Nanograined/Ultrafine-Grained Structure and Tensile Deformation Behavior of Shear Phase Reversion-Induced 301 Austenitic
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INTRODUCTION
THE development of high strength steels characterized by high strength–high ductility combination is a subject of significant interest worldwide. The grain refinement approach continues to dominate in the attempt to achieve this target.[1,2] In this regard, thermomechanical controlled processing (TMCP) is one of the key methods to obtain grain refinement.[3–6] A typical example is that of bcc ferrous alloys, where TMCP and microalloying approaches were combined and a ferrite grain size of order of 5 lm was obtained.[3–6] However, recent studies have indicated that the limitation in grain refinement achievable by TMCP can be largely overcome by severe plastic deformation, and thereby submicron/ultrafine grain structures can be acquired.[7,8] The laboratory scale methods attempted to obtain nanograined/ultrafine-grained (NG/UFG) R.D.K. MISRA, Distinguished University Professor, Director, Center for Structural and Functional Materials, and Endowed Chair in Metallurgy, is with the Chemical Engineering Department, University of Louisiana at Lafayette, Lafayette, LA 70504-1430. Contact e-mail: [email protected] S. NAYAK, Postdoctoral Fellow, and P.K.C. VENKATASURYA and V. RAMUNI, Graduate Students, are with the Center for Structural and Functional Materials, University of Louisiana at Lafayette, Lafayette, LA 70504-1430. M.C. SOMANI, Senior Researcher, and L.P. KARJALAINEN, Professor, are with the Department of Mechanical Engineering, The University of Oulu, 90014 Oulu, Finland. Manuscript submitted October 6, 2009. Article published online May 7, 2010 2162—VOLUME 41A, AUGUST 2010
structure are equal channel angular pressing,[8,9] accumulative roll bonding,[10–12] high pressure torsion,[13–16] multiple compression,[17] and upsetting extrusion.[18] One drawback of these methods, however, is that they involve multiple stages of deformation to generate large plastic strain and the ductility is quite low in relation to the conventional coarse-grained metals. One of the fundamental causes for low ductility in NG/UFG metals is instability.[1,2] In materials with grain size exceeding approximately 1 lm, the unit dislocations control plasticity, while in smaller grain sizes, grain boundary shear is considered to be the operating deformation mechanism[1,2] that encourages the activation of shear bands or localized deformation during the early stages of straining.[19–23] It is also proposed that extremely small grains are unable to house high density of dislocations,[1] which leads to significant reduction in the strain hardening ability of UFG metals. These nonequilibrium boundaries produce long-range stress fields, and the strain localization (shear band/slip lines) is not limited to a single grain but extends to grain boundaries with high atomic mobility.[24] Based on the preceding viewpoint, uniform strain during deformation of UFG materials can be obtained by controlling instability and delaying the necking process.[1,2,24,25] One plausible approach in the attempt to control instability is to obtain UFG structure with
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