Nickel-Based / Superalloy - Investigating the Microstructural Evolution of Inconel 718 under a Controlled Thermal Gradient
Investigating the Microstructural Evolution of Inconel 718 under a Controlled Thermal Gradient
Abdalrhaman Koko[1]
and Maria Lodeiro
National
Physical Laboratory, Hampton Road, Teddington TW11 0LW, UK
In this study, we demonstrate the ability of the Electro-Thermal Mechanical Testing (ETMT) system to investigate the thermo-mechanical behaviour of Inconel 718 (IN718) under a heating rate of 1 °C/s up to 1000 °C. The unique parabolic temperature profile, achieved through ETMT, allowed the single 65 x 3 x 1 mm3 sample to experience a temperature gradient that effectively replicated the entire phase diagram of IN718 up to 1000 °C. Upon reaching 1000 °C, the sample was rapidly quenched, freezing the microstructure and preserving the phase transformations that occurred at various temperatures. This resulted in a complex, gradient microstructure that provided a comprehensive map of the phase transformations in IN718.
Detailed microstructural analysis was carried out using Electron Backscatter Diffraction (EBSD) and Energy Dispersive X-ray spectroscopy (EDX). This revealed a rich tapestry of phases, from fine dendritic structures in the high-temperature central region to more equiaxed grains towards the edges, reflecting the lower temperatures experienced there. The study also identified precipitates of various secondary phases, and their distribution provided further insights into the local thermal history. The combination of thermal imaging, COMSOL modelling, EBSD, and EDX allowed for a thorough characterisation of the microstructural evolution in IN718, linking the observed phases to the specific temperatures experienced during the ETMT process.
Our findings underline the potential of ETMT as a powerful tool for material testing and characterisation, providing valuable insights into the behaviour of IN718 under thermal gradients and allowing comprehensive microstructural time-temperature-transformation maps to be efficiently generated.