Analysis of the Deformation and Damage Mechanisms of Pearlitic Steel by EBSD and in-situ SEM Tensile Tests

The processes governing the deformation and damage of C70 pearlitic steel were investigated in nanometer and micrometer scales using electron backscatter diffraction technique and in-situ scanning electron microscope tensile testing. The ferrite behavior was identified by in-situ x-ray tensile tests. Investigations were carried out on annealed microstructure with two interlamellar spacings of Sp = 170 and Sp = 230 nm. It is shown that pearlite yielding is controlled by the deformation mechanisms occurring in ferrite. Deformation and damage mechanisms were proposed. At low strain, pearlite deforms homogeneously with low misorientation (< 5A degrees) inside the pearlite colonies and elongates the cementite plates. At high strain, pearlite deforms heterogeneously in intense localized shear bands inside the more favorably oriented pearlite colonies. Misorientation reaches values up to 15A degrees. Cementite deforms by an offset of lamella along the shear bands. The nucleation of these shear bands occurs at strain level of E (11) = 7% for coarse pearlite and at a higher value for fine pearlite. Damage occurs by brittle fracture of the elongated cementite lamellae parallel to the tensile axis and which are developed by shear micro-cracks along the slip bands. The plastic-induced damage is thus delayed by the fine pearlite structure.