Investigation of the tensile deformation behaviour in Ni-based superalloy inconel alloy 617 using EBSD-based finite element simulations and optical flow method

Abstract

To improve component design, the fundamental understanding of the fatigue behaviour of gas turbine materials is essential. Since Ni-alloys exhibit pronounced elastic anisotropy, the local grain orientation strongly affects the stress and strain distribution in the material under mechanical loadings. This work addresses the characterisation of anisotropic elastic–plastic deformation and its consequences for crack initiation of nickel-base superalloy IN617 under tensile loading. Samples were loaded in situ in a scanning electron microscope (SEM) to correlate the deformation behaviour with the grain structure and the grain orientation determined by electron backscatter diffraction (EBSD) measurements. To calculate the resulting stresses and strains, the EBSD data were used to develop a model by finite element method (FEM) considering the grain structure and orientation. The results of the elastic–plastic finite element (FE) simulation were compared with the theories of the E⋅m model based on the Schmidt factor (m) and anisotropic Young’s modulus (E). A mathematical image registration method called “optical flow method” (OFM), which is capable to calculate the transformation of EBSD measuring points during deformation, was applied to the EBSD data. The strains calculated by the optical flow method and by FE simulation were compared for two samples. The findings revealed large strains in the later crack initiation area found in both the OFM and FEM. The developed FEM model was verified by the successful correlation of hypotheses of the E·m model with the simulated mechanical behaviour. Furthermore, the impact of the microstructural neighbourhood on the mechanical behaviour was emphasised. Graphical Abstract

Fatigue Analysis of the Nickel-Based Superalloy Inconel 617 by Fatigue Experiments and EBSD Data-Based Finite Element Simulations in Correlation with E·m Theories

“…In previous work on the deformation behaviour under quasistatic loading, a continuum mechanics-based, elasto-plastic FE model has been developed for Inconel 617, considering the grain structure and the elastic anisotropy. This model was successfully validated up to the onset of plastic deformation and confirmed the abovementioned results of the E•m model for a quasistatic load [21]. In the work presented here, these methods were transferred to cyclic loadings, accompanied by much smaller total deformations.…”

“…Notably, the lower Al and Ti contents in comparison to cast alloys like René80 (approximately 3 wt% Al and 5 wt% Ti) [23] suggest limited Ni3(Al,Ti) γ ′ precipitation hardening, simplifying plastic deformation. Data processing and analysis are based on a two-dimensional approach, based on EBSD measurements, as described in [21]. To achieve an approximately two-dimensional grain structure of the samples, a coarsening thermal treatment of Inconel 617 sheets with a thickness of 3 mm and an average grain size of ~100 µm (see Figure 1a,b) was performed at 1200 • C for 170 h in air.…”

“…Hence, the FE model and the analysis of the stiffness and Schmid factors considering the E•m model are based on a two-dimensional approach. The EBSD data were transferred to an FE model in several steps using MTEX 5.8.1, gmsh 3.0.6, Python 3.6.0, and Dassault Simulia Abaqus 2022 using the workflow developed in previous work [21]. The EBSD data underwent initial processing within the MATLAB Fatigue testing was performed at room temperature (20 °C) using the electro dynamic fatigue testing machine "TA Instruments EletroForce LM2 TestBench" that was operated with force-controlled sinusoidal loadings at a frequency of 20 Hz and a load ratio of 𝑅 = 0.1.…”

“…Hence, the FE model and the analysis of the stiffness and Schmid factors considering the E•m model are based on a two-dimensional approach. The EBSD data were transferred to an FE model in several steps using MTEX 5.8.1, gmsh 3.0.6, Python 3.6.0, and Dassault Simulia Abaqus 2022 using the workflow developed in previous work [21]. The EBSD data underwent initial processing within the MATLAB Electron backscatter diffraction (EBSD) mappings of the whole gauge section were achieved by a Tescan Gaya III Dual-Beam FIB with an integrated EDX-EBSD system with an EBSD camera-type Hikari Plus at 20 kV acceleration voltage.…”

“…Hence, the FE model and the analysis of the stiffness and Schmid factors considering the E•m model are based on a two-dimensional approach. The EBSD data were transferred to an FE model in several steps using MTEX 5.8.1, gmsh 3.0.6, Python 3.6.0, and Dassault Simulia Abaqus 2022 using the workflow developed in previous work [21]. The EBSD data underwent initial processing within the MATLAB toolbox MTEX, as per the guidelines provided in [26].…”