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.

High‐cycle and very‐high‐cycle fatigue behavior and life prediction of Ni‐based superalloy at elevated temperature

“…Obviously, a 150-μm pore can be seen clearly, which usually forms at the junction of grain boundaries and plays a key role in the process of inducing crack nucleation. 38 Particularly, there is no crystallographic facet around the pore, but a small rough area is observed in Figure 5C, which is similar to the phenomenon observed by Cervellon et al 39 They believe that the high local plastic activity in this area is a precursor of macro crack growth. Meanwhile, the rough area of the Ni-based superalloy is highly close to the fine granular area (FGA) in the steel structure.…”

Recent Advancements in the Field of Ni‐Based Superalloys

“…is was found to pave the way for the initiation of cracks at the time of LCF testing, in Goss and Cube oriented grains. An identical study [86] was performed, but with cast bars made of IN713C, which was tested under both LCF and HCF conditions. e results similar to the ones seen in the previous study were observed in this study for specimens subjected to both LCF and HCF testing conditions.…”

“…us, many studies have been done to investigate how freckling occurs and how it can be handled. Studies regarding freckling [85][86][87][88][89][90][91][92] showed that freckles were formed as a result of interdendritic microsegregation caused by thermosolutal convection, which in turn is mainly caused by density inversion taking place in the mushy zone. Preferential formation of freckles was seen when the thermal gradient and solidification rate during casting were low.…”

“…e PDAS and SDAS values were seen to decrease as the rate of withdrawal increased, and this was seen to cause a reduction in the number of freckles formed. Studies [5,[85][86][87][88][89][90][91][92] also showed that freckles only formed on internal or external surfaces, as thermosolutal fluid flow/convection and channel segregation were only seen to develop a stable flow at casting surfaces and not in the bulk of the material. Some studies [26,85] also revealed that convex edges are preferred locations for freckling to occur.…”

“…It was observed that while undergoing loadings under HCF conditions, the strain fatigue limit was seen to decline as the mean strain went up. But once the value of the average strain LCF [1, 11-13, 21, 41, 47, 67, 86, 96] HCF [3,21,47,86] VHCF [20][21][22]58] was seen to hit a particular limit, going beyond it was seen not to induce any significantly higher mean stresses. If the superimposed loading's amplitude was high enough, the lifetime during superimposed HCF loading was observed to have significantly reduced compared to pure LCF loadings.…”