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Abstract

Transformation induced plasticity (TRIP) steels exhibit high strength and ductility at the same time. These properties are assumed to result from the TRIP steel microstructure. Metastable retained austenite is embedded in a ferritic matrix and transforms to martensite under mechanical loading.

With the help of a multiscale constitutive model we want to map the characteristic micromechanical processes and predict the deformation behavior of TRIP steels.

The continuum model for the martensitic transformation is based on the well-known work of Olson and Cohen [1] and modified in the following aspects: With the help of the crystal plasticity approach the shear amount in each slip system of austenite is calculated and projected to the twin systems to estimate the shear amount of the fault bands. Hence, the fault band intersections are specified and by comparing the shear amount of the different fault pairs to the specific eigen strain, the nucleation probability is determined. The driving forces for the strain induced nucleation and stress assisted martensitc growth are derived within the thermodynamics framework considering elastic and plastic deformation as well as the difference in the Gibb’s free energy. The total Gibb’s free energy change is composed of a chemical and mechanical term. The chemical term is equivalent to the Gibb’s free energy difference between austenite and martensite, dependent on temperature and chemical composition and calculated by THERMOCALC . The mechanical term incorporates the work done resulting from the applied stress state. To calculate the macroscopic material properties we make use of a micromechanical approach, by setting up a representative volume element (RVE), which represents all TRIP steel phases according to their volume fraction. Consequently, the properties of the RVE can be homogenized to yield the macroscopic mechanical properties that result from the given microstructure.