The crystallographic orientations of grains in metallic materials play an important role on the microstructures and properties of materials. The determination of crystallographic orientations of grains on the metal surface has long relied on backscattered electron diffraction (EBSD), which is costly and time-consuming. It has been reported that the corrosion behaviours of various metal materials show significant crystallographic orientation dependence. By testing the reflection intensity of light from different corrosion morphologies, the crystallographic orientation can be deduced based on the relationship between corrosion morphologies and crystallographic orientations. In this work, with Inconel 690 alloy as a model alloy, its corrosion behaviours are investigated by immersing the alloy samples in a solution of 1 mol/L H2SO4 and 0.1 mol/L NaCl at 58 ℃ for 24 h. EBSD and scanning electron microscope (SEM) are used to characterize the microstructures and correlate the corrosion morphologies and crystallographic orientations. The statistical results of surface morphologies reveal that grains with orientations near {111} exhibit triangular pyramidal features, grains with orientations near {001} show corrosion pits, and those with orientations near {110} present fish scale-like morphology. According to the corrosion depths obtained from AFM analysis, the dissolution rate of the grains follows the following order: {111}> {110}> {100}. That is to say, the correlation between grain orientations and corrosion morphologies is successfully established. Furthermore, a custom-made OM setup is developed to explore the optical reflection characteristics of different corroded surfaces, i.e., aggregates of microstructure units. The optical reflection intensity of the surface grains at 396 angles (12 elevation × 36 azimuth) is obtained by the OM setup and plotted as directional reflection profile (DRP) to assess the optical reflection characteristics. Our findings reveal that surfaces exhibiting triangular pyramids and fish scale-like features demonstrate significant reflection at specific incident light angles due to the orientation of their maximum exposure facets. In contrast, surfaces characterized by corrosion pits show relatively weak reflection across all incident light angles. Therefore, a clear relationship among grain orientation, corrosion morphologies, and optical reflectance is established. This means that the crystal orientation can be derived from the optical reflection characteristics after corrosion. Compared with previous reports, the work shows remarkable creativity. To be specific, a programmed LED light source is used with each LED controlled by software to turn on or off, minimizing the effects of mechanical vibrations during optical micrograph acquisition. The inner surface of the dome-shaped LED light source is coated in black to eliminate interference from external light sources. All the LEDs are high-brightness and strategically aligned to the center of the dome, where the sample is positioned. This configuration ensures that the incident light is uniformly directed onto the sample surface and thus no additional background removal is required. Furthermore, the OM setup achieves higher resolution in imaging grains due to the use of an objective lens with greater magnification and larger numerical apertures. Based on this, it is expected to perform a fast and low-cost determination of the surface grain orientation using a simple optical setup in the future.
Key words
crystallographic orientation /
corrosion /
optical microscope /
reflection /
690 alloy
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Funding
National Natural Science Foundation of China (52127801, 52425101, 52471012)
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