The effects of grain size, dendritic structure and crystallographic orientation on fatigue crack propagation in IN713C nickel-based superalloy

Abstract

The polycrystalline IN713C produced via investment casting is one of the widely-used nickel-based superalloy in automotive and aerospace industries. This alloy, however, has an apparent inhomogeneous microstructure generated during casting and contains dendritic structure that gives rise to strain localisation during loading. Yet, the effect of dendritic structure, grain size and shape as well as crystallographic orientation, which directly influence fatigue property and deformation micromechanism in the components, is rarely studied. In the present study, IN713C cast bars are tailored with three different grain structures, i.e., transition, equiaxed and columnar, with substantial grain size variations. The produced bars were tested under strain controlled LCF (Low Cycle Fatigue) and stress controlled HCF (High Cycle Fatigue) conditions at 650 °C. The results showed that most of fatigue cracks initiated from casting pores and fatigue life extended in the microstructure with a small grain size during both HCF and LCF loadings. It is also demonstrated that fatigue striations were mainly observed within dendritic areas during crack propagation, whereas the higher GND (Geometrically Necessary Dislocation) density were predominantly observed in the interdendritic 2 areas. Here, we propose a concept of 'Crack Propagation Unit (CPU)' for better description of deformation mechanism at local scale during fatigue loading by combining fracture surface characteristic methodology and dislocation distribution analyses within the dendritic structural unit. Furthermore, this model to understand the deformation micromechanism can provide a new perspective on the interpretation of Hall-Petch relationship in casting materials that contain dendritic structure. This is further demonstrated via direct correlation of the high crack propagation resistance with the crack path divergence instead of the dislocation pileup at the grain boundary or in-between the γ/ γ' channels. Moreover, by utilising serial sectioning method followed by layered EBSD scanning, quasi-3-D grain orientation mappings were obtained, and crystallographic texture information were directly correlated with the fracture surface observations. This allowed an investigation of the influence of orientation of individual grains and micro/macro texture on crack propagation rate. The critical stage of crack propagation in fatigue life and its correlations with microstructural features is established, offering potential practical applications by controlling the investment casting process parameters.

Continuous texture evolution during staged solid solution of Al‐Cu‐Mg and the influence on fatigue resistance

“…Moreover, the twin parents grain (with low SF and high TF) and near twin parents grain (with both high SF) trend to induce the inter‐granular crack extension along the GB between with the neighbouring common grain (not necessarily the TGB and NTGB), especially with small grain size. Additionally, the TF difference of texture grains in the {001} cluster may be striking, even though the SF is similar, which may be the cause of the controversy in references 14–16 …”

“…Combining the texture intensity shown in Figure 3A-F, the maximum variation of Goss texture was addressed with increasing temperature in one-step T4, whereas that of Cube and near Cube (Cube 15 ) texture occurred in two-step T4, with the enhancement of preannealing temperature in the first stage of two-step T4. These mean that Goss has a significant advantage during single one-step, especially at high temperature and a preannealing at mediumtemperature followed by a solid solution at normal temperature (498 ± 2 C) is inclined to promote {001} texture.…”

“…As shown in Figures 5 and 3G-H, the Brass texture was dominant in the initial cold sheet. After one-step T4 at 498 ± 2 C for 5 min, the {001} texture increased significantly, including Cube and Cube 15 , whereas the β-fibre texture, such as Brass, Cu and S, reduced correspondingly. This suggests the deformation texture has transformed swiftly into recrystallization orientations at solid solution temperature.…”

“…Meanwhile, the intensity of β-fibre texture varied just slightly. These mean that the Goss, ideal Cube and P may possess higher thermal stability than Cube 15 . In addition, at 520 C-525 C, the Goss texture gradually played the dominate role, see in Figure 2D-E.…”

“…12 Zhang et al consider that the large difference of Schmid factors (DSFs) would lead to the twin GB cracking in copper alloy. 13 Recently, some researchers report the effect of texture on fatigue life (IN713C nickel-based super-alloy casting ingot bar) 14 and the short crack propagation behaviour (Ni-based polycrystalline super-alloy forged pancake) 15 involving the SF. 16 Although the influence of SF on the fatigue behaviour was not prominent than expected due to the possible difference of material system and processing state, such reports doubtlessly suggest that the influence of texture on fatigue behaviour needs further research.…”