The effect of reduced temperatures on microstructure development in tensile tested high-manganese steel

The current research is part of a project covering several grades of high-manganese steels (20-26 wt% Mn) with different chemical compositions, which were deformed at room and reduced temperatures. In the present paper, the microstructure development is analyzed at the onset of plastic deformation in Fe-Mn-Al-Si steel containing 21.2 wt% Mn. The examination was conducted under conditions of lowering the deformation temperature and aimed to show the influence of reduced temperatures on the occurrence of strain induced phase (martensitic) transformation. The steel samples were subjected to uniaxial deformation in a tensile test at gradually reduced temperatures, starting from 20 degrees C and then applying the temperatures 0 degrees C, -50 degrees C and -150 degrees C. The present paper includes the analysis of microstructure development and orientation relationships as well as determination of operating deformation mechanisms for samples after 10% of tensile strain. The main part of the research was carried out by means of scanning electron microscopy (SEM), using the method of electron back-scatter diffraction (EBSD). The carried out analysis indicates that within the range of small strains the mechanism controlling deformation behavior of high-manganese steel Fe-21Mn-3A1-3Si was slip of partial dislocations. This was immediately followed by the occurrence of strain induced phase transformation gamma (fcc) -> epsilon (hcp) and then locally epsilon (hcp) -> alpha' (bcc). With decreasing deformation temperature a noticeable increase of the volume fraction of the martensitic phases was observed, first of all the epsilon-phase and to a much lesser extent alpha'-martensite. The epsilon-martensite appeared on the background of a deformed austenite initially in the form of relatively narrow bands. The alpha'-martensite was formed predominantly at the intersections of the epsilon-bands. An interesting result, however, is the fact that numerous places were also found where alpha'-martensite was formed in single set of parallel epsilon-phase bands. The preferential crystallographic orientation relations (ORs) between the gamma-austenite and the strain induced epsilon- and alpha'- phases were successively followed, showing first of all the appearance of Shoji-Nishiyama (S-N) and Kurdjumov-Sachs (K-S) ORs respectively.