TEM investigations of the structural evolution in a pearlitic steel deformed by high-pressure torsion
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RODUCTION
COLD deformation of pearlitic steels occurs in many technical applications, for instance during wire drawing or at the surface of rails during service.[1,2] Especially in the latter example, it is important to relate the deformation and the resulting microstructure to changes of mechanical properties and damage processes. A key feature is therefore the behavior of the cementite lamellae (Fe3C) under shear deformation. In most cases, methods of severe plastic deformation are used as a new route to produce ultrafine grained materials with improved mechanical properties.[3,4,5] In the current study, high-pressure torsion (HPT) is used to obtain clearly defined deformed specimens of a low alloyed steel with a initially pearlitic microstructure. The microstructural evolution was analyzed by different transmission electron microscopy (TEM) techniques. The aim of the study is to characterize the microstructure, especially of the cementite lamellae, resulting from the severe plastic shear deformation. II.
EXPERIMENTAL DETAILS AND MATERIALS
A pearlitic rail steel 260 (UIC 900A) was deformed by HPT to obtain severely deformed material. The chemical composition and the mechanical properties of the material are given in Table I. Details of the HPT technique are given elsewhere.[6,7] The samples for HPT had a diameter of 8 mm and a thickness t of 0.8 mm (0.7 mm after the deformation). These discs where deformed at room temperature under a hydrostatic pressure of 5.7 GPa. The number of turns n was calculated according to Eq. [1] to reach equivF. WETSCHER and R. PIPPAN are with the Erich Schmid Institute for Materials Science, Austrian Academy of Sciences, Leoben, Austria, and the CD-Laboratory for Local Analysis of Deformation and Fracture, Leoben, Austria. Contact e-mail: [email protected] S. STURM is with the Max Planck Institute for Materials Research, Stuttgart, Germany. F. KAUFFMANN is with Materialpru¨fungsanstalt, University of Stuttgart, Germany. C. SCHEU is with the Department of Physical Metallurgy and Materials Testing, University of Leoben, Austria. G. DEHM is with the Department of Materials Physics, University of Leoben, Austria. Manuscript submitted November 17, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
alent von Mises strains eeq of 2 and 8, calculated for a radius r of 3 mm. eeq ¼
2"p"n"r pffiffiffi t" 3
[1]
TEM specimens from the deformed samples as well as from undeformed material were prepared by cutting severalmillimeter-small sections of the deformed material parallel to the torsion axis at a radius of 3 mm. The material was mechanically polished to a final thickness of ;50 mm and further reduced in size to keep its magnetic volume as small as possible (Figure 1). Finally, several electron transparent regions of ;10 3 10 mm2 were made using a Zeiss XP 1540 focused ion beam microscope. The TEM samples were then glued onto a copper grid and subsequently analyzed with a JEOL 2000 FX equipped with a Gatan imaging filter and a VG HB 501 UX scanning TEM. The VG HB 501UX has a cold-field-emission
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