Quantitative phase characterisation of nickel-based superalloy Inconel 718 using ultrasound

Phase composition in nickel-based superalloy Inconel 718 is a determinant factor of its overall mechanical properties. Ultrasound has been shown to have sensitivity to the phases through its wave speeds in a previous qualitative feasibility study. This paper demonstrates the capability of ultrasonic wave speed (UWS) measurements for the quantitative characterisation of delta (δ) phase in the nickel-based superalloy Inconel 718, which is fundamentally due to the zeroth spherical harmonic coefficient of velocity, V00, being sensitive to changes in microstructure. A variety of samples having undergone different processing histories were used for the UWS measurements, alongside extensive metallurgical investigations – including advanced microscopy techniques such as high-speed atomic force microscopy (HS-AFM), as well as hardness testing to fully understand the microstructures of each sample and corroborate the UWS results. By combining the data from these experimental techniques, and through an iterative parameter search, it was confirmed that the V00 measurements could be used to deduce the δ phase content in the Inconel 718 samples; more significantly, the strength of a combined materials characterisation framework – incorporating UWS measurements, hardness testing and microscopy – was demonstrated, establishing a powerful tool for the phase characterisation of nickel-based superalloys for the future.