Microstructural mapping of polycrystalline metallic alloys is essential for predicting their macroscopic mechanical performance. Among existing techniques, non-destructive subsurface imaging offers a promising but technically challenging pathway for advancing the characterization of metallic materials. This study introduces a water-immersion ultrasound full waveform inversion (FWI) framework for nondestructive reconstruction of subsurface polycrystalline microstructure. By integrating spectral-element wavefield simulations with adjoint-based inversion, this method simultaneously recovers anisotropic elastic coefficients and crystal orientations. Benchmark tests on Ti-6Al-4V alloy models demonstrate that the proposed method accurately images grain orientations and anisotropic elastic coefficient distributions. It achieves high-resolution reconstructions of polycrystalline microstructures and outperforms direct-contact ultrasound methods in accuracy and artifact suppression. Further applications to equiaxed polycrystals and microtextured regions verify the robustness and applicability of the approach.
Xie et al. (2026) studied this question.