Introduction of application field and imaging principle of metallographic microscope
Metallographic examination of ferrous metals, nonferrous metals, powder metallurgy, identification and evaluation of the structure after surface treatment of materials.
Material selection: there is a certain correspondence between the microstructure and properties of materials, so suitable materials can be selected.
Check: raw material check and process check.
Sampling inspection: metallographic examination of semi-finished products is carried out in the product manufacturing process to ensure that the microstructure of products meets the processing requirements of the next working procedure.
Process evaluation: to judge and identify the qualification of product process.
In-service evaluation: provide basis for the safety, reliability and service life of in-service parts.
Failure analysis: technological and material defects are found, thus providing macro and micro analysis basis for the analysis of failure reasons.
Imaging principles of metallographic microscope
1. Bright field of view and dark field of view
Bright field of view is the most basic way to observe samples by microscope, and it presents a bright background in the field of view of microscope. Its basic principle is that when the light source irradiates the sample surface vertically or nearly vertically through the objective lens, it is reflected back to the objective lens through the sample surface to make it image.
The illumination mode of dark field is different from that of bright field in that it presents a dark background in the field of view of microscope, and the illumination mode of bright field is vertical or vertical incidence, while the illumination mode of dark field is to illuminate the sample obliquely through the periphery outside the objective lens, so that the sample will scatter or reflect the irradiated light, and the light scattered or reflected by the sample will enter the objective lens to image the sample. Dark field observation, you can clearly observe colorless, fine crystals or fine fibers with lighter colors that are not easy to observe in bright field.
2. Polarized light and interference
Light is an electromagnetic wave, and electromagnetic wave is a shear wave, and only shear waves have polarization. It is defined as light whose electric vector vibrates in a fixed way with respect to the propagation direction.
The polarization of light can be detected by means of an experimental device. Take two identical polarizers A and B, and pass the natural light through the first polarizer A first. At this time, the natural light also becomes polarized light, but the human eye cannot distinguish it, so the second polarizer B is needed. Fixing the polarizer A, placing the polarizer B on the same horizontal plane as A, and rotating the polarizer B, we can find that the intensity of transmitted light changes periodically with the rotation of B, and the intensity gradually decreases from the maximum to the darkest every 90 degrees, and then gradually increases from the darkest to the brightest after rotating for 90 degrees. Therefore, the polarizer A is called a polarizer and the polarizer B is called an analyzer.
Interference is the phenomenon that two coherent waves (light) are superimposed in the interaction zone to increase or decrease the light intensity. The interference of light is mainly divided into double slit interference and thin film interference. Double-slit interference is that the light emitted by two independent light sources is not coherent light. The device of double-slit interference makes a beam of light pass through the double slit and become two coherent light beams, which communicate with each other on the light screen to form stable interference fringes. In the double-slit interference experiment, when the distance difference between a point on the light screen and the double-slit is even multiple of half wavelength, bright stripes appear at that point; When the distance difference between a point on the screen and the double slit is odd times of half wavelength, the dark fringe at that point is Young's double slit interference. Thin-film interference refers to the interference phenomenon caused by two reflected lights after a beam of light is reflected by two surfaces of the thin film. In thin film interference, the distance difference of reflected light from the front and back surfaces is determined by the thickness of the film, so the same bright stripe (dark stripe) in thin film interference should appear where the thickness of the film is equal. Because the wavelength of light wave is extremely short, the dielectric film should be thin enough to observe interference fringes when thin films interfere.
3. Differential interference contrast DIC
Metallographic microscope DIC uses the principle of polarized light. Transmission DIC microscope mainly has four special optical components: polarizer, DIC prism I, DIC prism II and polarizer. The polarizer is directly installed in front of the condenser system to linearly polarize the light. A DIC prism is installed in the condenser, which can decompose a beam of light into two beams (X and Y) with different polarization directions, and the two beams form a small included angle. The condenser adjusts the two beams of light to the direction parallel to the optical axis of the microscope. * The first two beams of light are in the same phase. After passing through the adjacent area of the specimen, the optical path difference between the two beams of light occurs due to the different thickness and refractive index of the specimen. DIC prism Ⅱ is installed at the back focal plane of the objective lens, which combines two light waves into one beam. At this time, the polarization planes (x and y) of the two beams still exist. Finally, the beam passes through a polarizing device, that is, an analyzer. Before the beam forms an eyepiece DIC image, the analyzer is at right angles to the polarizer. The analyzer combines two vertical light waves into two beams with the same polarization plane, so that they interfere with each other. The optical path difference between X and Y waves determines the amount of light transmission. When the optical path difference is 0, no light passes through the analyzer; When the optical path difference is equal to half the wavelength, the light passing through reaches a large value. So on the gray background, the specimen structure presents a bright and dark difference. In order to make the contrast of the image reach a good state, the optical path difference can be changed by adjusting the vertical fine adjustment of DIC prism II, which can change the brightness of the image. Adjusting DIC prism ⅱ can make the fine structure of the specimen present a positive or negative projection image, usually one side is bright and the other side is dark, which causes the artificial three-dimensional sense of the specimen.
