Nanoscale Deformation Behavior of Phase-Reversion Induced Austenitic Stainless Steels: The Interplay Between Grain Size
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THERE is a continued interest to fundamentally understand the deformation behavior of nanograined/ ultrafine-grained (NG/UFG) materials, where the basic mechanisms are expected to be fundamentally different from those operating in the coarse-grained (CG) materials.[1–3] This is especially crucial for developing materials with high strength-high ductility combination. The mechanical behavior of NG metals has led to the proposition that the dominant mode of plastic deformation of ductile CG materials governed by the grouped activity of dislocations inside grains (viz., dislocation glide leading to the formation of dislocation pile-ups or cells) is suppressed in NG materials, where partial dislocation emission from grain boundaries may operate instead.[2,4] Thus, with decrease in grain size and consequent increase in yield strength, we anticipate a gradual transition in the mechanism of deformation from those operating in the CG domain to those in NG materials. In recent years, we have adopted a novel processing route of producing NG/UFG structure in metastable austenitic stainless steels involving cold rolling and R.D.K. MISRA, Distinguished Professor and Director, and P.K.C. VENKATSURYA, Graduate Student, are with the Center for Structural and Functional Materials, University of Louisiana at Lafayette, P.O. Box 44130, Lafayette, LA 70504. Contact e-mail: [email protected] M.C. SOMANI, Senior Researcher, and L.P. KARJALAINEN, Professor, are with the Department of Mechanical Engineering, The University of Oulu, P.O. Box 4200, 90014 Oulu, Finland. Manuscript submitted March 27, 2012. Article published online August 9, 2012 5286—VOLUME 43A, DECEMBER 2012
controlled phase reversion annealing.[5–8] In this approach, in certain metastable austenitic stainless steel grades, heavy cold rolling of austenite at room temperature leads to the formation of predominantly dislocation-cell type martensite.[6] Upon annealing, the deformed martensite reverts to austenite with NG/ UFG structure through a diffusional reversion mechanism depending on the chemistry of the steel.[5,8] The uniqueness of this concept is that it enables us to obtain a spectrum of grain size from NG to CG regime through a single set of parameters (percentage of cold deformation and annealing temperature–time sequence). High temperature annealing promotes grain growth with grain size in the micrometer range. Thus, the approach facilitates large sampling of grain size effect to elucidate deformation mechanisms as a function of grain size in a single material using identical set of parameters. NG/ UFG stainless steel of Type 301LN was characterized by high yield strength and elongation, typically, 600 MPa to 800 MPa and 40 to 30 pct, respectively,[5–8] which exceeds the yield strength of 350 MPa but has comparable elongation of ~40 pct of the annealed CG steel. In austenitic stainless steels, the mechanical stability of the austenite phase and strain-induced martensite transformation governs ductility. The underlying reason is that gradual transformation
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