Advanced crystallography measurements by dIrectional reflectance microscopy

• Main Author: Zhu, Chenyang
• Other Authors: Xiao Zhongmin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Metals; Crystallography; Directionary reflectance microscope; Digital quality control; Forward model
• Online Access: https://hdl.handle.net/10356/181518
• Institution: Nanyang Technological University

Characterizing crystallographic orientation is crucial for understanding the structure-property relationships in crystalline solids. This information is used in academia, to interpret the behavior of existing materials and design new ones, as well as in industry, to qualify the performance of engineering components. Traditionally, crystal orientation mapping relies on diffraction-based methods, which are expensive, have limited measurement throughput, and require involved sample surface preparation. To address these limitations, our research group has proposed a complementary optical technique, called directional reflectance microscopy (DRM). DRM enables grain orientation mapping of crystalline solids by capturing and interpreting optical reflectance signals generated by chemically etched surfaces. In this thesis, I have developed a more reliable and robust approach to grain orientation mapping by DRM and I have extended the capabilities of this technique to characterizing large-scale parts with complex geometry and to measuring complex crystallographic quantities in polycrystalline materials. Conventional orientation mapping by DRM relies on fitting the optical signals captured from a flat, chemically etched sample using different image processing algorithms. This approach is inevitably error-prone, material-dependent, and limited to small-scale research samples. These limitations hinder the adoption of DRM as a universal characterization method in materials science and in industry. In contrast, my approach has been to build a physics-based reflectance model for chemically etched surfaces to simulate the reflectance signals produced by the chemically etched crystalline surfaces. This approach, which I name dictionary indexing DRM (DI-DRM), uses a dictionary of simulated reflectance profiles that are compared against the DRM measurement to identify the correct crystal orientation. DI-DRM yields measurements with improved accuracy compared to those enabled by fitting models, both on different materials as well as across all crystal orientations considered. Moreover, the measurement error (~3°) shows mild sensitivity to experimental variabilities, such as noise, measurement settings, and the quality of the sample surface. Leveraging this versatile approach, I demonstrated grain orientation mapping on curved surfaces, and I have characterized the precision of the measurement for an arbitrary surface orientation. I showcased this new capability by capturing a DRM measurement on the airfoil region of a nickel-base turbine blade. The results from this analysis highlight the potential of DRM as a technique for part-specific quality control in digital manufacturing. Finally, I have investigated and proven the suitability of DRM as a high-throughput technique to study complex crystallography-property relationships in materials science. First, I have shown that DRM can be used on specially produced samples to reconstruct the quasi-3D grain boundary network of polycrystals (i.e., how the internal interfaces in the material are connected together in 3D) and assess their 5D crystallography. Second, I have shown that DRM can be used to detect signals associated with the plastic deformation of polycrystalline metal samples. The combination of these two features opens the path to high-throughput, in situ mechanical testing of polycrystals to investigate their plasticity—a field which has fueled research in materials science since the Bronze age.