An Atom Probe Study of Kappa Carbide Precipitation and the Effect of Silicon Addition
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CAST lightweight austenitic Fe-Mn-Al-C steels have both low melting points, less than (1623 K) 1350 °C, and good filling characteristics which are similar to cast irons.[1] Addition of 9 to 10 wt pct aluminum reduces the density by up to 15 pct when compared with traditional steels and may be of interest to the transportation industry as corporate average fuel economy is increased to 54.5 mpg by 2025. These high aluminum
LAURA N. BARTLETT, Assistant Professor, is with the Texas State University Department of Engineering Technology, San Marcos TX. Contact e-mail: [email protected] DAVID C. VAN AKEN, Curator’s Teaching Professor, is with the Metallurgical Engineering, Missouri University of Science and Technology, Rolla MO. JULIA MEDVEDEVA, Associate Professor, is with the Physics Department, Missouri University of Science and Technology, Rolla MO. DIETER ISHEIM, Research Assistant Professor, is with the Northwestern University Center for Atom Probe Tomography, Evanston IL. NADEZHDA I. MEDVEDEVA, Senior Research Scientist, is with the Institute of Solid State Chemistry, Ural Branch of the Russian Academy of Science, Ekaterinburg, Russia. KAI SONG, formerly Senior Research Specialist with the Missouri University of Science and Technology, is now Senior Applications Engineer with the FEI Company, Hillsboro, OR. Manuscript submitted June 7, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
steels can be competitive in terms of strength with quenched and tempered steels when age hardened. However, the high manganese (20 to 30 wt pct) required to stabilize an austenitic matrix[1,2] may relegate this class of steel to castings, since electrolytic manganese is required to limit phosphorus and may be too costly for current steelmaking practices.[3] Grades that contain from 5 to 11 wt pct aluminum and from 0.3 to 1.2 wt pct carbon are age hardenable when heat treated in the range of 723 K to 973 K (450 °C to 700 °C).[2–4] All compositions in the following text are in weight percent unless otherwise stated. Depending on the heat treatment, cast alloys can attain strengths as high as 1100 MPa in the age-hardened condition and good ductility in the solution-treated condition with total elongations greater than 64 pct.[3,5] Excellent strengths and high work hardening rates with up to a 15 pct reduction in density make these alloys very attractive for high energy absorbing applications.[2] However, the mechanical properties are a function of age hardening and knowledge of how alloying additions and impurities affect aging is of primary interest when qualifying these steels for high energy absorbing or low temperature applications. Hardening is attained by the homogeneous and coherent precipitation of nano-sized j-carbide, (Fe,Mn)3AlCx. j-carbide has the E21 cubic perovskite crystal structure in
which aluminum occupies corner positions, iron and manganese occupy face-centered positions, and carbon is at the body center interstitial octahedral site. j-carbide has a cube on cube orientation relationship with the austenitic ma
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