The cyclic response of dilute iron alloys
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I.
INTRODUCTION
THE relationship
between the cyclic behavior of single crystals and that of polycrystals, expressed generally through the cyclic stress-strain (CSS) curve, is of interest because the relative simplicity of the mechanisms that operate in single crystals helps in understanding the more complex behavior of polycrystals. 1-10It has been found, for example, that the fatigue limit of single crystals (the threshold for the formation of persistent slip bands (PSBs)) coincides, to some extent, with that of polycrystals. ~l Furthermore, Laird and co-workers 5'6'7 have demonstrated that, for certain testing conditions, the CSS curves for polycrystals of fcc metals show a plateau region similar to, but not as marked as, that found for single crystals. These plateaus, however, are strongly dependent on the material characteristics, testing mode, and many other factors. 5 Most of the studies reported to date on the connection between mono- and polycrystalline cyclic deformation have been concerned with fcc metals and alloys, and studies of cyclic response in bcc metals are, in general, more rare. 12' 13' 14The cyclic response ofbcc metal is more complex than that of fcc metal. At low temperatures and high strain rates, the cyclic deformation of bcc metal is dominated by the difference in the mobilities of edge and screw dislocations. 12'13However, at low strain rates and in the presence of interstitial impurities, bcc metal can behave in a fashion rather similar to that of fcc metal, including the formation of PSBs. 12,13In this context, it was extremely interesting that Kim 15 reported a plateau in the CSS curve of a niobium bearing, polycrystalline HSLA steel, in spite of the fact that the curve was obtained in a multiple step test at high strain rate and under total strain control. Furthermore, it was obvious that this plateau was suppressed when the curve was measured by the engineering test known as "the incremental test", which contains envelopes of strain decrements. Plateaus in fcc metals are also suppressed by decremental tests involving small differences between steps] Kim and Fine later published a paper 16in which they G. GONZALEZ, formerly with the University of Pennsylvania, Philadelphia, PA, is now Assistant Professor, Departamento de Mecanica, Universidad Simon Bolivar, Apartado Postal No. 80659, Caracas, Venezuela. C. LAIRD is Professor, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. Manuscript submitted February 28, 1983. METALLURGICALTRANSACTIONS A
interpreted the plateau of their HSLA steel in terms of localized slip ("prominent slip band formation"). PSB behavior was judged from observations of the surface by SEM, and no observations of the dislocation structures were reported. In the present paper, we report the cyclic behavior of a Nb-bearing HSLA steel subjected to different loading programs in plastic strain-controlled tests at low strain rates.
II.
EXPERIMENTAL DETAILS
The HSLA steel used in this research was found to contain t
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