Defect Recovery in Severely Deformed Ferrite Lamellae During Annealing and Its Impact on the Softening of Cold-Drawn Pea
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INTRODUCTION
COLD-DRAWN pearlitic steel wires belong to the most successful engineering materials. Due to their excellent mechanical properties, i.e., ultra-high strength and tolerable ductility, these wires are used in a variety of engineering fields, such as suspension bridge cords, tire cords, springs, etc.[1] Because of their wide applications in engineering fields, the cold-drawn pearlitic steel wires have induced a number of investigations analyzing many aspects, such as strengthening mechanism,[2–5] microstructure evolution upon cold drawing,[2,3,6–9] Y.Z. CHEN, Professor, is with the State Key Lab. 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: yzchen@ nwpu.edu.cn G. CSISZA´R, Post-doc, is with the Department of Materials Physics, Eo¨tvo¨s University, Budapest, 1518, Hungary, and also with the Max Planck Institute for Intelligent Systems (formerly: Max Planck Institute for Metals Research), Heisenbergstraße 3, 70569, Stuttgart, Germany. J. CIZEK, Associate Professor, is with the Department of Low-Temperature Physics, Charles University in Prague, V Holesovickach 2, 18000, Praha 8, Czech Republic. X.H. SHI, Graduate Student, and F. LIU, Professor, are with the State Key Lab. of Solidification Processing, Northwestern Polytechnical University. C. BORCHERS and Y. J. LI, Research Scientists, are with the Institut fu¨r Materialphysik, Universita¨t Go¨ttingen. R. KIRCHHEIM, Professor, is with the Institut fu¨r Materialphysik, Universita¨t Go¨ttingen, and also with the International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka, Japan. Manuscript submitted July 2, 2015. Article published online December 14, 2015 726—VOLUME 47A, FEBRUARY 2016
decomposition of cementite lamellae,[8–15] and strain aging.[16–18] The outstanding mechanical properties of the cold-drawn pearlitic steel wires are believed to originate from their unique microstructure formed upon heavy cold deformation.[2–5,7,8,10,19] It was reported recently that when the total wire drawing strain reaches 6.52, an ultra-high strength up to 7 GPa can be achieved in a hypereutectoid pearlitic steel.[20] So far, it has been known that besides the nanoscale lamellar spacing,[2,3,8,19] the high density of lattice defects formed in ferrite during cold wire drawing, such as vacancy-like defects and dislocations, also plays an important role in the strengthening of the wires,[4,5,21] for example, leads to a deviation of the strength from a standard Hall–Petch relationship.[5] Commercial pearlitic wires are often subjected to heat treatments at temperatures between 573 K and 723 K (300 C and 450 C) for different practical purposes, e.g., bluing and galvanizing.[18,22] Since the deformation-induced lattice defects which contribute significantly to the strength of these wires are highly susceptible to recovery at elevated temperatures, the knowledge of the evolution
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