1. What are the general observation modes of metallographic microscopes for metallographic analysis, material detection, and failure analysis?
A: Bright field, dark field, polarized light (polarized light), differential interference DIC, fluorescence, some high-end models with additional oblique illumination and laser confocal auxiliary observation.
2. What is the difference between the commonly used BF bright field, DF dark field, Pol polarized light, and DIC differential interference effects?
Answer: The bright field lays out dark tissue with a bright matrix; the dark field has a black bottom, and the cracks mixed with holes are highlighted; the polarized light distinguishes the crystal orientation from the non-metallic phase by color; the DIC converts the small height difference of the surface into a three-dimensional relief, focusing on the observation of morphological fluctuations.
3. Different observation methods of metallographic microscopy Summarize what scene samples to look at?
BF Brightfield: post-corrosion metallographic structure, grain, carburized/decarburized layer, nodular graphite rating
DF dark field: non-metallic inclusion rating, micro-cracks, loose pores, polishing minor damage, corrosion micropores
Polarized light: magnesium titanium zinc alloy grains, graphite, oxide scales, plating, corrosion products, ceramic metallography
DIC differential interference: fracture failure topography, fatigue cracks, worn surfaces, coating steps, uncorroded polished specimens
4. What is the difference between dark field observation with metallographic microscopy and dark field observation with biological microscopy?
Metallographic dark field: reflective, the light source beats the metal specimen obliquely from the ring above the objective lens, imaging the uneven scattered light on the surface; the specular reflected light of the flat metal cannot be returned to the objective lens, and the background is black, mixed/scratched. Biological dark field: transmission type, the bottom parabolic condenser specimen is obliquely transmitted, the direct light is blocked by the center, and only the cellular particles refract the scattered light into the objective lens, which is used for transparent living body samples.
Metallographic dark field: Look at the opaque metal surface and check for inclusions, micro-cracks, pores, polishing defects, and failure defects. Biological Dark Field: Look at clear liquids/slices and observe live bacteria, protists, ultra-thin bioparticles.
Metallography relies on the annular beam splitter in the lens barrel to achieve dark field switching; biological relies on the understage dark field condenser (central light blocking plate) to achieve, and the optical path modules of the two cannot be used universally.
Metallographic: All the reflections of the flat metal escape, only the concave and convex defects scatter and emit light; Biological: The direct light of the transparent medium is blocked, and only the refracted light of the sample particles is developed.
5. If you look at the metallographic structure, do you not need dark field, polarization and differential interference?
A: No, conventional metallographic organization observation is mainly based on bright fields, but many scenarios must be assisted by dark fields, polarization, and DIC, and cannot rely solely on bright fields.
Simply put, the observation of the basic structure of ordinary corroded steel is only sufficient for the bright field; when it comes to defects, light metal, surface layer, and failure morphology, one of the three is indispensable.
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