Microscopic Shear Deformation Characteristics of the Lüders Front in a Metastable Austenitic Transformation-induced-plasticity Steel

To elucidate the mechanisms of deformation and a state of plastic stability at the front of Lüders bands during a tensile test, metastable austenitic transformation-induced plasticity (TRIP) steels with different dislocation densities and ferritic steels were characterized via macroscopic-DIC-based stress–strain investigations and scanning electron microscopy (SEM). A direct correlation between stress–strain curves and measured strain distributions in the tensile specimen indicated that the Lüders front represents a transition region from a state of plastic instability to one of stability, whereby a general rule relating the Lüders strain (ΔεL*) and increments in the true stress in the Lüders band (ΔσL*) to a lower yield stress (σy0*) can be described as σy0*=ΔσL*ΔεL* irrespective of the amount of deformation-induced martensite in the band or crystal structure of the steel. The inclination angle of the Lüders front with respect to the tensile direction changed from 55° to 90° with a reduction in the measured strain ratio (−εyy/εxx) in the Lüders band, and the change agreed with the tendency calculated by the plasticity model assuming the pure shear deformation under the minimum shear strain criterion. SEM observations of the sheet surface and the front cross-section in the TRIP steel revealed the formation of multiple inclined ~20 μm-wide shear deformation zones that accompanied a reduction in thickness. All the observed geometrical characteristics of the Lüders front are qualitatively explained by minimizing the misalignment from the fixed tensile axis caused by shear mode deformation.