Mechanical properties of iron processed by severe plastic deformation
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INTRODUCTION
THE mechanical behavior of bulk nanostructured Fe with grain sizes of ⱕ100 nm, processed via consolidation of milled Fe powders[1–6] or by severe plastic deformation,[7] has been the topic of recent work. Inspection of the literature, however, reveals that most of these studies have been limited to microhardness testing,[1] miniaturized-disk bend tests,[2,3] compression,[8] or tension with a microsample,[5] primarily as a result of the limited availability of bulk nanostructured Fe. Moreover, in many cases, tensile deformation of the bulk nanostructured Fe led to failure in the elastic regime, without any macroscopic ductility, presumably due to the existence of processing defects.[5,8] Therefore, no tensile properties from convincing standard or near-standard tensile tests are available to clarify the deformation mechanisms of nanostructured Fe for grain sizes around 100 nm. In related studies, Takaki et al.[4] and Sakai et al.[6] used near-standard tensile samples of ultrafine-grained (UFG) Fe, processed via consolidation of milled Fe powders, and reported a high tensile strength of 1.8 GPa in Fe and 1.768 GPa in Fe-1.5 pct O, a low ductility of 3.5 pct in Fe and 0.2 pct in Fe-1.5 pct O, and work softening. These studies involved consolidated pure Fe with grain sizes of about 180 nm[4] and a consolidated UFG Fe–0.2 to 1.5 pct O alloy with grain sizes from 600 to 200 nm.[6] Park et al. processed carbon steel (Fe-0.15 pct C-0.25 pct Si-1.1 pct Mn) via a technique of equal-channel angular pressing BING Q. HAN, Senior Research Associate, and FARGHALLI A. MOHAMED, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. Contact e-mail: [email protected] ENRIQUE J. LAVERNIA, Professor, is with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5294. Manuscript submitted February 25, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
(ECAP) for four passes at 623 K.[9] A tensile strength of 940 MPa was obtained from near-standard tensile testing on specimens with equiaxed ultrafine grains of 200 nm. Similarly, an interstitial-free steel (Fe-0.15 pct Mn-0.049 pct Ti) was processed by an accumulative rolling bond,[10] and a tensile strength of 870 MPa was achieved from nearstandard tensile testing on specimens with equiaxed ultrafine grains of 420 nm. Despite these recent studies and the implications of their results, the deformation behavior of UFG steels is not fully understood, partly due to complexities associated with different processing approaches (e.g., milling vs ECAP) and partly due to chemical effects: many of the systems studied contain multiple elements. The preceding information indicates that in order to provide insight into the deformation behavior of Fe in the UFG range, it is necessary that (1) microstructural complications arising from major alloying elements or processing flaws be minimized or eliminated, (2) Fe be tested in the bulk form using standard procedures, (
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