Surface engineering of metals via metallographic etching for microstructural characterization

• Main Author: Jain, Ekta
• Other Authors: Matteo Seita
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering; Engineering::Materials::Material testing and characterization
• Online Access: https://hdl.handle.net/10356/154406
• Institution: Nanyang Technological University

Metallography is the most utilised practice to analyse the microstructure of materials and interpret their properties. Most of the typically used characterisation methods are known to be complex and high-cost equipment setup and they often have limited field of view and low through-put. In this work, a simple and in-expensive novel optical-based method called directional reflectance microscopy (DRM) is used for inferring the grain orientation and compositional gradients in chemically etched metals and alloys. DRM relies on acquiring a series of optical micrographs of chemically etched metallic surfaces illuminated under different angles. Later, the angle-dependent reflectance of the sample surface enables us, to measure the orientation of the underlying crystal grains in metals and compositional heterogeneity in alloys. The thesis initially focuses on understanding and controlling the formation of the etch-induced surface structures in aluminium (Al), which underpin the DRM measurements. Al undergoes a complex etch-pit formation based on the competing oxidation and reduction reactions occurring at the interface between sample surface and chemical etchant. Variable etchant chemistry and etching time evolves etch-pits with different crystallography and have important implications on DRM measurements. The measurements revealed that sometimes, these etch-pits exhibit different geometries than those traditionally expected, suggesting that the orientation-dependent corrosion-resistance of Al may be function of the etching process itself. As such, this work provides the ability of controlling surface structure to access more information than that available by a single DRM data set (e.g., using a single etchant). The repertoire of correlating DRM analysis with chemically etched surfaces is later extended to additive manufactured metal alloys such as Inconel 718, produced via directed energy deposition (DED). DED is an additive manufacturing technique that enables rapid production and repair of metallic parts. The inherent non-uniform cooling rates imparted by the process, however, often yield parts with long-range compositional gradients, which exhibit large scatter in mechanical properties. Given the low-throughput and limited field-of-view of conventional characterization techniques, assessing this compositional heterogeneity in additively manufactured metals is challenging. Because local difference in alloy composition yield differences in surface structure upon chemical etching, DRM is an ideal candidate to assess compositional gradients over large areas. We demonstrate this capability and map changes in the local niobium (Nb) concentration across different Inconel 718 builds by quantifying the surface reflectance, which is directly related to the size and distribution of the Nb-rich secondary phase precipitates. These results demonstrate the capability of inferring compositional information using a low-cost and high-throughput optical technique like DRM.