Factors Affecting Martensite Tetragonality during EBSD Analysis

Recent years show a significant interest in the study of martensite tetragonality in steels [1].The tetragonality of martensite in general depends on the carbon content of the austenite before quenching.Almost 100 years of research have shown a linear dependence of tetragonality (the ratio c to a, c/a) on the carbon content of the steel.c/a = 1+0.045Xwhere X is the carbon content mass percentage.Studies of tetragonality were so far mainly carried out using X-ray diffraction.In X-ray diffraction studies, the signal comes from a large area of the sample, typically more than 1 mm 2 averaging the obtained result.The most promising technique for the local measurement of tetragonality appears to be Backscattered Electron Diffraction (EBSD), which allows surface measurements with steps of tens of nanometers or more.In recent years, new algorithms have been developed to analyze local changes in lattice parameter ratios based on EBSD patterns [2,3].Newly obtained results show that martensite tetragonality can be decreased by exposure to Ga + ions from focused ion beam (FIB).Small doses of 50 pC/μm 2 lead to decrease of the local tetragonality of the martensite at all investigated acceleration voltages (2-30 kV).Moreover, the retained austenite treated with gallium ions transformed into ferrite/or martensite which was not tetragonal.These results suggest that special attention should be paid for transmission electron microscopy (TEM) lamellae and atom probe tomography (APT) sample preparation of tetragonal martensite.Another factor leading to change of the measured tetragonality is deformation which can happen during sample preparation or on purpose and it is causing decrease of the martensite tetragonality.Examples of both ion beam and deformation induced tetragonality loss will be discussed during the presentation [4].