Crystallographic Analysis of Proeutectoid Ferrite/Austenite Interface and Interphase Precipitation of Vanadium Carbide i
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INTRODUCTION
DURING proeutectoid ferrite (a) and pearlite transformation in steels that contain strong carbide-forming elements such as Nb, Ti, V, Cr, and Mo, fine alloy carbides nucleate on migrating a/austenite (c) boundaries repeatedly, leading to characteristic rows of carbides or carbides on sheets parallel to the a/c interface. This is called interphase precipitation.[1] For weight saving of automobiles, demands for increasing strength of steel parts are increasing. Nanosized carbide dispersion via interphase precipitation has been applied to increase strength effectively. Funakawa et al.[2] have recently developed a high-strength, low-carbon steel with good elongation by using nanosized interphase precipitation of Ti and Mo carbides. It was also reported that vanadium addition to medium-carbon steels for forging parts can strengthen proeutectoid a and pearlite by interphase precipitation of vanadium carbide (VC).[3]
GORO MIYAMOTO and TADASHI FURUHARA, are with the Institute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japan, and with the Consortium of the Japan Research and Development Center for Metals for Fundamental Studies on Technologies for Steel Materials with Enhanced Strength and Functions, Tokyo, Japan. Contact e-mail: [email protected] RYOTA HORI, formerly Graduate Student, Tohoku University, is now with Toyota Motor Corporation, Toyota, Aichi 471-8571, Japan. BEHRANG POORGANJI is with Robert R. McCormick School of Engineering & Applied Science, Northwestern University, Evanston, IL 60208-3100, and with the Consortium of the Japan Research and Development Center for Metals for Fundamental Studies on Technologies for Steel Materials with Enhanced Strength and Functions. Manuscript submitted October 8, 2012. Article published online March 22, 2013 3436—VOLUME 44A, AUGUST 2013
Several mechanisms of interphase precipitation have been proposed so far.[1,4,5] Initially, Davenport and Honeycombe[4] supposed that carbon is rejected from proeutectoid a to c, and the resultant enrichment in c adjacent to a causes nucleation of carbide at the a/c interface. However, this model cannot explain why carbides are precipitated on sheets parallel to the interface because nucleation of carbide on immobile interface is necessary to form carbides on sheets. Later, Honeycombe[1] proposed ledge mechanism (Figure 1(a)). He proposed that the a/c interface consists of immobile coherent plane (terrace) and mobile incoherent plane (ledge or riser), and then carbides nucleate at immobile terrace planes. In this ledge mechanism, it was assumed that a holds Kurdjumov-Sachs (K-S) orientation relationship (OR) such as ((111)c // (011)a, ½101c // ½111a ) and the coherent terrace plane corresponds to close-packed planes in parallel relation, (111)c // (011)a. This model can explain straight sheet planes because sheet planes of carbide are fixed to be (011)a. On the other hand, it was frequently observed that sheets of interphase precipitation are curved as schematically shown in Figure 1(b). The
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