D. L. Chen, Ryerson University, Toronto, ON, Canada; L. Xiao, M. C. Chaturvedi, University of Manitoba, Winnipeg, MB, Canada

Summary:

Inconel 718 (IN 718) superalloy is being extensively used as a structural material in aero-engine turbine disks, blades, and shafts, and in powerhead components in the main rocket engine of the space shuttles, due to its excellent corrosion resistance coupled with superior mechanical properties at elevated temperatures. Resistance to low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) is an important consideration in these applications,. This investigation was, therefore, aimed at evaluating the LCF and TMF behavior of Inconel 718 at room temperature and at 650°C temperature, with an emphasis on evaluation of fatigue deformation substructures via transmission electron microscopy (TEM) and scanning electron microscopy (SEM), and studying the fatigue fracture mechanisms.

The LCF results at room temperature (RT) showed that at higher cyclic strain amplitudes (Δε_t/2³0.8%), IN 718 experienced a short period of initial cyclic hardening, followed by a continuous cyclic softening until fracture. At Δε_t/2=0.6% only cyclic softening occurred, and a nearly stable peak stress was observed at Δε_t/2=0.4%. However, as the test temperature increased from RT to 650°C, the initial cyclic hardening disappeared. The LCF life versus the cyclic plastic strain amplitude was observed to exhibit a linear relationship at 650°C, but a “two-slope” relationship at RT was observed (Fig.1). The Coffin-Manson equation parameters obtained at RT and at 650°C are shown in Table 1.

The TMF lifetime, Nf, as a function of the cyclic stress amplitude, Δσ/2, was observed to follow a linear relationship in the single logarithmic coordinates under the in-phase and out-of phase thermal cycling between 350°C and 650°C, as shown in Fig.2. It is evident that, the TMF lifetime under in-phase loading is much smaller than that observed under out-of-phase loading.

In the LCF regime at RT, a transgranular fracture with well-developed fatigue striations was the predominant feature, and the fracture surface was characterized by transgranular mode with some intergranular cracking at 650°C. However, in the TMF regime intergranular cracking was the primary fracture mechanism. TEM examinations revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111} slip planes in the LCF and TMF, as shown in Fig.3. The cyclic deformation mechanisms for IN 718 under LCF and TMF will be presented.