材料期刊网

The influence on the apparent martensitic morphology and substructure of tilting a thin-foil specimen through various angles in the transmission electron microscope was studied in detail. It was found that the amount of laths with internal twins in low-carbon steel after quenching from high temperature is more than 40 pct; the microstructure of medium- and high-carbon steel after quenching from high temperature is completely twinned martensite. Distinguishing twinned martensite by observation of martensitic morphology and substructure using a thin-foil transmission electron microscope seems rather unreliable. Previous reports that the volume fraction of lath martensite is about 100 pct in low-carbon steel and 75 to 90 pct in medium-carbon steel quenched from high temperature perhaps need reassessment. The conditions under which internal twins within martensite could be observed in the transmission electron microscope are also discussed.