Deformation Microstructure Of Fe-Mn-Si-Al TRIP/TWIP Steels

The deformation microstructures Fe-30Mn-4Si-2Al TWIP/TRIP steel (A12) and Fe-30Mn-3Si-3Al TWIP steel (A13) with different amount of tensile deformation (2%,5%, 10% and 30%) were investigated by the combined use of optical microscopy (OM), atomic force microscopy(AFM) and electron backscatter diffraction (EBSD) technique. The work hardening mechanism of the A12 and A13 steels were also discussed.Three kinds of deformation structures (the planar dislocation band, the s martensite and the deformation twin) are commonly formed on the{111} habit planes and exhibit plate-like morphology during tensile deformation. The surface tilt angles were determined by AFM for 10% tensile deformed A12 steel. It is found that surface tile angles of most banded structure determined by AFM are closed to the theoretical value of deformation twin. While, the surface tilt angles of the new plates formed in austenite matrix are close to the theoretical value of theεmartensite. Those results indicate, for A12 steel, in the case of small defomation, stress-induced martensite transformation is the dominant deformation mechanism, which alters into deformation twinning as deformation stain increases. Most of banded structures formed in A13 TWIP steel were too thin to precisely determine the surface tile angle by AFM. A few plates with relatively large thicknesses could be selected for the quantitative surface analysis. The values of their surface tilt angles were closed to the theoretical values of deformation twins.Microstructures of tensile deformed A12 steel (with deformation strains of 10% and 30%) were investigated by means of EBSD technique. For 10% tensile deformed A12 steel, fullεmartensite microstructure was observed and no deformation twin was found. As the deformation strain increased to 30%, a small amount of deformation twins were observed between s martensite plates. For 10% tensile deformed A13 steel, it was found that only a small amount of deformation twins was formed during deformation. Based on the TEM analysis taken by Ogawa.Kazuyuki and Sawaguchi.Takahiro, it may conclude that, under small deformation, the microsturcture of A12 steel consists mainly of s martensite, while the microstructure of A13 steel is mainly composed of deformation twin and planar dislocation. Although there is too much deference in microstructures between the two alloys, they exhibit a similar work hardening behavior during tensile deformation.By means of EBSD, the microstructures at intersections between twoεmartensite plates on different{111} habit planes in A12 steel and those between two deformation twin plates in A13 TWIP steel were examined. The following conclusions were made based on crystallographic analysis. It was found that the secondary FCC twin was formed at the intersection between two HCP plates, which could be probably attributed to the combination of two half-twinning shears on different{111} planes.{1012} HCP twinning was also observed inside the stress-induced s martensite plates. The intersecting of two deformation twin plates also brought about the formation of secondary austenite twin, but its formation mechanism may be different from that for the martensite intersecting. The detail is still under analysis.