The Study of Fatigue Behaviors and Dislocation Structures in Interstitial-Free Steel
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I.
INTRODUCTION
IT is generally accepted that microstructural change is accompanied by cyclic hardening and that wellannealed pure face-centered-cubic metals subjected to cyclic deformation at constant plastic strain amplitudes exhibit an initial rapid hardening stage, followed by a regime of cyclic saturation and, finally, suffer from fatigue failure.[1–8] By comparison, fatigued body-centered-cubic (bcc) metals have received less attention concerning the fundamental fatigue mechanisms. With regard to the cyclic hardening behavior for fatigued bcc metals, much of the literature has either merely paid attention to the early and saturated stage of cyclic deformation[9–15] or outright ignored the secondary cyclic hardening,[5,16] except for the studies of AbdelRaouf et al.,[17] Chopra et al.,[18] Ikeda,[19] and Shih et al.[20] It is well known that the dislocation-cell structure rapidly develops at higher strain amplitudes for cyclically deformed bcc metals.[15,17–19] However, for the lower strain amplitudes, only gliding behaviors of dislocations during early or saturated stage of cyclic deformation have been studied.[13–15,21] Hence, the dislocation development and dislocation arrangement during further cycling have not been analyzed and investigated in detail for pure bcc metals. In addition, Mughrabi and co-workers[14,21] indicated that once strain amplitudes are lower than the fatigue limit, the development of dislocation structures may be restricted. Nevertheless, they have not observed and analyzed CHIA-CHANG SHIH, Ph.D. Student, and NEW-JIN HO, Professor, are with the Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, Taiwan, Republic of China. HSING-LU HUANG, Professor, is with the Department of Mechanical Engineering, Chinese Military Academy, Kaohsiung, Taiwan, Republic of China. Contact e-mail: t0130553@ seed.net.tw Manuscript submitted May 1, 2009. Article published online May 20, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
the dislocation structures near fatigue limit in detail. Therefore, this study will attempt to understand what role the dislocation structures play in secondary cyclic hardening behavior and then to analyze the relationship between secondary cyclic hardening and fatigue limit.
II.
EXPERIMENTAL
A. Materials A hot-rolled polycrystalline IF steel plate with a chemical composition of C < 50 ppm, N < 50 ppm, S < 120 ppm, B 2 ppm, Mn 0.15 wt pct, Ti 0.04 wt pct, and balance Fe was used in this study. The material was annealed at 800 C for 2 hours and then cooled in a furnace to obtain an average grain size of about 80 lm in diameter. The preparation of specimens followed the ASTM E606 specification. B. Testing Conditions A computerized INSTRON* 8801 hydraulic testing *INSTRON is a trademark of Instron, Canton, MA.
machine was employed at a testing strain rate of 4 9 10 3 s 1 with R (strain ratio) = 0 at room temperature. The fatigue tests were performed with De/2 ranging from 0.05 to 0.6 pct. Fatigue test data are summarized in Table
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