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DUCTION

THE combination of high strength and good ductility is a key objective of the structural material design. Recently, the newly developed high manganese steels containing 15 to 25 mass pct Mn and additions of silicon and aluminum of about 2 to 4 mass pct exhibit large elongations of 60 to 95 pct and high tensile strengths of 600 to 1100 MPa[1] due to the twinning induced plasticity (TWIP) effect or the transformation induced plasticity (TRIP) effect via multiple martensitic transformations.[2–4] The product of strength and elongation for TWIP steel with Nb microalloying is high, up to about 70,000 MPa pct, in the tensile test at -75 C.[5] The study of TWIP steels indicates that the governing microstructural parameter is the stacking fault energy (SFE) of the fcc austenite. Low SFE (£ 20 mJ/m2) favors the cfcc (austensite) fi ehcp (martensite) transformation in metastable austenite, whereas the SFE of the order of 25 mJ/m2 (>20 mJ/m2) causes mechanical twinning in stable cfcc solid solution.[6,7] However, because the SFE in Fe-Mn-Si-Al TWIP steels is relatively low, it may be difficult to determine the SFE quantitatively by measuring the split-off width of the respective partial dislocations in transmission electron microscopy (TEM). Jiang et al. used X-ray diffraction (XRD) profile analysis (well known as peak broadening B.X. HUANG and X.D. WANG, Doctors, L. WANG, Senior Engineer, and Y.H. RONG, Professor, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, P.R. China 200030. Contact e-mail: [email protected] B.X. HUANG is with the School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, P.R. China 252059. L. WANG is with the Baosteel Research and Development Technology Center, Shanghai, P.R. China 201900. Manuscript submitted March 5, 2007. Article published online February 13, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A

method) to determine semiquantitatively the stacking fault formation probability (Psf) of the austenite (cfcc) matrix in Fe-Mn-Si–based alloys instead of the SFE measurement by using TEM and studied the effect of Psf on cfcc fi ehcp transformation.[8] The Psf measured by XRD is an average probability of stacking fault formation in the cfcc matrix, while in fact, the distribution of stacking faults is localized and such a probability of localized stacking faults (Plsf ) can be measured by using a new TEM method developed by the authors, which is closely relative to the cfcc fi ehcp transformation.[9] The effect of Mn, Si, or Al on the SFEs in both Fe-MnSi-Al TWIP and TRIP steels has been investigated.[1,2] Although the ab initio density functional theory calculations reveal that the addition of Al to Cu dramatically decreases the SFE,[10] whereas the addition of aluminum to Fe-Mn-Si alloys increases the SFE, and thus strongly suppresses the cfcc fi ehcp transformation in quasi-stable cfcc solid solutions.[1] Contrary to this, the addition of silicon decreases the SFE and sustains the cfcc fi ehcp transformation during cooling and de