The effect of cyclic loading on the dislocation structure of fully pearlitic steel
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I.
INTRODUCTION
A recent study on the relationship between microstructure and tensile properties in a fully pearlitic steel reconfirmed the strong influence of interlamellar spacing on yielding and flow; specifically, a threefold decrease of interlamellar spacing produced a twofold increase of the yield strength. The role of spacing on strengthening was shown to be a result of the higher stress necessary to move a dislocation between two impenetrable cementite plates, with the effective slip distance being considerably more limited in the more refined pearlite, tlJ Parallel studies were also undertaken to assess the role of microstructure on the fatigue behavior of the same steel, t27 Under this mode of loading, the fatigue limit was influenced only slightly by interlamellar spacing, suggesting that this parameter is not yield strength controlled. This observation stimulated the present research whereby the fatigue behavior and associated crack initiation of pearlite has been directly correlated with dislocation configurations in cyclically deformed pearlite of different interlamellar spacing. A further goal was to rationalize observed and apparent differences in deformation modes during monotonic and cyclic deformation, in terms of the specifics of the dislocation activity of and at cementite/ferrite interfaces during deformation. II.
EXPERIMENTAL PROCEDURES
A fully pearlitic steel conforming to AISI 1080 steel was used. The steel was provided by the Association of American Railroads in the form of rail sections having a chemical composition reported to be (in wt pet): C-0.80, Mn-0.84, Si-0.17, S-0.013, P-0.018, and Fe-balance. Cylindrical hour-glass fatigue samples with a midsection diameter of 5 mm were cut from the rail heads, with their longitudinal axis in the rolling direction. After machining, samples were austenitized at 1073 K for 1 hour in an argon furnace and M. DOLLAR is Assistant Professor, Academy of Mining and Metallurgy, 30059 Krakow, Poland; I.M. BERNSTEIN is Professor and Provost, Illinois Institute of Technology, Chicago, 1L 60616; M. DAEUBLER is Scientist, The Technical University of Hamburg, West Germany; and A.W. THOMPSON is Professor and Chairman, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted July 29, 1987. METALLURGICAL TRANSACTIONS A
subsequently isothermally transformed to nominally 100 pct pearlite at either 823 K or 963 K for 1 hour. The samples were electrolytically polished and fatigue tested in a servohydraulic closed loop testing machine, using a sine wave cycle of 10 Hz frequency and a load ratio R = 0.1 = O'mln/O'max. All tests were done at room temperature in air. Discs for transmission electron microscopy (TEM) studies were prepared from undeformed and fatigued samples. These were electrolytically thinned, using a window technique, in a solution of 1 part of anhydrous sodium chromate and 4 parts of glacial acetic acid. Thin foils were then examined in a JEOL 120CX microscope. In addition
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