Crack initiation sensitivity of wrought direct aged alloy 718 in the very high cycle fatigue regime: the role of non-metallic inclusions

Abstract

Fatigue crack initiation in the direct aged version of the nickel-based superalloy Inconel 718 has been investigated at room temperature in the low stress/very high cycle regime via ultrasonic fatigue testing. Three different microstructures have been examined at the same strain amplitude in order to understand the influence of non-metallic inclusions (NMIs), i.e. carbides, carbonitrides and nitrides, and Σ3 twin boundary density on lifetime and failure mode. A slight refinement in grain structure and a higher Σ3 twin boundary density is associated with substantial reductions in lifetime. Decreasing Σ3 twin boundary density for fine grain microstructures results in a change in crack initiation mechanism from strain localization within grains at the high end of the grain size distribution to cracking of NMIs. To study the early stages of crack initiation and growth, specimens with pre-cracked NMIs were also tested in order to examine the role of the surrounding grain structure. Pre-cracked NMIs mainly result in macroscopic failure initiation at NMIs independent of the wrought microstructure. However, pre-loading specimens within the plastic domain highlighted the competition in crack initiation mode between cracked-NMIs and favorably oriented twin boundaries. Crack arrest from most of the pre-cracked NMIs demonstrates that surrounding grain structure (grain orientation, local plasticity and roughness in the vicinity of crack tip due to pre-straining) play a key role in the fatigue life of components stressed in the nominal elastic regime.

High Cycle Fatigue Performance of Inconel 718 Alloys with Different Strengths at Room Temperature

“…The fracture mode and nucleation source of the secondary fractures on the fatigue fracture profiles of the two states were the same as those of the fatigue source fractures (Figure 7). The average SF of the grains where the secondary fracture source was located was lower than that of the surrounding grains, and the intragranular SF changed more (Figures 8 and 9), which is different from the fatigue surface fracture features observed by Texier et al [12]. There were more low angle grain boundaries near the secondary fracture of the ST state than that of the ST + A state, and the distribution was more uniform, indicating that the local plastic deformation of the cyclic ST state was more uniform than that of the ST + A state, and the degree of stress concentration was relatively low.…”

“…The laser scanning confocal diagrams for the vertical sections of the ST and ST + A fatigue fractures are shown in Figure 7. In Figure 7a, the plane of the ST state fatigue fracture source is 45 • to the stress axis, indicating that the fatigue fracture initiated on the <111> slip plane [34]; this fracture mechanism is different from the room temperature ultrahigh cycle fatigue surface fracture of the aged state Inconel 718 alloy, which initiated at the twin boundaries of the coarse grains [12]. The bumpy morphology of the main fracture shows plastic fracture features.…”

“…Therefore, the research on fatigue performance of the Inconel 718 alloy has been widely concerned. According to the specific service environment temperature and load of the parts, it can be roughly divided into several aspects: high temperature [2]/medium temperature [3]/room temperature [4,5] low cycle fatigue, high temperature [6,7]/room temperature [8]/low temperature [9,10] high cycle fatigue, and ultra-high cycle fatigue [11,12] performance. Fournier et al [13] found that the Inconel 718 alloy in the solution treated and aged (ST + A) state had cyclic softening while in the direct aged (DA) state [14] had cyclic hardening under high temperature.…”

In-plane and out-of-plane deformation at the sub-grain scale in polycrystalline materials assessed by confocal microscopy

“…10 and Table 2). Twins, and more precisely twin boundaries, were found particularly prone to crack initiation in high twin containing superalloys subjected to fatigue loading, either in VHCF or LCF loading conditions where strain amplitudes are low [2,3,36]. Due to relatively high elastic anisotropy at the grain level in Ni-based superalloys, intense strain/stress localization in the elastic domain raises near grain boundaries and even more near twin boundaries [32].…”

In-Situ SEM Deformation Testing of Ni-Based Superalloy Heated by the Joule Heat

“…In general, the failure of Ni-based superalloy material originates from the sites of stress concentration. The main cause of stress concentration is the presence of defects in the material, including, MC, holes, precipitates phase and so on [9,10]. Figure 4a,b showed the two different morphology, rod and block shape, of carbide inclusions after tensile failure for PWA1483 Ni-based superalloy at 900 °C.…”