Scattering of elastic waves in simple and complex polycrystals

The propagation of elastic waves in polycrystals is a classical topic with a rich history of research, with primary focus on attenuation in single phase materials with randomly oriented, equiaxed grains. Over the last decade, the need to non-destructively evaluate the degree of damage of engineering components has led to extension of the classical understanding to a number of more complex cases. These motivations include the desire to understand how the noise backscattered from microstructure, and limiting flaw detectability, is controlled by the measurement configuration and microstructure of the material, the desire to use the understanding of attenuation and backscattering in designing improved inspections and in assessing their capability as quantified by probability of detection, and the desire to develop improved procedures for characterizing microstructures. This paper provides an overview of this work. A brief review of the classical understanding of how elastic waves are attenuated and backscattered by scattering from grain boundaries in randomly oriented polycrystals is first presented. This is followed by the results of recent experiments and analysis concerning how these phenomena change in engineering materials with more complex microstructures. For single phase polycrystals, the paper presents results verifying the classical theories in copper, showing how these theories can be used to determine single crystal elastic constants from measurements in alloy polycrystals, demonstrating this technique on nickel-base superalloys, and providing evidence of multiple scattering effects that are not accounted for in the classical, first-order theory. Additional results are presented in titanium alloys having duplex microstructures that demonstrate the existence of fluctuations of beam amplitude and phase, and a simple two-dimensional theory is presented which qualitatively explains the results. The paper concludes with the presentation of some pitch-catch (bi-static) experiments that clearly illustrate the role of multiple scattering. (C) 2007 Elsevier B.V. All rights reserved.