Introduction of metallographic microscope application field and imaging principle
Application field of metallographic microscope
Metallographic examination of ferrous metals, metallographic examination of non-ferrous metals, metallographic examination of powder metallurgy, tissue identification and evaluation after material surface treatment.
Material selection: There is a certain correspondence between the microstructure and performance of the material, based on which the appropriate material can be selected.
Check: raw material check and process check.
Sampling inspection: The product manufacturing process conducts metallographic inspection on semi-finished products to ensure that the microstructure of the product meets the processing requirements of the next process.
Process evaluation: Judging and identifying the qualification of product process.
In-service evaluation: Provide basis for the reliability, reliability and in-service life of in-service parts.
Failure analysis: find process and material defects, so as to provide macro and micro analysis basis for failure analysis.
Various imaging principles of metallographic microscope
1. Bright field, dark field
Bright field is the most basic way to observe samples with a microscope, and it presents a bright background in the field of view of the microscope. The basic principle is that when the light source is irradiated vertically or nearly vertically through the objective lens to the sample surface, it is reflected back to the objective lens by the sample surface to make an image.
The difference between the dark field illumination method and the bright field is that there is a dark background in the microscope field area, and the illumination method of the bright field is vertical or vertical incidence, while the illumination method of the dark field is through oblique illumination around the objective lens. The sample, the sample will scatter or reflect the irradiated light, and the light scattered or reflected by the sample enters the objective lens to image the sample. Dark field observation can clearly observe colorless and small crystals or light-colored fine fibers that are difficult to observe in bright field in dark field.
2. Polarized light, interference
Light is a kind of electromagnetic wave, and electromagnetic wave is a kind of transverse wave, only transverse wave has polarization phenomenon. It is defined as light whose electric vector vibrates in a fixed manner with respect to the direction of propagation.
The polarization of light can be detected with the aid of experimental setups. Take two identical polarizers A and B, let the natural light pass through the first polarizer A first, then the natural light also becomes polarized light, but the first polarizer B is needed because the human eye cannot distinguish it. Fix the polarizer A, place the polarizer B on the same level as A, turn the polarizer B, you can find that the intensity of the transmitted light changes periodically with the rotation of B, and the light intensity will change from maximum to 90° per turn. Gradually weaken to the darkest, and then turn 90 ° light intensity will gradually increase from the darkest to the brightest, so the polarizer A is called a polarizer, and the polarizer B is called an analyzer.
Interference is a phenomenon in which two columns of coherent waves (light) are superimposed in the interaction area 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 means that the light emitted by two independent light sources is not coherent light. The double-slit interference device makes one beam of light pass through the double slit and become two beams of coherent light, which communicate on the light screen to form stable interference fringes. In the double-slit interference experiment, when the path difference from a point on the light screen to the double slit is an even multiple of the half-wavelength, bright fringes appear at the point; when the path difference from a point on the light screen to the double slit is an odd multiple of the half-wavelength , the dark fringe at this point is Young's double-slit interference. Thin-film interference is the phenomenon of interference between two beams of reflected light after a beam of light is reflected by the two surfaces of the film, which is called thin-film interference. In thin-film interference, the path difference of reflected light from the front and rear surfaces is determined by the thickness of the film, so the same bright fringe (dark fringe) should appear at the place where the thickness of the film is equal in thin-film interference. Due to the extremely short wavelength of light, when thin films interfere, the dielectric film should be thin enough to observe interference fringes.
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 analyzer. Polarizers are installed directly in front of the condenser system to linearly polarize the light. A DIC prism is installed in the condenser, and this prism can decompose a beam of light into two beams of light (x and y) with different polarization directions, which form a small angle. The condenser aligns the two beams of light parallel to the microscope optical axis. Initially, the two beams of light have the same phase. After passing through the adjacent area of the specimen, due to the difference in the thickness and refractive index of the specimen, the two beams of light have an optical path difference. A DIC prism II is installed at the back focal plane of the objective lens, which combines the two light waves into one. At this time, the polarization planes (x and y) of the two beams of light still exist. Finally, the beam passes through the first polarizing device, the analyzer. Before the beam forms the eyepiece DIC image, the analyzer is at right angles to the direction of the polarizer. The analyzer combines two perpendicular beams of light into two beams with the same plane of polarization, causing them to interfere. The optical path difference between the x and y waves determines how much light is transmitted. 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 the maximum value. Therefore, on the gray background, the structure of the specimen presents a difference between light and dark. In order to achieve the best image contrast, the optical path difference can be changed by adjusting the longitudinal fine-tuning of the DIC prism II, which can change the brightness of the image. Adjusting the 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.
