Key Features for Attention in Metallographic Microstructure Analysis
Metallographic observation under a metallurgical microscope reveals lath martensite in the optical microstructure. X-ray diffraction phase analysis and transmission analysis show that retained austenite also exists in the quenched microstructure, mainly distributed between martensite laths. Quantitative X-ray measurement determines the retained austenite content to be 4.5%. Low-temperature tempering after quenching can improve the stability of retained austenite among martensite laths and enhance the strength and toughness of the material. Moreover, the thin austenite films between martensite laths act as a tough phase. Under external load, plastic deformation and transformation-induced plasticity (TRIP effect) occur. This effect dissipates energy, hinders crack propagation or blunts crack tips, achieving an excellent combination of strength and toughness. For this reason, quenched and tempered steel possesses high strength together with favourable impact toughness, which is associated with retained austenite within the martensitic microstructure formed after quenching.
In practical metallographic analysis and research, paying attention to the following features of material microstructure is beneficial. It helps improve the systematicness and rigor of experimental design and reduces misinterpretation and unreasonable analysis of observed microstructural morphology.
① Multi-scale nature of material microstructure: including atomic and molecular scale, crystal defect scale such as dislocations, grain-scale microstructure, substructure scale and macrostructure scale.
② Heterogeneity of material microstructure: real microstructures often exhibit heterogeneity in geometric morphology, chemical composition and microscopic properties (e.g. microhardness, local electrochemical behaviour).
③ Anisotropy of material microstructure: covering grain morphological anisotropy, directional features of low-magnification structures, crystallographic preferred orientation and anisotropy of macroscopic material properties. These aspects shall be analysed and characterised separately.
④ Evolutionary nature of material microstructure: changes in chemical composition, external conditions and time-dependent phase transformation and structural evolution may alter the microstructure. Therefore, besides qualitative and quantitative analysis of static microstructural morphology, attention should be paid to whether solid-state phase transformation, microstructure evolution kinetics and evolution mechanisms need to be investigated.
⑤ Possible fractal characteristics of material microstructure and resolution dependence in specific metallographic observation: quantitative analysis results of microstructure may depend heavily on image resolution. This point deserves special attention during quantitative analysis of fracture surface morphology and the storage and processing of digital microstructure images.
⑥ Limitations of non-quantitative study on material microstructure: although qualitative microstructure analysis can sometimes meet the requirements of materials engineering, materials science research usually requires quantitative measurement of the geometric features of microstructure and error analysis of the quantitative results.
