Stabilization mechanisms of retained austenite in transformation-induced plasticity steel
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ARIOUS phase transformations, i.e., proeutectoid ferrite,[1] bainite,[2] martensite,[3] and intermetallic precipitation reactions,[4,5] may take place in a steel grade exhibiting transformation toughening.[6] The occurrence of a specific phase transformation depends on the alloy composition and heattreatment procedures,[7,8] while the amount of retained austenite at room temperature in such a steel grade depends largely on the martensite start (Ms) temperature of the dispersed austenite with a very small grain size. It is known that most engineering steels, after regular austenitizing, have an average austenite grain size ranging from 20 to 100 m. On such a size scale, the influence of the grain size on the Ms temperature is negligible, which is in contrast to that on the premartensitic reactions, such as the proeutectoid ferrite, pearlite, and bainite reactions. Results reported by Kajiwara et al.[9] suggested that ultrafine austenite particles (20 to 200 nm) transformed at the same Ms temperature as that for the corresponding bulk alloy. On the other hand, it was reported that the Ms temperature might rise by approximately 40 K when the austenitization temperature is raised from 1073 to 1473 K.[10,11] However, the use of this fact to support the significance of the grain-size effect is arguable, because it is difficult to disentangle the effect of changing austenite grain size from those of a changing defect structure, the homogeneity of the solid solution, and segregation.[12] Investigation of the decomposition of small austenite particles shows that the kinetics of isothermally formed martensite JIAJUN WANG, Researcher and Metallurgist, formerly with the Netherlands Institute for Metals Research, Rotterdamseweg 137, 2628 AL Delft, is with Philips Lighting B.V., 6026RX Maarheeze, The Netherlands. SYBRAND VAN DER ZWAAG, Professor, is with the Department of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands. Manuscript submitted May 16, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
depends largely on the grain size when it is down to 20 m.[12,13] Fine-grained austenite generally has a relatively low Ms temperature or high stability during either thermal or mechanical processing.[14,15] Fisher et al.[16] modeled the influence of austenite grain size on the extent of martensite transformation and concluded that a decrease in grain diameter lowers the experimental Ms temperature. Obviously, the influence of grain size on the martensitic reaction in steel remains unclear. With respect to the steels demonstrating a transformation-induced plasticity (TRIP) effect,[6,15] the question seems more important, because the average dispersed austenite grain size in question is between 0.1 and 20 m[17] due to the partitioning of the matrix by large portions of ferrite and bainite transformation products. The dimension of the dispersed austenite trapped inside bainitic ferrite sheaves may well be smaller than 0.1 m. If the Fisher–Hollomon diminishing-size effect[16] is also
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