Introduction
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.
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.
Grain Boundaries
Two important things to determine in microstructures are grain size and shape, because most engineering alloys are polycrystalline. These help in revealing details about the crystal structure, like BCC, FCC, or HCP or whether it is single crystal grains. The crystallographic orientation of each grain is related to its neighboring grain, which is divided by an interface. This interface is called the grain boundary. This exists between each grain because of the disordered crystal lattice, which occurs by an abrupt change in crystallographic directions. Spacing between grains, called grain size, is highly affected by many characteristics of a polycrystalline material. Therefore, it is a routine practice in material testing to observe grain boundaries for grain shape and size identification in each microstructure. In metals, average grain size is in the order of a few to 10mm, which microscopes with software solutions can easily measure.
It is interesting to know that grain shape and grain size within the grain boundaries are two distinct and important properties for any material. Size represents the absolute magnitude of the grain and shape explains the relative measures of grain to determine its geometry. In material characterization, both are important because many mechanical properties rely on these factors. For example, coarse grain size can increase hardness and brittleness in alloy and fine grain size increases the ductility. The size and geometry of grain is largely dependent on processing methods or heat treatments, if given to material. grain shape can vary with the size of grain therefore both are related. In short, it is important to know about grain which can be determined by light microscopes easily.
Second phase particles
Engineering materials mostly consist of multiple phases. Phase are defined as a distinct part of a material in each crystal structure like BCC, HCP, or FCC, or a composition. For example, martensite is a phase in steel, and iron is a phase in iron carbide Fe3C. The size of second phase particles in most engineering alloys is in the order of some micrometers and can be seen through light microscopes. Micrographs appear in optical microscopes at different gray scales because each phase reflects light at various intensities, depending on its optical properties.
In the same way, optical microscopes can be used to examine the microstructures and surfaces of non-metallic materials like ceramics and plastics or elastomers. These are like metal samples, but polymers are softer than metal and can cause difficulties. Therefore, experience is important in determining good results in polymer surface analysis when using optical microscopes.
Methods can reveal the structure of metal when a properly prepared mount is used for the analysis of a polymer or some other soft material. The structures that can be seen are knit lines, porosity, and other molding issues for polymer samples. It can also reveal the size and shape of reinforcing fibers or particles and can measure, observe, and document any changes throughout the part in their distribution. In the case of metals, crack profiles can be examined and reveal microcracking. Sometimes, in the strained part of ductile polymers that are not yet fully formed, cracks can be seen and provide information about how a part might be absorbing service stresses. An example of a normal photograph and one taken under an optical microscope are shown below. Useful quantitative data information about the surface can be seen in optical micrographs along with the more specific shapes of surface topography.
Light microscopes can analyze and examine many other materials than metals, alloys, and polymers. These are: 3D printed materials, Polycarbonates, Printed Circuit Boards, Semiconductor & Microelectronic components.
Contrasting methods of Light microscopes
The information extracted by the microscope can also vary greatly, depending on the different contrasting technique. There are several methods to assess the microstructural properties of materials. These methods are:
1. Brightfield:
Brightfield is the standard method for analyzing all types of materials. Porosity, voids, phases, and oxidation in metals can be seen without etching metals, because of the different manners of reflections compared to the base metal in brightfield.
2. Darkfield:
This method is mostly utilized for nonmetals. But for metals, it also has some benefits like presenting colored structures for coating and the base metal. It can also be used for corrosion analysis. It also has the ability to prominently display fine scratches that occurred during grinding processes.
3. Polarization:
Polarization is mostly utilized for metals with hexagonal crystal structures like titanium and aluminum. Aluminum when etched electrolytically can be easily seen in polarized light with tetrafolouroboron acid.
References
- Di Gianfrancesco, A., 2017. Technologies for chemical analyses, microstructural, and inspection investigations. In Materials for ultra-supercritical and advanced ultra-supercritical power plants (pp. 197-245). Woodhead Publishing.
- Ebnesajjad, S., 2011. Adhesives for medical and dental applications. In Handbook of Polymer Applications in Medicine and Medical Devices (pp. 103-129). William Andrew Publishing.
- Church, M.J. (1978). Grain size and shape. In: Sedimentology. Encyclopedia of Earth Science. Springer, Berlin, Heidelberg . https://doi.org/10.1007/3-540-31079-7_104
Related Products
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The PA53MET holds all brightfield, darkfield and DIC options along with easy-to-use software for measurement and recordkeeping. The microscope also offers motorized xyz for more advanced imaging applications. |
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