Plastic Localization Phenomena in a Mn-Alloyed Austenitic Steel
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TRODUCTION
HIGH manganese austenitic alloys have a remarkable combination of strength and ductility, which makes them interesting for the automotive industry. Steels used for automotive components, such as anti-intrusion bars and structural parts, must display high tensile strength, biaxial ductility, and energy absorption capability for improved fuel saving, crash-test performance, and capability to deep draw complex shapes. Highmanganese steels exhibit a better combination of high strength and ductility than most present automotive steels, such as ferritic deep-drawing steels, high-strength low-alloy steels, or multiphase steels. This is due to the mode of deformation: in addition to the dislocation slip mechanism, high-manganese steels also deform by mechanical twinning (this is often called the twinninginduced plasticity, or TWIP, effect), and the twin boundaries behave as obstacles to dislocation movement, similarly to grain boundaries or dislocation forests. Works have already been performed in alloy development, microstructure analysis, and mechanical properties of TWIP steels;[1–9] moreover, some recent articles reported the occurrence of plastic instabilities during G. SCAVINO, Associate Professor, P. MATTEIS, Assistant Professor, and D. FIRRAO, Full Professor, Department of Materials Science and Chemical Engineering (DISMIC), and P. RUSSO SPENA, Research Assistant, Department of Production Systems and Business Economics (DISPEA), are with the Politecnico di Torino (Torino Technical University), Torino, It-10129, Italy. Contact e-mail: donato.fi[email protected] F. D’AIUTO, Specialist, is with Engineering & Design, Fiat Group Automobiles, Torino, It-10135, Italy. Manuscript submitted January 22, 2009. Article published online March 18, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
uniaxial deformation tests;[10–12] the latter phenomenon was attributed either to plain dynamic strain aging, due to interactions between mobile dislocations and pointdefect complexes including interstitial C atoms,[10] or to not yet clarified, more complex atomic interactions, also involving twinning phenomena.[11] In the present article, the mechanism of plastic deformation of a batch produced 1.5-mm-thick TWIP sheet steel (with 0.5 pct C) has been studied in-depth, with the purpose of describing and explaining the occurrence of plastic instabilities, with both uniaxial and biaxial deformation experiments. To investigate plastic instabilities, several tensile tests were performed at low to medium crosshead speeds, thus varying the applied strain rates, accordingly. Moreover, standard and reduced-width Erichsen tests were done to investigate the influence of biaxial stresses on plastic instabilities.
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EXPERIMENTAL PROCEDURES
The chemical composition of the TWIP steel is reported in Table I. Manganese is the main alloy element and stabilizes the austenitic matrix at room temperature; carbon is employed to increment the yield and tensile strength by interstitial solute hardening. The microstructural characterization was carried o
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