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.

Introduction

The direct aged version of the polycrystalline nickel-based superalloy Inconel 718 (DA718) is widely used to manufacture structural components requiring extremely high mechanical performance at intermediate and high temperatures. The δ-subsolvus final forging step followed by aging heat treatment with no intermediate solution treatment enables a significant improvement of the yield strength and the low cycle fatigue (LCF) properties of this wrought alloy [1], [2]. However, variability in microstructure and metallurgical state within an individual wrought component, i.e. grain size and δ and γ”-phase content, are more pronounced in the direct aged version than in the standard annealed version of the alloy. Such material variability has recently been shown to strongly impact the low cycle fatigue life in the low-strain amplitude regime [3]. Non-metallic inclusions (NMIs), i.e. carbides, nitrides or carbonitrides, at the high end of the size distribution combined with low δ-phase content (f_δ-phase < 3.2%) – and inversely high γ” precipitation content – can lead to a ten- to hundred-fold reduction in fatigue life under these particular conditions [3]. Furthermore, fine grain microstructures (ASTM >10) were demonstrated to be particularly sensitive to crack initiation when brittle surface or near-surface NMIs were present, especially at low temperatures [1], [4], [5], [6], [7], [8], [9], [10], [11]. While NMIs in fine grain IN718 alloys are important in this cycling regime, other microstructural features such as coarse favorably oriented grains or twin boundaries can also play a key role in the crack initiation process [12], [13], [14], [15]. At lower cyclic stress amplitudes, i.e. for stress levels preventing the development of persistent slip bands (PSB), internal defects such as NMIs, favorably oriented large grains or twin boundaries generally lead to failure of structural materials [15], [16], [17], [18], [19], [20]. Near-surface crack initiation is mainly attributed to the greater probability of finding a defect inside the material, compared to the surface. In the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime, the influence of NMIs in Alloy 718 remains incompletely understood [20], [21], [22], [23], [24], [25]. Cracks have been shown to initiate within grains surrounded by Nb-rich carbides [23], “casting defects” [24] as well as favorably oriented grains [20], [21], [22], [25]. Therefore, trade-offs in terms of microstructural features are required to ensure reliable design of structural components subjected to cyclic deformation, i.e. sufficiently high yield strength with a low sensitivity to strain localization. VHCF testing is an efficient means to survey critical microstructural features in materials in the low-strain/stress alternating loading regime. The aim of the present study is to examine the influence of surface and near-surface NMIs as well as coherent twin boundaries on the VHCF fatigue life of DA718 alloys. Microstructural variability has been examined via slight changes in forging parameters of DA718 pancakes in order to highlight the competition between crack initiation mechanisms involving NMIs compared to Σ3 twin boundaries. In addition, the effect of the prior damage on VHCF lifetime was examined by means of introducing “natural cracks” within NMIs prior to VHCF testing.