Probing the Characteristic Deformation Behaviors of Transformation-Induced Plasticity Steels
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WITH increasing demands for materials with good comprehensive mechanical properties, transformationinduced plasticity (TRIP) steels have attracted much attention in recent years.[1–5] The TRIP steels are expected to have an excellent combination of high strength and tensile ductility due to their improved work-hardening ability.[6] It is well known that workhardening ability in a single-phase alloy is often dictated by dislocation pinning/tangling and multiplication.[7] However, as the dislocation density accumulates to a certain degree, dynamic recovery occurs, and then further hardening becomes difficult. The TRIP steels use a totally different hardening mechanism as opposed to dislocation multiplication. The TRIP steels normally contain a metastable phase, e.g., retained austenite (RA), which transforms into martensite under applied load. As the strength of martensite is much higher than its austenite parent, the strain-induced martensitic transformation work hardens the material, thus ensuring a high ductility.[8,9] In addition, as the mechanicallySHENG CHENG, Postdoctoral Research Associate, HAHN CHOO, Associate Professor, and PETER K. LIAW, Professor, are with the Department of Materials Science and Engineering, the University of Tennessee, Knoxville, TN, 37996. Contact e-mail: [email protected] XUN-LI WANG, Instrument Scientist, Neutron Scattering Sciences Division, and ZHILI FENG, Staff Scientist, Materials Science and Technology Division, are with the Oak Ridge National Laboratory. BJORN CLAUSEN, Instrument Scientist, is with the Los Alamos Neutron Science Center, Los Alamos National Laboratory, Los Alamos, NM, 87545. This article is based on a presentation given in the symposium entitled ‘‘Neutron and X-Ray Studies for Probing Materials Behavior,’’ which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee. Article published online August 5, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
induced martensite introduces a volume expansion during transformation, this geometrical change has an effect of strain hardening the surrounding ferrite matrix at the microscopic level. Consequently, the failure due to necking is shifted to a higher strain. However, in addition to RA, multiple phases are usually present in TRIP steels due to the thermomechanical processing, which renders the deformation of TRIP steels very complicated. So far great effort was devoted to optimizing the thermomechanical processing parameters for improved mechanical properties,[10–14] but the influence of strain-induced martensitic transformation in TRIP steels on the work-hardening behavior was not well understood. In particular, the micromechanical character of individual phase on the work-hardening process in TRIP steels was far from being well explored due to the complexity from the multiple phases. As the strength of the co-existing phases is
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