Fatigue behavior and bilinear Coffin-Manson plots of Ni-based GH4169 alloy with different volume fractions of δ phase
Introduction
The precipitation-strengthened nickel-based superalloy, GH4169, has similar microstructure and mechanical properties to those of 718 alloy. This alloy has been widely used in aviation, aerospace and nuclear industries in China due to its high performance/price ratio, good formability and weldability [1], [2]. The components fabricated by nickel-based superalloy, such as aircraft turbine disks, often undergo fatigue loading, which could cause the low cycle fatigue (LCF) failure. Hence, it is very important to identify the LCF behavior and mechanism of GH4169 superalloy.
Several investigations have been concentrated in reflecting the effects of microstructure, temperature, loading condition on the LCF behavior of Inconel 718 alloy at room temperature (RT). Earlier studies regarding microstructural effect on the deformation behavior and LCF of IN718 alloy have been reported by Merrick [3], Fournier and Pineau [4], Sanders et al. [5], Worthem et al. [6], Rao et al. [7], Bhattacharyya et al. [8], Xiao et al. [9], [10], and recently Maderbacher et al. [11]. Merrick [3] investigated the effect of grain size on LCF life of Inconel 718 alloy at RT. It was found that the fatigue life decreased with increasing grain size. On the other hand, the GH4169 or IN718 alloy was mainly strengthened by Ni3Nb type γ″ precipitation and partially by Ni3Al type γ′ precipitation. A small degree of fatigue hardening quickly followed by fatigue softening was often found during the fatigue process, which might be attributed to the formation of intense slip bands and the shear and possible dissolution of or precipitates [4], [11]. The regularly spaced arrays of deformation bands were often observed at high strain amplitudes [5], [7] and their density generally increased with increasing number of cycles [8]. The experimental results by Worthem et al. [6] and Xiao et al. [9] indicated that the deformation band spacing was obviously influenced by the distribution and movement of dislocations. In addition to the and precipitates, the precipitates at grain boundaries in IN718 alloy were often formed during heat treatments (HTs) [12]. The phase has similar chemical compositions to the primary strengthening phase , implying that the increment of volume fraction of phase could lead to the decrement of volume fraction of phase. It was generally believed that the presence of phase with moderate volume fraction would improve the creep resistance of IN718 alloy since it was beneficial in enhancing ductility and toughness [13], [14].
Coffin-Manson (C-M) law has been widely used to predict the LCF lives of different metallic materials. It also provided the foundation of time-dependent fatigue life prediction approach [15]. In Coffin-Manson law, it was assumed that there was a linear relationship between the plastic strain amplitude, , and the number of reversals to fatigue failure, , in double log coordinates. However, the non-linear relationship between and was also observed in Nickel based superalloys [5], [10], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], Nimonic alloys [27], [28], [29], [30], and Haynes alloy [31] during LCF tests. Moreover, no detailed report regarding the effect of amount and distribution of precipitates on the LCF behavior and C-M relationship of GH4169 or IN718 alloy at RT was available. The aim of this paper was to identify the influence of volume fraction of δ phase on the LCF behavior of GH4169 alloy at RT. The volume fraction of δ phase was changed through changing the parameters used in HTs.
Section snippets
Brief review of non-linear - relationship of Nickel-based alloys [5,10,16–26]
LCF behavior of many materials characterized by the C-M relationship generally exhibits a linear behavior in log-log coordinates, i.e.,where and are the fatigue ductility coefficient and exponent, respectively. However, the nonlinear relationship between and has been often reported for Nickel-based alloys. Moreover, the – curve is generally convex upward. For simplicity, it is often assumed that the C-M plots follow bilinear relationship between and
Material
The material used in this study was Nickel-based GH4169 alloy which was provided by Fushun Special Steel Shares CO., LTD, China. The as-received alloy, which was produced by using vacuum induction melting process, was supplied in the form of hot rolling bar with diameter of 44 mm and average grain size of 6 µm. The chemical compositions of the alloy were listed in Table 1. The alloy had the microstructure consisting of an austenitic face-centered cubic (FCC)-Ni matrix strengthened by the
Microstructure and tensile stress-stain curve
The microstructures of as-received and heat-treated GH4169 alloys are shown in Fig. 8. By comparing with the as-received material in Fig. 8a, the grain sizes of the alloy are almost not changed after heat treatments. However, the heat treatment will lead to the precipitation of δ phase at the grain boundaries, as indicated by the arrows in Fig. 8b to d. Some discretely short-rod like and granular δ phase precipitates at the grain boundaries within the alloys after HT1. After HT2, the continuous
Effect of heat treatment on microstructure and fatigue resistance
In the present paper, the HT of GH4169 alloy includes solution annealing and ageing. The purpose of solution annealing is to dissolve and phases back to matrix and the subsequent double-ageing treatment makes the adequate and uniform precipitation of and phases. The HT1 leads to dissolution of and phases and precipitation of a small amount of δ phase along the grain boundaries. In HT2 and HT3, the annealing treatments at temperature of 900 °C for 4 h and 20 h are respectively
Conclusions
The aim of this paper was to investigate the effect of volume fraction of δ phase on the low cycle fatigue of Nickel-based GH4169 alloy at room temperature. The volume fraction of δ phase was changed through changing the processing parameters used in heat treatments. The volume fractions of δ phase in three groups of low cycle fatigue specimens were respectively 2.9%, 9.31% and 15.43%. The following conclusions can be obtained:
- (1)
The volume fraction of δ phase had a significantly effect on the
Acknowledgements
The authors would like to acknowledge gratefully for the financial support through National Natural Science Foundations of China (51371082 and 51322510). The author X.C. Zhang is also grateful for the support by Shanghai Pujiang Program, Shanghai Technology Innovation Program of SHEITC (CXY-2015-001), and Fok Ying Tung Education Foundation.
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