[2B4] Phase characterisation of nickel-based superalloys using ultrasound
J Jobling
Imperial College London, UK
Nickel-based superalloys are widely used across the engineering industry, due to their superior mechanical properties at elevated temperatures (such as downstream of the combustor) and excellent resistance to creep and fatigue under harsh working conditions. The microstructure of the material greatly affects these characteristics, including the phase composition and grain size (smaller grains result in better fatigue resistance, whereas larger sizes give better creep resistance). A notable superalloy within this material family is Inconel 718, where the amount of the gamma double prime phase precipitated within the main gamma phase matrix is most important for material performance (though other phases are also present). Current existing methods for material characterisation are destructive, expensive and time-consuming (such as scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD)), as are techniques for determination of the fatigue life of components (which require surface preparation for subsequent hardness testing); hence it would be highly beneficial to develop a non-destructive, rapid and reliable method to enable quick inspections for characterising the microstructure of a material and determining its suitability for a particular application.
This project aims to develop a non-destructive inspection method using ultrasound for phase characterisation of nickel-based superalloys. A total of eight samples of Inconel 718 were acquired, with half remaining in as-received condition and the others undergoing a heat treatment to simulate heat damage (comprising a four-hour hold at 1100°C followed by an oil quench). Specimens were machined off each sample and prepared for SEM to confirm that the expected microstructural changes (dissolution of precipitates) had occurred. Two ultrasonic testing methods were trialled on these samples: wave speed measurements (by taking an average in all 3D propagation directions, then using a spherical convolution technique to calculate the zeroth coefficient); and measurement of normal incident attenuation, to explore which would be most sensitive to detecting phase changes in nickel-based superalloys. The feasibility of using the zeroth coefficient methodology in characterising the phase composition of the material has been demonstrated, which will now be applied to specific phases within the material.
This project aims to develop a non-destructive inspection method using ultrasound for phase characterisation of nickel-based superalloys. A total of eight samples of Inconel 718 were acquired, with half remaining in as-received condition and the others undergoing a heat treatment to simulate heat damage (comprising a four-hour hold at 1100°C followed by an oil quench). Specimens were machined off each sample and prepared for SEM to confirm that the expected microstructural changes (dissolution of precipitates) had occurred. Two ultrasonic testing methods were trialled on these samples: wave speed measurements (by taking an average in all 3D propagation directions, then using a spherical convolution technique to calculate the zeroth coefficient); and measurement of normal incident attenuation, to explore which would be most sensitive to detecting phase changes in nickel-based superalloys. The feasibility of using the zeroth coefficient methodology in characterising the phase composition of the material has been demonstrated, which will now be applied to specific phases within the material.
