Deformation Microstructure and Deformation-Induced Martensite in Austenitic Fe-Cr-Ni Alloys Depending on Stacking Fault
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stable austenitic stainless steels are of high technical interest due to their pronounced work hardening behavior, providing an attractive ductility in combination with their inherently high corrosion resistance. Part of the high work hardening is due to the deformation-induced martensitic transformation (DIMT), which has been the subject of extensive research in commercial steels.[1–4] Experimental research considering the microstructure of carefully tailored model alloys that can more directly be compared with theoretical predictions of stacking fault energy (SFE) using, for instance, first-principles calculations[5] and
YE TIAN, Ph.D. Candidate, ANNIKA BORGENSTAM, Professor, and PETER HEDSTRO¨M, Assistant Professor, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden. Contact e-mail: [email protected] OLEG I. GORBATOV, Postdoctoral Fellow, is with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, and also with the Nosov Magnitogorsk State Technical University, 455000 Magnitogorsk, Russia. ANDREI V. RUBAN, Professor, is with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, and also with the Materials Center Leoben, 8700 Leoben, Austria. Manuscript submitted June 14, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
thermodynamics-based modeling[6] is, however, more scarce in the literature. The body-centered cubic (bcc) martensite (a¢-martensite, hereinafter referred to as a¢) has been claimed to contribute most to the mechanical properties of these alloys, but also the hexagonal close-packed (hcp) martensite (e-martensite, hereinafter referred to as e) is important since it is known to contribute to the nucleation of the a¢ as well as the mechanical properties. Consensus has not been reached about this to date. Two transformation sequences of the a¢ have been reported: (i) c (austenite, face-centered cubic(fcc)) fi e fi a¢ and (ii) c fi a¢,[3,7–9] and it is today generally accepted that e can be a transient phase,[10,11] but it is not a necessary precursor for a¢.[12,13] Furthermore, it is likely that the transformation sequence differs with the variation of chemical composition, temperature, and strain rate,[14] and that it can be related to SFE and the austenite stability.[15] Clearly, the discussion about the existence of a transient phase relates to the nucleation and growth of a¢. In the most commonly applied model of DIMT,[7] intersections of shear bands (SBs, consisting of bundles of intermixed faults, twins, and e[16,17]) are assumed to be the preferred nucleation sites for a¢,[18] but it has also been shown that nucleation at individual SBs,[19–23] grain boundaries,[24] and SB–grain boundary intersections[3] may occur. Another interesting aspect of nucleation of deformation-induced martensite is the suggested difference between stress-assisted and strain-induced nucleation.[7] Perdahcioglu et al.[25,26] have suggested that the main effect of the deformation is the additional
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