Effect of heat treatment on low cycle fatigue of IN718 superalloy at the elevated temperatures

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Abstract

Low cycle fatigue (LCF) performance of IN718 superalloy under different heat treatment conditions are investigated with controlled strain amplitudes from 0.6% to 1.0% at 350 °C and 650 °C. Standard heat treatment (ST) and delta-phase aging treatment (DA) are applied to produce two types of precipitation microstructures. Under the same strain amplitude and temperature, the fatigue lives of ST specimens exceed that of DA specimens. And strikingly, ST specimens exhibit cyclic softening behavior in contrast to cyclic hardening behavior for DA specimens. The different stress responses can be rationalized by the dislocation interaction mechanisms with the precipitates. For the ST specimens, the dislocation shearing of γ′′ precipitates governs the microscopic deformation process, while the micrometer-sized δ phases are not shearable but act as the obstacles against dislocation slipping and lead to the cyclic hardening. Our TEM observations clearly show that recrystallization nucleation is prevalent in the DA specimens under 650 °C, which is interpreted by the high dislocation density induced by the δ phases. The results shed new lights on the fatigue microscopic deformation mechanisms of IN718 superalloy.

Introduction

Owing to an extraordinary combination of mechanical properties and corrosion resistance at elevated temperature [1], [2], polycrystalline nickel base superalloy has hitherto been deemed as the most successful high temperature material to fabricate the turbine disc in aeroengine. In engineering practice, the in-service mechanical reliability and fatigue life of polycrystalline nickel base superalloy are controlled by the LCF performance. Under operating condition, the fatigue cyclic stress arises from the fluctuation of rotating component under cyclic loading [3], [4]. In particular, the stress imposed on the rotating component decreases during cruise period and typically suffers from high cycle fatigue (HCF) [5], [6]. On the contrary, in the course of take-off and landing operations, the stress is required to be higher and LCF performance is considered for the fatigue design. Inconel 718 superalloy is a widely used high temperature material that consists of two types of strengthening precipitates of the metastable body centered tetragonal γ′′ (Ni3Nb) precipitate and the face centered cubic coherent γ′ (Ni3Al) precipitate. Improving its LCF properties normally requires fine grain size distribution, but it is very difficult to be realized solely by the recrystallization process [7]. Ruiz et al. [8] reported a novel route through introducing the δ phases during delta-phase aging treatment, which are utilized to act as the grain size controller in the subsequent thermomechanical process. In this article, we intentionally employ the alloys with a high density of δ phases for the fatigue tests and compare the mechanical responses with γ′′ precipitate-dispersed alloys.

Previous literatures have widely documented that precipitation microstructure plays a critical role in determining the LCF performance of IN718 superalloy. Fournier and Pineau [9] investigated LCF behavior of Inconel 718 at room temperature and 550 °C, exhibited that both increasing temperature and cyclic frequency decrease fatigue life remarkably. H. Maderbacher et al. [10] reported the fatigue strength reduced with the increasing of grain size of hot-forged Inconel 718 at room temperature. Prasad et al. [11] employed fast and slow asymmetrical waveforms within a cycle and found that the lower strain rates exhibited serrated behaviors in the hysteresis loop, which was explained by the dynamic strain aging during fatigue process in the plastic region under low strain rate.

The previous results also showed that the IN718 superalloy occasionally manifested by the hardening behavior within the several beginning cycles and followed by cyclic softening [12], [13] or direct softening [14], [15]. It was proposed [15] that the cyclic softening behavior was controlled by the shearing and dissolution of γ′′ precipitates, where the successive dislocation slip on {111} planes leads to the gradual decrease of fatigue resistance. Sundararaman et al. [16] reported that the passage of deformation twinning shears γ′′ precipitates without destroying the ordered atomic arrangements when the size of γ′′ precipitates exceeds a critical value of 10 nm. Moreover, increasing the volume fraction of δ phases is accompanied with the decrease the volume fraction of the γ′′ precipitates considering the total amount of Nb/Al elements are constant. Recently, An et al. [17] reported that granular δ phases acted as the resistance of dislocation slipping but readily piled up around needle-like δ phase and decreased fatigue crack propagation resistance. The spherical and short-rod δ phases form on both the interior or grain boundaries during aging [7], [18] and contribute a critical role in the fatigue performance, together with other strengthening phases, such as γ′′ and γ′ precipitates. In between, both of γ′′ precipitates and δ phases comprise the similar chemical composition (Ni3Nb). It is believed that δ phases in a moderate volume fraction on grain boundaries [19] could improve the creep property of IN718.

In this article, the effect of precipitate microstructures of IN718 superalloy during different heat treatment processes on LCF behavior and failure mechanism are investigated. The mechanical test results indicate that the fatigue lives of ST specimens significantly exceed those of DA specimens at the same test conditions. We find that transgranular fractures govern the deformation processes at 350 °C for both ST and DA specimens, in contrast to the intergranular factures at 650 °C. The successive dislocation shearing of γ′′ precipitates is the dominated deformation mechanism for the ST specimens, while the dislocation pile-up in the vicinity of the interface of δ/matrix lead to a cyclic hardening.

Section snippets

Materials and experimental procedures

The nominal composition of nickel base IN718 superalloy used in this study was shown in Table 1. The master alloy of IN718 superalloy was produced by vacuum induction melting and then forged at 1020 °C. Two types of heat treatment routines, ST and DA, were used to produce different precipitates microstructures. The detailed heat treatment parameters of ST and DA were schematically illustrated in Fig. 1a and b, respectively. Strain-controlled LCF tests were conducted at 350 °C and 650 °C using

Results and discussion

The microstructures of pristine IN718 superalloy before heat treatment, standard heat treated and delta-phase aged are shown in Figs. 3a, b and c, respectively. Fig. 3a shows that the pristine alloy consists of an average grain size of 26 µm using linear intercept method. It can be found that the grains are irregularly-shaped and carbides/borides are dispersed both on grain boundaries and grain interior, wherein, no obvious δ phases can be detected. After ST, the average grain size is refined to

Conclusions

Through imposing two types of heat treatments on the IN718 superalloy, two groups of precipitation microstructures are produced and their effects on the LCF performance are investigated.

  • (1)

    The fatigue life of ST specimens significantly exceeds that of DA specimens. Cyclic softening is the primary deformation mode of ST specimens, in contrast to the cyclic hardening behavior of DA specimens.

  • (2)

    The failure mode changes from transgranular fracture at 350 °C to intergranular at 650 °C. Ductile fracture

Conflicts of interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgment

The authors would like to appreciate the financial support from the National Key Research and Development Program of China (2016YFB0700300). The authors are grateful to Chengxin Wu for the experimental assistance.

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