Atom probe and transmission electron microscopy investigations of heavily drawn pearlitic steel wire
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INTRODUCTION
HEAVILY drawn pearlitic wire has been investigated extensively over the years for its unusual strain-hardening behavior and for the high strengths that can be attained, with a toughness sufficient for many engineering applications.[1] Widely used for tire cord, springs, wire rope, and suspension bridge cable, it is typically produced by rolling or drawing wire of an approximately eutectoid composition to an intermediate diameter, patenting it to produce a fine pearlitic microstructure, and then cold drawing it to strains between 1.5 and 5.0. Previous microstructural investigations have established a number of salient characteristics associated with the wire drawing process.[2,3,4] The strength of the wire increases exponentially with the drawing strain, and, despite the limited ductility of monolithic cementite crystals,[5–8] the cementite lamellae in pearlitic wire co-deform with ferrite. The ferritic component develops a strong ^110& wire texture,[9] and the cementite lamellae appear to fragment into planar arrays of small particles.[3,10,11] No preferred texture has been identified for the cementite phase. The pearlite interlamellar spacing decreases in proportion to the wire diameter, and the range of spacings broadens markedly with increasing strain.[3] In addition to these microstructural traits, drawn pearlitic wire has several interesting chemical characteristics. Atom probe field ion microscopy (APFIM)[12–16] studies indicate that silicon in undeformed (as-patented) wire is piled up at ferrite/cementite boundaries, while manganese shows no significant tendency to segregate or partition to either ferrite or cementite. Internal friction[17] and Mo¨ssbauer[18] experiments suggest that a substantial proportion of the cementite M.H. HONG, JST Fellow, and K. HONO, Head of 3rd Laboratory, are with the Materials Physics Division, National Research Institute for Metals, Tsukuba 305-0047, Japan. W.T. REYNOLDS, Jr., Associate Professor, is with the Materials Science and Engineering Department, Virginia Tech., Blacksburg, VA 24061-0237. T. TARUI, Senior Researcher, is with the Steel Research Laboratories, Nippon Steel Corporation, Futtsu 293-0011, Japan. Manuscript submitted March 4, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
(from 20 to 50 vol pct) dissolves during deformation at room temperature. This phenomenon is intriguing, since cementite is stable at room temperature and the solubility of carbon in ferrite is quite low. Direct support for cementite dissolution is provided by the aforementioned APFIM investigations,[12–16] as well as by several analytical transmission electron microscopy (TEM)[11,19] studies show that deformed ferrite lamellae contain significantly more carbon than ferrite in undeformed pearlite. Explanations for why cementite dissolves during cold drawing are based upon either interactions between carbon and dislocations or upon thermodynamic arguments. In the first category, Gridnev et al.[18] suggested that a larger binding energy between interstitials and dislocation
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