PSI - Issue 7

L.L. Liu et al. / Procedia Structural Integrity 7 (2017) 174–181 L. L. Liu et al. / Structural Integrity Procedia 00 (2017) 000–000

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not pay attention to the effect of grain size to estimate LCF lifetime. So, it is necessary to develop a new LCF lifetime prediction model taking into the grain size of the alloy account, which is of great significance to the selection of heat treatment process and the actual lifetime of the alloy. According to previous study, Xing [23] proposed that when the d ≤10 um, there was an exponential relationship between the grain boundary energy and grain size as follow: = g (2) Where γ is the grain boundary energy of polycrystal, b is average width of grain boundary, γ g is average interfacial energy of grain boundary. The finer the grain with more grain boundary can prevent the movement of dislocation effectively so as to improve the strength of material. According to this view, the grain boundary energy and the fatigue life have a certain relationship. The SWT model describes the relationship between damage parameters and fatigue lifetime, in which the damage parameter can be regarded as an energy form. Based on the energy theory, an exponential-form average grain size based on the grain boundary energy was introduced into the damage parameters to describe the relationship between grain size and the fatigue lifetime, and a new LCF model shown as formula (3) is proposed for fatigue lifetime prediction of compressor disc. b d γ γ

1 2

(2 )

f kd N α

β

t max ε σ ∆ =

(3)

Where d is grain size, k , α and β are material parameters. The LCF lifetime of a compressor disc is analyzed by using the SWT model and developed SWT model as shown in the Fig.5. The results of computational median lifetime agree well with the experimental one, and forecasting precision is higher than the result by the original SWT model with the scatter band reducing by about 10 times and the scattered band is 5.49. It is shown that the modified method based on SWT model can be used to estimate the LCF lifetime of compressor disc of GH4169 superalloy.

Fig.5 The relationship between predicted lifetime and experimental data. (a) modified SWT model (b) original SWT model

4. Conclusion

Low cycle fatigue experiments were conducted at 600°C on smooth specimens from an actual compressor turbine disc of GH4169 superalloy. The microstructure of superalloy GH4169 and LCF behaviors under different maximum strain loading were investigated. Fracture surface analysis and LCF lifetime prediction compared with the experimental results were performed. The contributions of this study are summarized as follows: (1) The different grain sizes and a large number of short rod-like δ phases are observed in the metallographic examination. Based on the measurement of microscopic parameters, the scatters of grain size and the content of δ phase are found in different specimens which cut from the same disk. A dimensionless analysis shows that the LCF lifetime of GH4169 is closely related to grain size, and little correlation with the content of δ phase. (2) A negative correlation between the grain size and LCF lifetime was found induced by the grain refinement