Load Partitioning and Strain-Induced Martensite Formation during Tensile Loading of a Metastable Austenitic Stainless St
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METASTABLE austenitic stainless steels are used as engineering materials due to their combination of good formability, as-formed strength, and corrosion resistance. Their good mechanical properties are, to a large extent, due to the three-phase composite structure evolved during deformation, in which the original ductile austenite partial phase transforms to the stronger a¢ martensite (bcc or bct) and e martensite (hcp). This martensitic transformation is strain induced at ambient temperature, which means that the martensite is formed at nucleation sites generated by the plastic deformation, e.g., shear-band intersections.[1] The strain-induced martensite strengthens the original austenite and can improve the ductility via the transformation-induced plasticity (TRIP) effect.[2] Bressanelli et al.[3] found both the onset and the rate of martensite formation to be vital P. HEDSTRO¨M, formerly Graduate Student, Division of Engineering Materials, Lules University of Technology, SE-971 87, Lules, Sweden, is Researcher, Department of Materials Science and Engineering, Royal Institute of Technology (KTH), SE-100 44, Stockholm, Sweden. Contact e-mail: [email protected] L.E. LINDGREN, Professor, is with the Division of Material Mechanics, Lules University of Technology. J. ALMER and U. LIENERT, Physicists, are with the Advanced Photon Source at Argonne National Laboratory, Argonne, IL 60439. J. BERNIER, Researcher, is with the Lawrence Livermore Laboratory, Livermore, CA 94551. M. TERNER, formerly Postdoctoral Researcher, Division of Engineering Materials, Lules University of Technology, is Researcher, Rolls Royce Fuel Cell Systems, Loughborough LE113GR, United Kingdom. M. ODE´N, Professor, Nanostructured Materials, is with the Department of Physics, Chemistry, and Biology, Linko¨ping University, SE-581 83 Linko¨ping, Sweden. Manuscript submitted September 26, 2007. Article published online March 25, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
for obtaining good ductility. Furthermore, Hedstro¨m et al. recently suggested that autocatalytic martensitic transformation improves ductility.[4] These characteristics of the strain-induced martensitic transformation can to some extent be tailored by the stability of the austenite, which is controlled by the alloy composition and microstructure.[5] However, there are additional parameters, such as temperature,[6] stress state,[7] and strain rate,[8] that affect the strain-induced martensitic transformation. Increasing the strain rate will alter the strain-induced martensitic transformation behavior, due to the exothermic martensite transformation and the heat generated from the plastic deformation. In addition, it has been proposed that an increase in the strain rate increases the number of shear bands that have been proposed as favorable nucleation sites for the martensite and, thus, enhances the degree of martensitic transformation.[1] Because the strain-induced martensitic transformation contributes to the considerable complexity in the deformation behavior of metastable austenit
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