INFLUENCE OF MICROSTRUCTURE ON ULTRASONIC BACKSCATTERING NOISE IN TITANIUM ALLOYS

Abstract Backscattering from microstructural discontinuities such as grain boundaries in metals produces noise which is observed in most ultrasonic experiments. These signals provide information concerning the microstructure of the material. The utilization of the absolute values of the noise signals in materials characterization is described in this paper. A quantitative model for the dependence of the noise signals upon the radiation pattern of the transducer is presented, including such measurement variables as transducer radius, frequency and ultrasonic propagation paths. The microstructure enters into the problem via a scattering parameter, or material figure of merit (FOM) for grain noise severity. For a simple case such as a single phase polycrystal, this may be thought of as the root mean square value of the scattering amplitude of a grain embedded in an effective medium representing the polycrystal. After reviewing this model, the paper will concentrate on experimental tests of its accuracy. Included will be tests of the model's ability to predict the dependence of noise signals on measurement parameters such as those mentioned above, as well as comparison of its absolute predictions to measurements made on single-phase alloys. The use of the model to interpret noise observations on commercial titanium alloys having more complex microstructures is then discussed. KEY WORDS: Ultrasonicsbackscattered noisegrain structuretitaniumfigure-of-merit