In situ observations on the austenite stability in TRIP-steel during tensile testing

In-situ deformation tests have been performed on a steel displaying the transformation-induced plasticity (TRIP) effect, while monitoring the phase transformation by means of X-ray diffraction. A tensile stress is applied to 0.4 mm thick samples of this steel with mass contents of 0.26 % Si, 1.5 % Mn, and 1.8 % AI in a transmission geometry for a synchrotron-radiation beam of 25 mum . 25 mum. On the diffraction patterns every grain appears as a discrete spot. The austenite (200) reflections are analysed during this investigation. The diffraction patterns are treated like a powder pattern for five different eta-angles, with eta representing the angle between the tensile direction and the normal direction of the diffracting {200} planes. The results of the analysis show that eta = 0degrees and eta = 90degrees are the preferential orientations for the transformation to martensite. The Ludwigson and Burger model [9] is used to gain more information about the stress dependence of the deformation induced martensite formation. The microdiffraction patterns also reveal the changes in carbon concentration in austenite at each retained austenite fraction.