Some characteristics that should be paid attention to in the analysis of material microstructure by metallographic microscope
The optical metallographic structure of the metallographic microscope is lath-like, which is a lath martensitic structure. X-ray diffraction phase analysis and transmission analysis show that there is still retained austenite in the quenched structure, and the retained austenite mainly exists in the martensitic structure. Between body laths, the residual austenite content was quantitatively tested by X-ray method to be 4.5%. After quenching, low-temperature tempering can improve the stability of retained austenite between martensite laths and improve the strength and toughness of the material. In addition, the austenite film between the martensite laths is a ductile phase, and the metallographic microscope will undergo plastic deformation and phase transformation-induced plastic effect (TRIP effect) under the action of external force, which consumes energy, hinders the expansion of cracks or makes Cracks are completely passivated to obtain a better combination of strength and toughness. Therefore, while the strength after quenching and tempering is high, the impact toughness value is also high, which is related to the presence of retained austenite in the martensite formed after quenching. In practice In metallographic analysis research, it is very beneficial to pay due attention to the following characteristics of the material microstructure, especially to the systematic and rigorous design of the experimental plan, and to reduce misunderstanding and unreasonable analysis of the apparent microstructure possibility.
1. Multi-scale of material microstructure: atomic and molecular level, crystal defect level such as dislocation, grain microstructure level, mesostructure level, macrostructure level, etc.;
2. Inhomogeneity of material microscope structure: the actual microstructure often has inhomogeneity in geometry, inhomogeneity in chemical composition, inhomogeneity in microscopic properties (such as microhardness, local electrochemical potential), etc. ;
3. The directionality of the microstructure of the material: including the anisotropy of the grain shape, the directionality of the low-magnification structure, the crystallographic orientation, the directionality of the macroscopic properties of the material, etc., which should be analyzed and characterized separately ;
4. The variability of the microstructure of materials: changes in chemical composition, external factors and time changes that cause phase transitions and tissue evolution may lead to changes in the microstructure of materials. Therefore, in addition to the need to qualitatively and In addition to quantitative analysis, attention should be paid to whether there is a need for research on solid-state phase transition process, microstructure evolution kinetics and evolution mechanism;
5. The possible fractal characteristics of the material microstructure and the resolution-dependent characteristics that may exist in specific metallographic observations: it may cause the quantitative analysis results of the microstructure to strongly depend on the image resolution. More attention should be paid to this point when conducting quantitative analysis of morphology and storing and processing digital image files of microstructure;
6. The limitations of non-quantitative research on the microstructure of materials: Although the qualitative research on microstructure can meet the needs of materials engineering, the analysis and research of materials science always requires quantitative determination of the geometry of the microstructure And the error analysis of the obtained quantitative analysis results.
