| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 105 === The crystallography and microstructure of lenticular martensite in Fe-0.7C-13r stainless steel has been studied by EBSD, TKD, and TEM in this research. In Fe-0.7C-13r stainless steel, after austenization treatment at temperatures 1200°C for three days, austenite was stable at room temperature and would transform to lenticular martensite after subzero treatment. That lenticular martensite consisting of three regions: the midrib, the twinned region, and the untwinned region was observed by TEM. EBSD analysis shows that the orientation relationship between austenite and martensite is Kurdjumov-Sachs orientation relationship and the traces of midrib of many martensite variants can be used to calculate the normal of habit plane. At initial stage of the formation of lenticular martensite, one random plate group and twin variants of this plate group will be dominant. Other plate groups can only form in retained austenite and the area near grain boundaries. Specific variant selections of lenticular martensite show particular morphologies. Generally, V1-V16 coupling mostly forms from M/A interface while V1-V6 and V1-V17 coupling usually form from the endpoint of lenticular martensite. In this case, the angle between V1 and V6 is about 38° while the angle between V1 and V17 is about 72°; however, different cross-section and morphology will make the angles change.
During analyzing of EBSD results of lenticular martensite, some peculiar microstructures of lenticular martensite were observed. Because of the large interaction volume and the limited resolution of EBSD, TKD and TEM were used to confirm that those microstructures were actual microstructures rather than misleading results due to EBSD limitations. In Fe-0.7C-13r stainless steel, those microstructures could be considered atypical lenticular martensite but the mechanism is not clear.
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