Corporate Effects of Temperature and Strain Range on the Low Cycle Fatigue Life of a Single-Crystal Superalloy DD10

doi:10.1007/s40195-014-0179-3

Abstract

Abstract:

Low cycle fatigue behavior of a nickel-based single-crystal superalloy DD10 was investigated at 760 and 980 °C under different strain ranges. Results show that the fatigue life (N _f) of DD10 alloy exhibits different temperature dependence under various strain ranges. Under low strain range, the alloy exhibits a longer N _f at 760 °C than that at 980 °C. However, under high strain range, a reverse result is obtained. This difference can be attributed to the change of dominant damage modes under various test conditions, which is manifested in different modes of crack initiation (crack nucleation and its early propagation). At 760 °C, the crack initiates at pores in subsurface due to local stress concentration. This process is mainly controlled by plastic amplitude and plastic property, but not affected by oxygen-induced damage before the crack propagates to the surface. At 980 °C, the crack initiates at surface instead of pores due to the more homogeneous plastic deformation and the disharmony between the external oxidation layer and the bulk material when the strain amplitude is high. At that temperature, the process is mainly controlled by oxidation damage and strain amplitude simultaneously. Therefore, under high strain range, the crack initiation is much easier at 760 °C due to plastic deformation and the poor plasticity, while under low strain range obvious oxidation damage at 980 °C may accelerate the crack initiation.

Key words: Low cycle fatigue, Single-crystal superalloy, Temperature Strain, Crack initiation

Zhi-Dong Fan, Dong Wang, Lang-Hong Lou. Corporate Effects of Temperature and Strain Range on the Low Cycle Fatigue Life of a Single-Crystal Superalloy DD10[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(2): 152-158.

Figures/Tables 10

Fig. 1 OM image a, SEM image b showing the microstructure of DD10 after solution heat treatment

Fig. 2 Total, elastic, and plastic strain amplitude versus number of cycles to failure of DD10 at 760 °C a, 980 °C b

Table 1 Strain fatigue parameters of DD10 alloy at 760 and 980 °C

Temperature (°C)	\( \varepsilon_{\text{f}}^{'} {\text{ (\% )}} \)	c	\( \sigma_{\text{f}}^{'} \, ({\text{MPa}}) \)	b	E (GPa)	K (MPa)	n′
760	0.523	0.36	1,084	0.0669	102	1,540	0.0751
980	1.054	1.04	1,832	0.1946	73	1,572	0.1685

Table 2 Tensile properties of heat-treated DD10 alloy at 760 and 980 °C

Temperature (°C)	ε _f (%)	Reduced area (%)	ε _yield (%)	σ ₀ (MPa)	E (GPa)
760	15.9	15.9	0.89	1,038	98.2
980	30.4	30.7	0.42	392	79.3

Fig. 3 Total strain amplitude (∆ε totoal/2) versus number of cycles (2N f) to failure of DD10 at 760 and 980 °C

Fig. 4 SEM images of fracture after LCF at 760 °C with ∆ε total/2 = 1.2%: a macroscopical fracture topography; b crack initiation site; c slip bands on outer surface

Fig. 5 SEM images of fracture surfaces after LCF at 980 °C with ∆ε total/2 = 1.2%: a macroscopical fracture topography; b crack initiation site; c longitudinal section; d cracks at the exterior surface

Fig. 6 SEM images of longitudinal section for specimens interrupted after 1,500 cyc at 760 °C with ∆ε totoal/2 = 0.7% a, b, that after 30 cyc at 980 °C with ∆ε totoal/2 = 1.2% c, d

Fig. 7 SEM images of longitudinal section for failure specimens at 760 °C with ∆ε totoal/2 = 0.7% a, 980 °C with ∆ε totoal/2 = 0.4% b

Fig. 8 Illustration of crack initiation for alloys DD10 at 980 °C: a original surface; b exterior oxidation layer; c oxide ruptures due to disharmony when deforming; d air passage to crack tip

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