In-Situ Synchrotron X-ray Diffraction Studies on Effects of Plastic and Elastic Loading on bcc Phase Transformations of
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ADVANCED High Strength Steels (AHSSs) are under continuous development to meet various requirements, including combinations of strength, formability, and crashworthiness. In these steels, the combination of mechanical properties is achieved by a carefully designed multi-phase microstructure.[1] The volume fractions of the different microstructural constituents, such as ferrite, bainite, martensite, and austenite, determine the final properties. During steel production and in sheet forming processes as well as joining processes like resistance spot welding, loading is applied that affects the behavior of the solid-state phase transformations in the steel. It has been reported that in hot pressing processes, non-isothermal deformation in austenite promotes ferrite and/or bainite formation prior to the martensitic
P. EFTEKHARIMILANI, R.M. HUIZENGA, B. KIM, and M.J.M. HERMANS are with the Department of Materials Science and Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands. Contact e-mail: p.eftekharimilani@ tudelft.nl A. BERNASCONI is with the European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP 220, 38043 Grenoble Cedex, France. Manuscript submitted April 11, 2017. Article published online November 21, 2017 78—VOLUME 49A, JANUARY 2018
transformation and that the martensite start temperature (Ms) decreases.[2–5] If the austenite is plastically deformed, it will be strengthened. With an increase in yield strength of the austenite, Ms decreases and this can be explained by a dislocation stabilization mechanism.[6–9] He et al. reported that Ms increases with strain after small deformations and decreases with strain after large deformations of austenite in the plastic regime.[10] In resistance spot welding, a constant load (stress range of 100–200 MPa) is applied by two electrodes. It is expected that this applied load will affect the solid-state phase transformations occurring after the formation of the weld. In-situ synchrotron diffraction is a suitable technique to study these solid-state phase transformations.[11–17] Dutta et al. observed that loading conditions below the yield strength of austenite lead to elastic strains upon martensitic transformation and Ms increases with an increase in the applied external tensile load.[18] From a thermodynamic point of view, a reduction in Gibbs free energy of a system indicates whether it is favorable for phase transformations to take place. The total Gibbs free energy is the sum of a chemical and a mechanical component. The chemical free energy component of austenite and martensite as a function of temperature is shown schematically in Figure 1. At T0, both austenite and martensite have equal Gibbs free METALLURGICAL AND MATERIALS TRANSACTIONS A
energies. At a certain temperature (Ms) lower than T0, martensite starts to form spontaneously. At temperatures between Ms and T0 (for example T1), although the martensite Gibbs free energy is lower than that of austenite, the transformation does not occur, as there is a free energy bar
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