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NTRODUCTION

TRANSFORMATION-INDUCED plasticity (TRIP) steels are composite materials with ferritic matrix containing bainite and meta-stable retained austenite. Many previous studies have demonstrated that TRIP steels possess favorable mechanical properties such as high strength and good ductility. This excellent strength-ductility combination is the result of the multiphase nature as well as the phase transformation from the metastable austenite into martensite during the deformation process. Over the past 30 years, various approaches to simulate the TRIP effect have been performed and reported in the literature.[1–5] All these approaches share the same goal, which is to predict and to control the material properties during the deformation process. The austenite-to-martensite phase transformation is the predominant phenomenon that explains the fact that TRIP steels represent a good A. SOULAMI and K.S. CHOI, Postdoctoral Research Associates, W.N. LIU and X. SUN, Scientists, and M.A. KHALEEL, Division Director and Laboratory Fellow, are with the Computational Science and Mathematics Division, Pacific Northwest National Laboratory, Richland, WA 99352. Contact e-mail: [email protected] Y. REN, Scientist, is with the X-ray Science Division, Argonne National Laboratory, Argonne, IL 60439. Y.D. WANG, Professor, is with the Department of Materials Science, Northeastern University, Shenyang 110004, P.R. China. This article is based on a presentation given in the symposium ‘‘Neutron and X-Ray Studies of Advanced Materials,’’ which occurred February 15–19, 2009, during the TMS Annual Meeting in San Francisco, CA, under the auspices of TMS, TMS Structural Materials Division, TMS/ASM Mechanical Behavior of Materials Committee, TMS: Advanced Characterization, Testing, and Simulation Committee, and TMS: Titanium Committee. Article published online March 4, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A

compromise between ductility and toughness. This typical property of TRIP steels results from the strong coupling between plasticity by dislocation motion and martensitic phase transformation through the internal stresses generated by the inelastic processes. Since the macroscopic behavior of TRIP is strongly dependent on the microstructure (grain size, volume fractions of the different phases, geometrical distribution, etc.), modeling of TRIP steels needs to be accomplished on the microstructure level. From this point of view, micromechanics-based models have been used to understand the local deformation mechanics and microscopic mechanisms governing the macroscopic behaviors of TRIP steels[5–10] Recently, Choi et al.[8,11] developed a microstructure-based finite element modeling procedure for TRIP steels in which the actual microstructures are used in predicting the complex deformation behavior of TRIP steels up to the ductile failure point. In these studies, ductile failure is predicted as the result of plastic instability in the form of strain localization. Analyzing the crack propagation behavior in TRIP steels is also of great int