Phase characterisation of nickel-based superalloys using ultrasound

The nickel-based superalloys are a class of materials with excellent mechanical properties and resistance to degradation, oxidation and corrosion at extremely high temperatures. This ability to withstand harsh operating environments makes them invaluable across many engineering applications, and their superior properties means that these superalloys are often used for highly critical components, where failure is catastrophic and unacceptable. It is therefore essential to ensure that no microstructural alterations or defects are present which could cause a loss in integrity of a part, as the microstructure of the material itself strongly influences properties including fatigue life, toughness and tensile strength. One key microstructural characteristic is the material’s phase composition, as the amount and morphology of the different phases present can drastically influence overall mechanical behaviour. There are many techniques available for investigating microstructure, but those typically deployed in industry today are time-consuming, costly, and ultimately destructive, including scanning electron microscopy and hardness testing. In addition, only a very localised region can be inspected, and thus measurements are unlikely to be representative of the whole part. This thesis presents the research undertaken towards developing a robust, rapid and reliable non-destructive technique, using ultrasonic wave speed measurements, enabling one bulk measurement to quantify the phase composition within a given nickel-based superalloy. Building on a method successfully utilised in quantifying texture, it has been found that the ultrasonic wave speed measurements in conjunction with a spherical harmonic framework is able to detect phase changes within a range of nickel-based superalloys with varying microstructures and manufacturing methods, including Inconel 718, Udimet 720Li and RR1000. This thesis details the investigations and analyses carried out on each of these materials, including the ultrasonic experiments and concurrent metallurgical tests, towards building a robust materials characterisation framework which could be deployed across the engineering industry in the future.