Temperature-dependent micromechanical behavior of medium-Mn transformation-induced-plasticity steel studied by in situ synchrotron X-ray diffraction

The temperature-dependent micromechanical behavior of medium-Mn transformation-induced-plasticity (TRIP) steel with a nominal chemical composition of Fe-0.1C-10Mn-2Al (mass%) fabricated by intercritical annealing 650 °C for 1 h after cold-rolling, was investigated using in situ high-energy X-ray diffraction (HE-XRD) with uniaxial tensile tests at temperatures of 100, 25 and −50 °C. We find that as the deformation temperature decreases, the Lüders strain decreases and more austenite transforms to martensite during Lüders band propagation. The Lüders bands are associated with sudden changes of lattice strain in austenite. At 100 °C, austenite is too stable to transform to martensite, resulting in limited work-hardening capability and a relatively low strain to failure. At 25 °C, the austenite is found to transform in bursts during applied loading. These transformations correlate with stepwise peak broadening in the austenite phase and are attributed to Portevin-Le Châtelier (PLC) band propagation. At −50 °C, we observe a more intense TRIP effect which suppresses PLC band formation and leads to a high ultimate tensile strength. In addition to the TRIP effect, we find that work hardening in the deformation-induced martensite phase plays an important role in the plastic stability at low temperature, by accommodating stress compatibility between grains and phases. Our studies provide the first direct experimental evidence for the existence of large stress concentrations in the austenite phase near the Lüders band propagation front. These observations help to deepen understanding of the complex temperature-dependent micromechanical behaviors of advanced medium-Mn TRIP steels.

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