Three-dimensional computational-cell modeling of the micromechanics of the martensitic transformation in transformation-
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INTRODUCTION
TRANSFORMATION-INDUCED-PLASTICITY (TRIP)–assisted multiphase steels show remarkable mechanical properties of formability and crashworthiness, resulting from the occurrence of concurrent modes of plastic deformation: essentially, dislocation strengthening and mechanically induced martensitic transformation. There is an urgent need, especially coming from the automotive industry, to develop constitutive models that properly account for the couplings between phase transformation, loading conditions, and evolution of the material properties, while remaining sufficiently simple to allow tractable implementation and use with finite-element codes. Such models are essential for both the optimization of forming operations and the control of the properties after forming and bake-hardening. Since the martensitic transformation is associated with considerable shape and volume changes, the local-stress state varies drastically during the transformation both inside the newly
TIM VAN ROMPAEY, formerly Doctoral Researcher, with the Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Leuven, Belgium, is Project Leader, Umicore Research, B-2250 Olen, Belgium. FREDERIC LANI, Doctoral Researcher, formerly with the Department of Materials Science and Processes, Université Catholique de Louvain, IMAP, B-1348 Louvain-la-Neuve, Belgium, is Group Leader, Centre of Excellence in Aeronautical Research, CENAERO, Gosselies, Belgium. BART BLANPAIN, Professor, and PATRICK WOLLANTS, Professor and Head of Department, are with the Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, PASCAL J. JACQUES, Qualified Researcher of FNRS, and THOMAS PARDOEN, Professor, are with the Department of Materials Science and Processes, Université Catholique de Louvain, IMAP, Place Sainte Barbe 2, B-1348 Louvain-la-Neuve, Belgium. Contact e-mail: [email protected] Manuscript submitted February 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
formed martensitic inclusion and in the surrounding austenite and ferrite phases. These changes of morphology and strain energy strongly affect the transformation evolution. The product of the transformation strain by the local-stress inside the retained austenite corresponds to a mechanical driving force, whereas the accommodation of the transformation strain in the surrounding material opposes the transformation. Mechanical considerations must, thus, be included in the thermodynamic description of the martensitic transformation. The goal of this work is to provide a better understanding and quantify more precisely these mechanical contributions to the transformation kinetics in the case of multiphase TRIP steels. The starting point of this study is the local thermodynamic condition for the growth of a martensitic region, proposed by Fischer and Reisner.[1] This transformation criterion is an energy balance, which compares driving and dragging forces (or energies). The transformation is initiated when the sum of the chemical and the m
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