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Acta Metall Sin  2017, Vol. 53 Issue (11): 1453-1460    DOI: 10.11900/0412.1961.2017.00169
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Grain Boundary Oxidation Effect of GH4738 Superalloy on Fatigue Crack Growth
Chao XU, Qiliang NAI, Zhihao YAO, He JIANG, Jianxin DONG()
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
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Chao XU, Qiliang NAI, Zhihao YAO, He JIANG, Jianxin DONG. Grain Boundary Oxidation Effect of GH4738 Superalloy on Fatigue Crack Growth. Acta Metall Sin, 2017, 53(11): 1453-1460.

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Abstract  

The low cycle fatigue (LCF) experiments of nickel-based turbine disc alloy GH4738 have been carried out at different temperatures in air to investigate the influence of temperature on fatigue crack growth (FCG) behavior of GH4738 alloy. The FCG curves (da/dNK and a-N) and their regularity have been obtained. The results show that there is a sensitive range of temperature in which the fatigue life for GH4738 decreases sharply. The microstructures and fracture surface morphologies of the GH4738 samples tested at different temperatures were observed by FE-SEM, and changes of the mechanical properties of GH4738 at high temperature were also taken into account through modifying the stress intensity factor amplitude, ΔK. The interruption experiments were carried out at 700 ℃ and room temperature, respectively, to investigate the crack growth mode and oxidation degree at the crack tip and grain boundary of the samples. And the essential reason of temperature influence on FCG behavior of GH4738 was discussed. The result showed that as the temperature increases, the fatigue crack growth rate (FCGR) of GH4738 accelerates, the fracture surface tends to coarse, and the failure mode converts from a mixed transgranular and intergranular fracture to totally intergranular fracture. The fatigue crack growth lifetime decreases remarkably at 650~700 ℃, existing a temperature-sensitive region under ΔK=40 MPam1/2 and 30~40 μm grain size conditions, which is mainly caused by the oxidation at elevated temperature, independent of the microstructure and mechanical property. Oxygen diffuses into the grain boundary through crack tip and slip band, or penetrates directly into the grain boundary, reacts with active elements (Co, Ti, Al) and generates brittle oxides. These brittle oxides result in weakening of grain boundary and significant decrease of fatigue property of GH4738.

Key words:  nickel-based superalloy      GH4738      fatigue crack growth      temperature-sensitive region      oxidation     
Received:  04 May 2017     
ZTFLH:  TG146  
Fund: Supported by National Natural Science Foundation of China (No.51371023)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00169     OR     https://www.ams.org.cn/EN/Y2017/V53/I11/1453

Fig.1  Compact tension specimen of fatigue crack growth (FCG) test (unit: mm)
Fig.2  Fatigue load waveform figure
Fig.3  Microstructure of GH4738 alloy(a) grain size distribution (b) intergranular carbides and γ' phase
Fig.4  Curves of fatigue crack growth lifetime (a) and fatigue crack growth rate (FCGR) (b) of GH4738 alloy at different temperatures (a—crack length, N—fatigue life, ΔK—stress intensity factor amplitude)
Fig.5  Curve of fatigue crack growth lifetime of GH4738 alloy at different temperatures
Fig.6  Microstructures of GH4738 alloy tested at 650 ℃ (a), 700 ℃ (b), 750 ℃ (c) and 800 ℃ (d)
Temperature / ℃ E / GPa σy / MPa
650 151 706
700 137 693
750 132 683
800 126 624
Table 1  Elastic modulus E and yield strength σy of GH4738 alloy at different temperatures[15]
Fig.7  da/dNKnorm curves of GH4738 alloy after modification of E and σyKnorm—modified ΔK)
Fig.8  Fracture morphologies of GH4738 alloy at fatigue crack initiation areas (a, c) and ΔK=45 MPam1/2 (b, d) at 650 ℃ (a, b) and 750 ℃ (c, d)
Fig.9  FCG path (a, b) and associated elemental linescan at the crack tip in Fig.9b (c) of GH4738 alloy at 700 ℃ (Fig.9b is the local enlarged image of the square area in Fig.9a)
Fig.10  FCG path (a, b) and associated elemental linescan at the crack tip in Fig.10b (c) of GH4738 alloy at room temperature (Fig.10b is the local enlarged image of the square area in Fig.10a)
Fig.11  Morphologies around the fatigue crack of GH4738 alloy at 750 ℃(a) secondary crack(b) crack tip zone(c) triple junction
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