Metallographic microscope fracture analysis technology
1. Observation method of metallographic microscope
The basic principle of a metallographic microscope used in materials science is to observe the surface state of an object by reflecting light from the surface of the specimen. Due to differences in the microstructure, crystal structure, chemical composition, and roughness of the material surface, the reflection of light varies, resulting in the formation of a lining as shown in Figures 3-5.
The ultimate resolution of an optical microscope is limited by the wavelength of visible light and can generally be determined by the Payleigh criterion, which is d=0.61 λ/(N • A)
In the formula: d is the resolution;
λ is the wavelength of visible light
N. A is the numerical aperture.
If a green filter is used in a metallographic microscope, the k value can be approximately equal to 0.5 μ m, and for larger allowable numerical apertures with a N · A of 1.4, it is impossible to distinguish fine structures smaller than 0.2 μ m. Due to the inherent drawbacks of low magnification and shallow focal depth, the samples of split rod optical microscopes are limited to flat fracture surfaces, while tough fracture surfaces with large undulations cannot be observed and analyzed using optical microscopes.
In fracture failure analysis, metallographic microscopy is mainly used to observe the microstructure of materials and the morphology of cracks, which belong to the scope of metallographic analysis and will not be elaborated here. When observing cracks, it is not only necessary to observe and analyze the morphological characteristics, orientation, properties, and the beginning and end of the crack itself, but also to observe and analyze the situation around the crack and the normal body, the changes in microhardness on both sides of the crack, the distribution of inclusions, and the morphological characteristics of oxides or corrosion products inside the crack.
Recently, new progress has been made in the application of optical microscopy to observe branches, with the emergence of new types of optical microscopes with confocal pan lenses; Additionally, plastic carbon replication technology can be used to observe the fracture morphology under optical microscopy.
2. Main optical instruments applied
The tools used in metallographic microscope fracture analysis technology mainly include optical instruments such as metallographic microscope and stereo microscope with dual lenses.
Due to the shallow depth of focus of the metallographic microscope, it is required that the studied fracture surface be quite flat, even very close to a plane. That is to say, it is usually not possible to examine rough and uneven fracture surfaces using an optical microscope.
When observing the fracture surface under a metallographic microscope, the commonly used magnification is around 100 to 500. When applying metallographic analysis to study the morphology characteristics of fracture surfaces, it is necessary to install a fracture specimen clamping device on the microscope stage to ensure arbitrary adjustment of the tilt constraint of the fracture observation surface, so that the observed part of the fracture is perpendicular to the microscopic dimming axis.
The undulating morphology of the fracture surface makes it difficult to fully focus the image under the metallographic microscope, which means that only clear images of smaller areas can be obtained under the metallographic microscope. To overcome this drawback, a very small area of the field of view can be selected under an x400 optical microscope environment to take polar photos, and then the focused parts in these same field of view photos can be cut off without having to paste these photos into one image according to the relative positions of each section. This method is quite cumbersome, but from the perspective of expanding the use of optical microscopes, it is still feasible. Especially for units that currently do not have electron microscopy, it has more practical significance.
Another type of optical microscope is a binocular stereo microscope, which typically uses a magnification of x l to x 100 and has a strong sense of stereo vision. It can be paired with photographic equipment.
Recently, Mclachin has developed an optical microscope with a greater depth of focus, which can be used to analyze and study the morphology of microscopic fractures.
