Modeling of the Austenite-Martensite Transformation in Stainless and TRIP Steels

The transformation of austenite to martensite is a dominant factor in the description of the constitutive behavior during forming of TRIP assisted steels. To predict this transformation different models are currently available. In this paper the transformation is regarded as a stress induced process based on the thermodynamic action of the local stresses during transformation. A threshold for the thermodynamic action, above which transformation will occur, can be easily measured in a properly instrumented tensile test. The martensitic transformation is a diffusionless lattice shear. It is characterized by a habit plane normal n and a shear vector m, which are both defined with respect to the austenite lattice coordinate system. Therefore the thermodynamic action in each material grain strongly depends on the orientation of the grain with respect to the applied stress. Uniaxial tensile tests on both a non-textured austenitic stainless steel and one with a strong crystallographic texture were performed in both the rolling and the transverse directions. Both materials show mechanically induced phase transformation from austenite to martensite. When a strong texture is present in the austenite, differences between transformations during deformation in different directions can be observed clearly. The stress induced transformation theory, in combination with the textures measured before and after deformation, is used to explain and model the difference in transformation behavior when straining in various directions. During deformation the texture changes. This can have consequences for modeling of the transformation during non-proportional deformation.