Optical microscopy is a widely used technique in materials science and metallurgy to examine and analyze the microstructure of metals. It allows researchers to study the surface of metal specimens, revealing important details about their composition, properties, and previous treatment.
In this article, we will explore the basics of optical microscopy, including its principles, techniques, and applications.
Principles of Optical Microscopy Optical microscopes use visible light to observe the microstructure of materials. They are capable of resolutions down to roughly the wavelength of light, which is about half a micron. Electron microscopes, on the other hand, are used for detail below this level, down to atomic resolution.
Optical microscopy can be used to look through specimens (in transmission) as well as at them (in reflection). However, for materials that are opaque to visible light, such as metals, many ceramics, and polymers, only the surface is subject to observation. In reflective mode, contrasts in the image produced result from differences in reflectivity of the various regions of the microstructure.
Techniques of Optical Microscopy To reveal the microstructure of a metal specimen, meticulous surface preparations are necessary. The specimen surface must first be ground and polished to a smooth and mirror-like finish, using successively finer abrasive papers and powders. Then, an appropriate chemical reagent is used to etch the surface of the specimen, revealing the important details of the microstructure.
Applications of Optical Microscopy Optical microscopy can give information concerning a material’s composition, previous treatment, and properties. Particular features of interest are grain size, phases present, chemical homogeneity, distribution of phases, and elongated structures formed by plastic deformation.
The information obtained through optical microscopy is critical for product development, quality control, and failure analysis. It enables researchers and engineers to gain valuable insights into the properties and behavior of metals and alloys, helping them make informed decisions about their design and application.
In conclusion, optical microscopy is a powerful tool that plays a critical role in the examination and analysis of metals and alloys. Its applications are vast, and the insights gained through this process can help researchers and engineers develop better materials and products.
Photography
Once the polished and etched sample is prepared, it is viewed under microscope. Microstructures of various magnifications can be viewed and every magnification viewed microstructures can be saved in the local hard disk as photographs to present in the failure analysis reports.
The microstructures can also be compared with ASM Handbook Volume 9 for cross references whenever required.
Grain Size Measurement
Grain size is measured with microscope by counting the number of grains within a given area,
by determining the number of grains that intersect a given length of random line, or by
comparison with standard charts. The average grain diameter D can be determined from
measurements along random lines by the equation;
D=L/N
where L is the length of the line and N is the number of intercepts which the grain boundary
makes with the line. This can be related to the ratio of the grain-boundary surface area S to the
volume of the grains, V, by the equation;
S/V=2N/L=4l/A
where l is the total length of grain boundary n a random plane of polish and A is the total area
of the grains on a random plane of polish. A very common method of measuring grain size is to
compare the grains at a fixed magnification with the American Society for Testing and Materials (ASTM) grain-size charts. The ASTM grain-size number n is related to N, the number of grains per square inch at a magnification of 100X by the relationship:
N*=2n-1
The table below compares the ASTM grain-size numbers with several other useful measures of
grain size.
Comparison of Grain-Size Measuring Systems (ASM Metals Handbook.)
