Heterogeneous Deformation of Pre-strained Lath Martensite

To verify the effect of anisotropy of internal stress on the heterogeneous deformation behavior in martensitic steel, in-situ digital image correlation (DIC) method was conducted on as-quenched and pre-strained Fe–18%Ni–0.02%C alloys with lath martensitic structure. Uniaxial tensile loading and unloading were repeated in several steps to apply stepwise pre-strain, and it is considered that the pre-strain makes decreases in anisotropy of internal stress or alters the state of internal stress. In the 1st step of deformation, during the transition from elastic to plastic deformation, the heterogeneous deformation begins block-by-block. In the 2nd step (after pre-strain of the 1st step) and the later steps, a similar strain distribution to that in the 1st step was observed. If the effect of anisotropic internal stress generated during quenching appear in the initial stage of deformation and be reduced or altered by pre-strain, the strain distribution should gradually change with increasing pre-strain. The unchanged strain distribution seems to have demonstrated the effect of internal stress on the heterogeneous deformation is relatively small. Slip system analysis was conducted on almost all blocks in the DIC field of view. It was confirmed that in-lath plane slip systems and in-habit plane slip systems were activated in blocks with higher strain concentrations. Based on these results for the ultra-low-carbon martensitic steel, it is concluded that the preferential activation of in-lath plane slip systems could be a more dominant factor for the heterogeneous deformation of lath martensite, compared to the anisotropy of internal stress introduced during martensitic transformation.