PSI - Issue 7

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

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

during service [1] . Owing to excellent performances in fatigue, oxidation and corrosion resistance at high temperature, the wrought nickel-based superalloy GH4169 (similar to Inconel 718 in the U.S. and NC19FeNb in France) has been widely used in the compressor disc of the aero-engine. The grain size at different locations of a compressor disc generally shows differences due to inhomogeneous forging deformation and the heat treatment cooling rate (which is difficult to precisely control for the compressor discs in large size), thus revealing the dispersion in LCF lifetime. Previous studies reported that the grain size has a significant influence on the LCF lifetime of GH4169 superalloy both at room and elevated temperatures. Hall [2] and Petch [3] firstly proposed the theory that polycrystalline material with fine grains processes higher yield strength, and developed the Hall-Petch formula based on cleavage strength of grain with different sizes to describe the relationship quantitatively. Pieraggi [4] found that the grain size was the most important factors in fatigue performance of Inconel 718 alloys. Spath [5] and Merrick [6] put forward that the fatigue lifetime of GH4169 at room temperature will be shortened with the increase of grain size. Comparative tests conducted by Jia [7] and Kobayashi [8] et al. showed the fine-grained GH4169 had a higher fatigue properties than the common GH4169 and the fatigue strength of the coarse-grain alloy decreased notably. For further investigating, there are two approaches to a grain size-related quantitative analysis attracting enormous research efforts. One approach is numerical simulation on a microscopic scale. For instance, the grain structure simulated from a Poisson–Voronoi model and a short crack growth model were applied to investigate the effects of various grain sizes on fatigue lives [9] . Based on physical dislocation mechanisms, Sweeney [10] et al. developed a strain-gradient crystal plasticity framework for finite element model to predict the fatigue crack initiation of a CoCr alloy. Moreover, a representative volume element (RVE) model consisting of a number of crystal grains was constructed to investigate the fatigue lives under different strain amplitudes [11] . This numerical simulation described the stress / strain behavior and lifetimes of materials under different fatigue loading from the crystal structure, which reflected the microscopic nature of grain size. However, the microstructure of the material is too complex to obtain relevant parameter accurately used for predicting crack initiation lifetime, which makes it uneasy to reflect real microstructure or achieve an ideal result. As a result, it is difficult to be widely used in the engineering. Another approach is to introduce grain size or other relevant parameter to the existing lifetime prediction model, based on crack initiation and propagation theory. Li [12] et al. developed a Multi-scale fatigue (MSF) life models by introducing grain size to estimate the fatigue lifetime of the magnesium castings. Similar methods which associate grain size with fatigue lifetime directly could also be found elsewhere [13,14] . Although a lot of comparative studies were carried out to investigate the influence of grain size on LCF lifetime, the quantitative descriptions of GH4169 are relatively few because of the complexity of material microstructures and the different grain size effect of various load, temperature and material type. As a consequence, it is necessary to develop a lifetime prediction model to study the relationship between grain size and LCF lifetimes of GH4169 superalloy. In this study, a series of LCF tests for the GH4169 smooth specimens cut from an actual compressor disc were carried out with optical microscope (OM) and scanning electron microscope (SEM) analysis. The relationships between the grain size and fatigue lifetime were also analyzed. Furthermore, a modified SWT lifetime prediction model was established to describe the dispersion of the fatigue lifetimes quantitatively.

2. Material and experiment procedure

2.1 Material

GH4169 is a precipitation strengthening superalloy based on Ni-Cr- Fe, mainly consisting of matrix γ phase, precipitations and inclusions. The precipitated phases include the main strengthening phase γ” (Ni 3 Nb), the auxiliary phase γ’ (Ni 3 AlTi) and the equilibrium phase δ (Ni 3 Nb) of γ” phase. The γ’ phase with face centered cubic (fcc) lattice is uniformly distributed in the matrix, and the interfacial energy between the matrix and the γ’ is small. The metastable γ” phase with body centered tetragonal (bct) lattice changes into δ phase under the high temperature, which reduces the strength of the material. δ phase that distributed at grain boundary can prevent the motion of dislocation and refine the grain to improve the strength of the material to some extent. The hard brittle NbC and TiC are the main forms of carbides in GH4169. These carbides are easy to cause stress concentration in the process of high temperature deformation of the alloy resulting in debonding interface between carbide and matrix or the crack