0.2	0.1	0.1

Fig. 1 Macrostructure a, microstructure b of nickel-based SC superalloy used in the experiment (the γ and γ’ phases were marked in (b))

Fig. 2 a LCF life of the alloy at different strain amplitudes. b-f Cyclic stress response curves of the alloy at different amplitudes: b Δε/2 = 0.7%, c Δε/2 = 0.8%, d Δε/2 = 0.9%, e Δε/2 = 1.0%, f Δε/2 = 1.2%

Fig. 3 Fracture characteristics of the longitudinal profile of the fatigue: a Δε/2 = 0.7%, NTH, near the fracture surface, b Δε/2 = 0.8%, NTH, ruptured by shearing γ’ and along γ, c Δε/2 = 1.2%, 60 s TH, near the fracture surface

Fig. 4 Dislocation configurations of the experimental alloy after LCF failure at different strain amplitudes: a, b Δε/2 = 0.7%, NTH; c, d Δε/2 = 0.9%, NTH; e Δε/2 = 1.0%, NTH; f Δε/2 = 1.2%, NTH

Fig. 5 Dislocation configurations of the experimental alloy after LCF failure at different strain amplitudes: a, b Δε/2 = 0.7%, 60 s TH; c, d Δε/2 = 0.9%, 60 s TH; e Δε/2 = 1.0%, 60 s TH; f Δε/2 = 1.2%, 60 s TH

Fig. 6 Fatigue fracture morphologies under different load conditions: a Δε/2 = 0.7%, none tensile hold specimen; b, c Δε/2 = 1.0%, none tensile hold specimen; d, e Δε/2 = 0.7%, 60 s tensile hold specimen; f Δε/2 = 1.2%, 60-s tensile hold specimen

Fig. 7 Edge and screw dislocation movement in different {111} at 760 °C

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