Orientation Dependence of the Deformation Microstructure in a Fe-30Ni-Nb Model Austenitic Steel Subjected to Hot Uniaxia
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IT is well known that the hot deformation microstructure evolution in industrial steels in the austenite state cannot be directly experimentally observed due to the phase transformation occurring on cooling to the room temperature. It has been surmised from the microstructural studies performed using Fe-30Ni and Ni-30Fe model alloys[1–5] that austenite hot deformation microstructure, relevant to the industrial steel finish rolling temperature range,[6,7] is expected to consist of ‘‘microbands’’ (MBs) for a majority of grain orientations. MBs are elongated substructure features bounded by pairs of parallel low-angle planar dislocation walls, often termed ‘‘geometrically necessary boundaries’’ (GNBs), typically characterized by opposite misorientation vectors.[1–3,5] It has also been suggested that the dislocation networks constituting MB walls might play an important role in providing the preferred nucleation sites for NbC precipitation during the industrial multi-pass rolling of Nb-containing DEBASIS PODDAR, formerly Ph.D. Student with the Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3216, Australia, is now Researcher with the Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208. PAVEL CIZEK and HOSSEIN BELADI, Senior Researchers, and PETER D. HODGSON, Professor, are with the Institute for Frontier Materials, Deakin University. Contact e-mail: [email protected] Manuscript submitted on July 3, 2015. Article published online October 1, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
microalloyed steels.[8–12] Thus, the detailed understanding of the MB evolution during hot deformation of austenite is critically important not only in relation to the softening processes and flow behavior, but also with respect to the strain-induced precipitation. The GNB/MB evolution during straining at ambient temperature has been extensively investigated, especially for aluminum alloys e.g.,[13–18] and the MB formation mechanisms involving dislocation multiple crossslip[19,20] or dislocation wall splitting[21–23] have been proposed. By contrast, detailed studies of the GNB/MB evolution during hot deformation of austenite are rather limited.[1–5] In particular, there is lack of information on such evolution for austenite deformation in uniaxial compression. There has been some controversy over whether the MB wall alignment is principally related to the macroscopically imposed deformation geometry[14,24] or, alternatively, to the underlying slip pattern.[15,25–28] It has recently been reported[5] that MBs in a Ni-30Fe austenitic model alloy subjected to hot plane strain compression tended to form preferentially on the highly stressed {111} slip planes containing slip systems having high Schmid factors. However, more experimental data for austenite subjected to different deformation modes are urgently needed to further clarify the above matter. There have been numerous reports[1–5,13–23,25,29] showing that MB walls tend to retain their crystallograp
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