Material Microstructural Analysis With Light Microscopes
Microscopy is an advanced way of observing samples in a special resolution range beyond the unaided eye. Light optical microscopy is considered a simpler microscopic technique when compared to electron and scanning probe microscopy for observing samples for microstructure analysis, but has its own advantages when viewing microstructures.
Microstructures are the geometric arrangement of grains and the various phases in a metal or alloy. The light optical microscope use a visible light and a system of lenses to magnify pictures of microscopic specimens. Using light optical microscopes like the Motic PA53MET Series involve the diffraction, reflection, or refraction of light beams interacting with the samples. The subsequent collection of this scattered radiation or another signal creates an image.
Typically, the magnification of an optical microscope ranges from 1-1000X, but 200X is sufficient for microstructural analysis. The contrast in the reflective image is generated by the difference in reflectivity between different microstructure areas. Optical microscope images are important in determining and identifying the microstructure of a given material. High-end microscopes provide brightfield, darkfield, and polarized light - optical options with high color fidelity and resolution. The resulting micrographs can be seen on computer screens without needing an eyepiece via a CCD camera for sample examination. Below are examplse of an optical micrograph showing the microstructure of two samples of grade 91 martensitic steel and A617 superalloy.
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High-end light microscopes are among the best options to assess physically work-hardened samples. Phases and coatings of different materials can also be observed other than microstructures. The cross-sectional method is utilized to identify and examine hidden defects, impurities, flaws, and cracks for root cause analysis.
For optical microscopy, samples are normally prepared by metallographic techniques. Metallography is a study of the physical structure and components of metals using microscopes. The surface of the samples is prepared by a series of methods, which include mounting, grinding, polishing, and etching. The microstructural constituents of polished and etched samples are easy to observe under optical microscopes. Some cases do not require etching, such as the non-cubic crystals of metal like Ti or Zn, which can be seen using polarized light in polarized light microscopes. High-end microscopes can also perform microstructural analysis, which includes grain boundaries and secondary phase identification.
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