Study of Microstructural Changes in an Fe-Mn-Al-Si-C Alloy
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INTRODUCTION
AN important characteristic of the family of steels containing a large amount of manganese (15 to 30 pct) is deformation twinning that leads to a combination of high ductility and high strength.[1,2] This effect is known commonly as a twin induced plasticity (TWIP) effect, which is a promising route for developing advanced high-strength steels for automotive application. Various studies have been conducted with such high manganese steel compositions.[1–8] Substantial work has also been carried out to understand the deformation behavior of these alloys.[9–13] The phase stability of these steels is yet to be understood completely, although significant research efforts have also gone into that area.[14–17] In a previous work, two Fe-Mn-Al-Si-C alloys were subjected to thermomechanical processing to achieve the TWIP property.[18] One of those alloys, containing approximately 0.5 pct C and 30 pct Mn, exhibited excellent TWIP property. The other composition, with a low carbon content (0.05 pct), failed under tensile test at zero elongation. This material consisted of two phases; along with a face-centered cubic (fcc) austenitic phase, there was a strong presence of an ordered bodycentered cubic (bcc) (DO3) phase, and the poor ductility of this material was attributed to the presence of this phase. The ordered DO3 phase is known to exist in ironbased intermetallic systems. The existence of DO3 phase in Fe-Al-Si ternary system and also in several iron-base ternary alloys was observed by Kozaki and Miyazaki.[19] The ambient temperature ductility of the ordered Fe3Al and Fe3Si based alloys, with DO3 structure, is known to be extremely poor.[20] A study carried out[21] on the mechanical properties of Fe3Al alloys revealed that the tensile elongation of nearly stoichiometric Fe3Al was almost zero. Transgranular cleavage fracture in an Fe3Si based alloy has also been reported in the literature.[22] The poor ductility of this type of ordered intermetallic alloys is related to the existence of superdislocations and BASUDEV BHATTACHARYA, Principal Scientist, is with the R&D Division, Tata Steel Limited, Jamshedpur 831001, India. Contact e-mail: [email protected] Manuscript submitted October 25, 2010. Article published online January 19, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
antiphase domain boundaries. The low-carbon alloy studied in the previous work, which was found to be brittle,[18] is the subject of the current report. Although a single austenite phase is desirable for developing TWIP steel compositions, the heavy amount of alloying may be an industrial bottleneck, and it may appear as a challenge to materials engineers to compromise with a two-phase situation, where the maximization of the beneficial austenite phase would have to be achieved through suitable thermomechanical treatment. From that point of view, it is important to know about the transformation behavior of the two phases and also whether the phase fraction of the ductile phase can be enhanced. Accordingly, the current work was undertaken
