Martensite formation during tensile deformation of high-alloy TRIP steel after quenching and partitioning route investigated by digital image correlation

High-alloy austenitic TRIP steel was manufactured using quenching and partitioning route (Q&P) for further enhancement in strength which leads to a microstructure consisting of thermally induced martensite, carbides and stabilized austenite. The austenite is stabilized during partitioning due to diffusion of carbon and nitrogen from supersaturated martensite into the austenite. For evaluation of deformation behavior micro tensile specimens were manufactured and deformed in situ inside scanning electron microscope. The evolution of the microstructure was characterized by X-ray diffraction before and after deformation. During in situ deformation two austenite orientations (< 001 > and < 101 > regarding load axis) were investigated and occurring strain localizations were observed by means of digital image correlation. The strain localizations could be correlated with formation of epsilon- and twinned alpha'-martensite. Using high resolution EBSD (HR-EBSD) the transformation behavior during tensile deformation is discussed. Due to the inhomogeneous internal stress distribution and the increased defect density in the austenite, preferred martensite formation is to be expected despite the increased chemical stability. This observation is attributed to the so-called constraining effect.