Ultrasonic backscattering method for characterizing the non-uniform microstructure of polycrystals

The characterization of polycrystalline microstructure is crucial for understanding and optimizing mechanical properties. Although ultrasonic backscattering has proven effective for uniform grain structures, it remains challenging to apply to non-uniform, multilayered grain distributions. In this paper, we present a novel ultrasonic backscattering method tailored to such heterogeneous microstructures. We develop a theoretical model to describe backscattering in such materials and propose a feature quantity, N_RMS, to capture time-domain amplitude changes caused by microstructure transitions. The approach is first validated on synthetic layered polycrystals, where 3D grain-scale finite element (FE) simulations confirm excellent agreement with theoretical predictions. A subsequent 2D FE parametric study demonstrates robust detection of interfaces between regions of differing grain sizes for various interface depths and grain-size ratios. We further corroborate the method's effectiveness through experiments on a welded sample and detailed FE simulations based on the electron backscatter diffraction data. Collectively, these results highlight the capability of ultrasonic backscattering for non-destructive evaluation of complex polycrystalline structures, especially in industrial applications that require rapid assessment of grain size variations (e.g. welded joints or additively manufactured metal parts).