Fatigue Crack-Growth Rates of Nickel-Based GH4742 Superalloy
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FATIGUE CRACK-GROWTH RATES OF NICKEL-BASED GH4742 SUPERALLOY H. G. Liu,1,2 W. Long,1 and Y. Y. Li1 We study the fatigue crack-growth behavior in the near-threshold region, middle section of the diagram, and high-amplitude region depending on the rates for the nickel-based GH4742 superalloy in tension with various stress ratios. The curves of fatigue crack growth rates are presented (from 10 – 7 to
10 – 2 mm/cycle). By using the presented dependences, we determine the characteristics of fatigue crack-growth resistance of GH4742 alloy and reveal a significant decrease in the value of the critical stress intensity factor as the stress ratios increase. Keywords: GH4742 nickel-based superalloy, fatigue crack-growth resistance, load ratio.
The nickel-based superalloy has excellent performance in fatigue resistance, creep resistance, corrosion resistance, and oxidation resistance under the action of severe service environments and, thus, is often chosen as an aeronautical material for fabricating turbine discs [1]. However, the fatigue life and fatigue crack-growth behavior of the nickel-based superalloy are controlled by numerous factors, such as its composition [2], grain sizes [3], microstructure [4], etc. Note that the stress ratio [5], mean stress [6], stress amplitude [7], and temperature [8] are external factors that affect the fatigue life. Under the influence of these factors, the mechanical properties of the alloy materials are uncertain and the fatigue crack-growth behavior of these materials also reveals a variety of complex random phenomena. Therefore, it is necessary to perform extended experimental investigation with an aim to give accurate predictions of the fatigue crack-growth resistance of the aero-engine turbine discs made of GH4742 alloy. In order to investigate the uncertainty of the fatigue crack-growth rate of GH4742 alloy in aero-engine turbine disks, we propose to describe the fatigue crack-growth rate of GH4742 alloy for different load ratios by using a modified Paris equation. Materials and Experimental Procedure The specimens of GH4742 alloy used for the fatigue crack-growth tests were taken from a turbine disc. The chemical composition of this alloy is as follows: 0.04–0.08 wt.% C; 13–15 Cr; 9–11 Co; 2.4–2.8Ti; 2.4–2.8 Al; 4.5–5.5 Mo; 2.4–2.8 Nb, and balance nickel. The process of heat treatment was realized as follows: 8 h at 1080°C, 16 h at 750°C, and cooling in air. The microstructure of the material after heat treatment is illustrated in Fig. 1. The specimens were machined into standard compact tension specimens (Fig. 2) with a thickness B = 12 mm. The fatigue crack-growth tests for compact tension specimens of GH4742 alloy were carried out in a CTM8020 microobservation electrostatic test machine produced by the CTM Shanghai Xieqiang Instrument Technology Co., Ltd. The tests were carried out at room temperature. 1 2
School of Manufacturing Science and Engineering, Sichuan University, Chengdu, China. Corresponding author; e-mail: [email protected].
Published in Fizyko-Khimichna Mekha
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