A feasibility study on phase characterisation of nickel-based superalloys using ultrasound

Abstract

The use of nickel-based superalloys, such as Inconel 718, Udimet 720Li and Waspaloy, is widespread across the engineering industry due to their superior mechanical properties at elevated temperatures and excellent resistance to creep and fatigue under harsh working conditions. The microstructure of these materials, such as the precipitates and grain sizes, strongly influence the materials’ performance. Currently, existing materials characterisation methods are time-consuming, costly and destructive, as well as only capable of taking measurements of surface or near-surface properties. This paper presents a novel ultrasonic technique for bulk materials characterisation of nickel-based superalloys, based on a spherical harmonic convolution theory which has been successfully used to quantify texture in various metals; it is applied here to study a nickel-based superalloy, with a specific focus on the phase changes when subject to thermal loading. The key parameter used to interpret phase information of a material is the zeroth spherical harmonic coefficient of ultrasonic wave velocities, V₀₀, which corresponds to the average velocities across all directions in 3D space and is independent of crystallographic texture. Experimental investigation using samples of a representative material, Inconel 718, with contrasting microstructures, has shown a clear and quantifiable sensitivity of this coefficient to microstructural changes occurring within the material, and has thus laid the foundations for the development of a rapid phase characterisation technique for nickel-based superalloys using ultrasound.