[6A2] Characterisation of microstructural changes in nickel-based superalloys using ultrasound
J Jobling¹, T Barden², E A Saunders², M Lowe¹ and B Lan¹
¹Imperial College London, UK
²Rolls-Royce, UK
Nickel-based superalloys are widely used in many engineering applications due to their superior mechanical properties at elevated temperatures and excellent resistance to creep and fatigue under harsh working conditions. These properties can be affected by the microstructure of the material, such as grain size (where smaller grains result in better fatigue resistance and larger grains are more favourable for creep resistance) and phase composition.
One example of a nickel-based superalloy is Inconel 718 (In718), in which the amount of gamma double-prime precipitate within the main gamma phase matrix will affect the material’s performance. Currently, the only methods suitable for material characterisation are destructive, costly and time-consuming (such as SEM/EBSD and X-ray synchrotron inspections) and thus it would be highly beneficial to develop a non-destructive, rapid and reliable method to enable quick inspections for characterising material microstructure and determining whether it would be suitable for a particular application. The aim of this project is to develop a non-destructive inspection method using ultrasound waves for the phase characterisation of nickel-based superalloys. Four dodecagonal samples of Inconel 718 were acquired for ultrasound experiments, two of which were left as received and two undergoing a heat treatment of a 4 h hold at 1100ºC followed by an oil quench. Small cubes were machined off each sample and prepared for SEM inspection, which confirmed that a microstructural change – the dissolution of gamma prime and gamma double-prime precipitates – had occurred.
A variety of ultrasonic testing methods have been trialled on these samples and the relative effectiveness of each has been discussed; these include normal incident attenuation (which confirms grain size change), ultrasound wave speed, or UWS, measurements (of the average wave speeds in all 3D propagation directions) and shear wave speed measurements, in order to determine which would be most sensitive to detecting phase changes in nickel-based superalloys.
One example of a nickel-based superalloy is Inconel 718 (In718), in which the amount of gamma double-prime precipitate within the main gamma phase matrix will affect the material’s performance. Currently, the only methods suitable for material characterisation are destructive, costly and time-consuming (such as SEM/EBSD and X-ray synchrotron inspections) and thus it would be highly beneficial to develop a non-destructive, rapid and reliable method to enable quick inspections for characterising material microstructure and determining whether it would be suitable for a particular application. The aim of this project is to develop a non-destructive inspection method using ultrasound waves for the phase characterisation of nickel-based superalloys. Four dodecagonal samples of Inconel 718 were acquired for ultrasound experiments, two of which were left as received and two undergoing a heat treatment of a 4 h hold at 1100ºC followed by an oil quench. Small cubes were machined off each sample and prepared for SEM inspection, which confirmed that a microstructural change – the dissolution of gamma prime and gamma double-prime precipitates – had occurred.
A variety of ultrasonic testing methods have been trialled on these samples and the relative effectiveness of each has been discussed; these include normal incident attenuation (which confirms grain size change), ultrasound wave speed, or UWS, measurements (of the average wave speeds in all 3D propagation directions) and shear wave speed measurements, in order to determine which would be most sensitive to detecting phase changes in nickel-based superalloys.
