Effect of environment and grain size on cyclic deformation and surface hardening of iron
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INTRODUCTION
IT has been known since the pioneering work of Gough and Sopwith 1'2 that the fatigue resistance of metals increases as the ambient pressure decreases and that this greater fatigue resistance in vacuum is due to the absence of chemically active constituents. Premature fatigue failure of some materials was attributed by some investigators to oxide films or oxygen adsorption. 3-6 Recent studies by Majumdar and Chung 7'8 on fatigue deformation of polycrystalline iron revealed a significant difference in the post-fatigue surface morphology and crack initiation modes in ultrahigh vacuum (UHV) and oxygen. Since most engineering materials are subjected to atmospheric oxygen for their entire service life, the study of the effect of oxygen on the fatigue behavior is of great importance. One of the important rnetallurgical factors controlling mechanical properties of polycrystalline materials is the grain size. Grain size is known to affect the yield strength, fracture characteristics, creep rate, ductilebrittle transition, hardness, and fatigue strength. 9-~3 However, information on the effect of the grain size on fatigue crack initiation in bcc metals is s c a r c e . 14'15 Furthermore, most of the experimental studies published so far on the effect of grain size on the fatigue properties were carried out with little environmental control. Therefore, in the present work, the aim is to investigate the dependence of cyclic deformation behavior of iron on the grain size and on the presence of oxygen by examining the details of the surface phenomena associated with crack initiation. The influence of the surface layer on the deformation of crystalline solids has been extensively studied. ~6-3~ It is now agreed that, during plastic deformation, there can be a substantial difference in the distribution of dislocations and flow stress near the surface and in the bulk. However, there have been some controversies over the nature and role W. J. LEE, formerly Graduate Research Assistant at Northwestern University, is Assistant Professor, Korea Institute of Technology, Korea. Y. W. CHUNG, Professor, and M. E. FINE, Walter P. Murphy Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60201-9990. Manuscript submitted January 5, 1987. METALLURGICAL TRANSACTIONS A
of the surface even in unidirectional deformation: some investigators ~s'19'2~have reported that surface regions become preferentially harder than the bulk while others 21'22'23 have reported the opposite result. In cyclic loading where failure initiates at or near the surface, greater differences in the dislocation structure between the surface layer and the interior can be expected than in static loading. Examination of foils taken from different depths below the surface showed that cyclic loading produced a dislocation arrangement in the surface layer different from that in the bulk. 24'25 The change of microhardness due to cyclic deformation depends on its structural condition. For annealed copper 2
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