Material characterization of nickel-based super alloys through ultrasonic inspection
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Nickel-based super alloys are widely used in aircraft engine components, mainly in turbine disks and the high-pressure compressor. Extensive data on the mechanical properties of these alloys is available. However, fundamental data relating micro-structural features to ultrasonic properties, in turn controlling the inspectability, are not available. Knowledge of these relationships would facilitate improvements in current Ni billet inspections. A single scattering model has been found to be very effective in simulating the effects of backscattered noise on the ultrasonic inspection of billets and forgings engine alloy materials. The work on this dissertation is focused on evaluating the effectiveness of the single scattering model in predicting backscattered noise in the nickel-based alloys. Attenuation, grain size and single-crystal elastic constants are important input parameters that need to be evaluated for making the predictions of noise levels. Determination of these quantities on a number of nickel-based super alloy samples is reported and studied how noise levels predicted with the current single scattering model compare with the direct measurements of noise on the same samples;In Chapter 1, experimental investigations of the relationships between ultrasonic properties (velocity, attenuation, and backscattered grain noise FOM) and the microstructure are reported. A close relationship was established between the measured attenuation and the Noise FOM, a result consistent with classical theories for backscattering and attenuation but inconsistent to observations in titanium. Chapter 2 describes a quantitative study of the relationship of the experimental observations reported in Chapter 1 to the predictions of classical theories, which require as input the single-crystal elastic constants of the grains and the grain size. The single-crystal elastic constants for these alloys are unknowns and an effort to infer the same for the nickel-based super alloys from ultrasonic measurements is reported. In Chapter 3, a detailed study of the accuracy of the single scattering model to predict the grain noise generated by the microstructure was carried out. Detailed measurements and model calculations were carried out to quantify the breakdown of the single scattering model the comparison of theory and experiment provided strong evidence for the presence of multiple scattering effects.