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Discrete and continuum micromechanical models of transformation-induced plasticity in multiphase steels

Date: 12.04.2010
Place: ICAMS, UHW 11/1102

S. Turteltaub, Department of Engineering Mechanics, Delft University of Technology, Delft, The Netherlands

Among the class of high-strength, high-ductility materials, multiphase steels occupy a promi- nent position. The microstructure of multiphase steels consists of grains of retained austenite embedded in a ferrite-based matrix. Upon mechanical loading, retained austenite may trans- form into martensite, as a result of which lastic deformations are induced in the surrounding phases. This solid-state martensitic transformation has been experimentally identifi ed as the underlying mechanism responsible for the improvement of the overall yield strength and duc- tility. A detailed understanding of these mechanisms is essential in order to optimize the properties of these materials. The microscopic mechanisms controlling the macroscopic properties are studied by means of numerical simulations of aggregates of grains of retained austenite embedded in a ferrite-based matrix. Two approaches are used to simulate the phase transformation and plastic deformation, namely applying a discrete and a continuum model. In the discrete model, plasticity is accounted for using discrete dislocations and the transformation is explicitly modelled through evolving transformation regions inside grains of retained austenite. The evolution of the martensitic regions is governed by a kinetic relation that relates the phase boundary velocity to the corresponding driving force. The continuum model is based on crystal plasticity coupled to a transformation model where the transforming regions are represented through internal variables (volume fractions). The driving forces for transformation and plasticity are derived from thermodynamical principles and include lower length-scale contributions from surface and defect energies. Both discrete and continuum models provide valuable insight, at somewhat different length scales, on the interaction between plastic deformation and transformation. The simulations are used to study the e ect of various microstructural parameters, such as crystal orientation, austenitic volume fraction and grain size, on the eff ective response of an aggregate of multiphase grains. This presentation is based on joint work with A. Suiker, D. Tjahjanto, J. Shi and E. van der Giessen.

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