Effect of stress triaxiality and Lode angle on the kinetics of strain-induced austenite-to-martensite transformation

The effect of the stress state on the transformation kinetics of stainless steel 301LN sheets at room temperature is investigated using newly developed experimental techniques for simple shear and large strain in-plane compression. In addition, uniaxial and equi-biaxial tension experiments are performed. Two-dimensional and stereo digital image correlation techniques are used to measure the surface strain fields. In situ magnetic permeability measurements are performed to monitor the evolution of martensite content throughout each experiment. The experimental results indicate that the martensitic transformation kinetics cannot be described solely by a monotonically increasing function of stress triaxiality: for instance, less martensite is developed under equi-biaxial tension than under uniaxial tension for the same increment in equivalent plastic strain. A stress-state-dependent transformation kinetics law is proposed that incorporates the effect of the Lode angle parameter in addition to the stress triaxiality. In the proposed model, the rate of martensite formation increases monotonically with the stress triaxiality and the Lode angle parameter. The comparison with the experimental data demonstrates that the proposed transformation kinetics law provides an accurate description of the evolution of the martensite content in stainless steel 301LN over a wide range of stress states. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.