PNNL

Abstract

The mechanical properties of polycrystalline materials depend on the individual physical properties of the constituent grains. When grains are randomly oriented with respect to a fixed coordinate system, the average elastic properties are isotropic. A polycrystalline aggregate possesses macroscopic texture, and thus exhibit anisotropic elastic properties, when the grains are preferentially oriented. Accurate knowledge of texture is important for a number of engineering applications. For example, it plays an important role in determining their subsequent formability into finished parts of complex shape by deep drawing. Textural information can also be exploited to assess plastic damage in a component due to fatigue or external impact. Current ultrasonic methods are based on the relationship between ultrasonic phase velocities and only the low-order orientation distribution coefficients (ODCs, a set of orthonormal basis functions to express crystallographic orientation of a grain), a consequence of the application of Voigt averaging method to infer aggregate elastic properties. Thus, such techniques provide only a partial description of texture. It is well-known from the scattering theories that while phase velocity of acoustic waves is controlled primarily by averages of single grain elastic constant fluctuations, attenuation due to scattering of such waves at the grain boundaries are controlled by two-point averages of these fluctuations. Since these two-point averages depend on higher order ODCs, inversion of measured attenuation is expected to yield a more complete and accurate description of texture. In this paper, we will present computed frequency and direction dependent attenuation coefficient of ultrasonic waves in orthotropic polycrystals with equiaxed cubic grains and mathematically investigate the inversion of computed attenuation coefficient to obtain higher order ODCs. We will also compute pole figures and compare them with those obtained by current ultrasonic methods.