Defects in Carbon-Rich Ferrite of Cold-Drawn Pearlitic Steel Wires
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INTRODUCTION
COLD-DRAWN pearlitic steel wires, with extremely high strength and tolerant ductility, have been used widely in engineering applications, such as tire cords, springs, suspension bridges, etc., and studied extensively for many years.[1–15] These wires are produced by colddrawn patented pearlitic steel wires, which are composed of ferrite and cementite lamellae, to a defined strain. Upon cold-drawing, the cementite lamellae are partially decomposed,[5–13] while numerous lattice defects, such as dislocations and vacancies or vacancy clusters, are introduced into the ferrite lamellae.[14] Since both the dislocations and the vacancies or vacancy clusters cause hardening,[16] these defects present in high densities may play a crucial role in strengthening the wires. Thus, a systematic investigation of the evolution of defect densities in ferrite lamellae with changing true strain (e) will be helpful in understanding the strengthening mechanisms of the wires. After deformation, a Y.Z. CHEN, Associate Professor, is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, 710072 Xi’an, P.R. China and also with the Institut fu¨r Materialphysik, Universita¨t Go¨ttingen, 37077 Go¨ttingen, Germany. Contact e-mail: [email protected] G. CSISZA´R, Post-Doc, and T. UNGA´R, Full Professor, are with the Department of Materials Physics, Eo¨tvo¨s University, Budapest 1518 POB, Hungary. J. CIZEK, Associate Professor, is with the Department of Low-Temperature Physics, Charles University in Prague, V Holesovickach 2 18000 Praha 8, Czech Republic. S. WESTERKAMP, Graduate Student, and C. BORCHERS, Research Scientist, are with the Institut fu¨r Materialphysik, Universita¨t Go¨ttingen. S. GOTO, Full Professor, is with the Department of Materials Science and Engineering, Akita University, Akita, 010-8502, Japan. F. LIU, Full Professor, is with the State Key Laboratory of Solidification Processing, Northwestern Polytechnical University. R. KIRCHHEIM, Full Professor, is with the Institut fo¨r Materialphysik, Universita¨t Go¨ttingen, and also with the International Institute for Carbon–Neutral Energy Research (WPI-I2CNER), Kyuchu University, Fukuoka, Japan. Manuscript submitted June 15, 2011. Article published online April 10, 2013 3882—VOLUME 44A, AUGUST 2013
large number of carbon atoms stemming from the decomposed cementite are released into ferrite.[6–13] Since the equilibrium solubility of carbon in ferrite at room temperature is merely several atomic ppm, it has been suggested that the carbon in ferrite is probably associated with the defects in ferrite.[8,13] However, the association of carbon with defects in these wires still lacks direct experimental evidence. Although atom probe tomography is capable of mapping carbon atmospheres around dislocations,[17,18] the extremely high dislocation density and the extremely fine width of ferrite lamellae prevent a precise mapping of carbon atmospheres at dislocations in this special case.[13] An investigation of the accommodation of carbon in f
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