Strain partitioning in ultra-fine grained medium-manganese transformation induced plasticity steel

A 7.1-Mn 0.1-C transformation-induced plasticity steel was intercritically annealed at 600 °C and 650 °C for 168 h. Ultra-fine-grained microstructures with annealing temperature dependent retained austenite fractions and tensile properties were produced. in situ neutron diffraction was used to investigate the change in tensile properties via measurement of phase fractions, elastic phase strains, and diffraction peak broadening during deformation. Austenite transformation to martensite controlled initial yielding in the 650 °C annealed steel and stress induced transformation was observed. In contrast, yielding after annealing at 600 °C was controlled by plastic deformation of ferrite, with austenite transformation initiating only after yield point elongation. The sequence of deformation between constituents was readily apparent in the lattice strain and peak width data. During deformation, compressive lattice strains were always developed in austenite, ferrite plastic deformation initiated around 700 MPa in both steels, and tensile stress was preferentially transferred to deformation-induced martensite. The development of compressive strains in austenite was related to constraint of the volume expansion during austenite transformation to martensite.