Abstract:The digital image correlation (DIC) technology was employed promptly to track the strain distribution and local strain evolution under different strain rates, and the propagation behavior of strain distribution and strain rate sensitivity were investigated. Electron backscatter diffraction (EBSD) was used to analyze the microstructure evolution, the distribution of stress-induced α" martensite transformation (SIMT), and martensite twinning after deformation. Besides, the scanning electron microscope (SEM) was adopted to observe the fracture morphology of the material. The following conclusions can be drawn. (1) The strain distribution is evolved from an approximate uniform distribution to a non-uniform distribution, then showing a phenomenon of strain concentration. Fracture occurs ultimately in the strain concentration area, and significant necking phenomenon are displayed at low strain rates. (2) The stress-strain curves exhibits a clear double yield phenomenon, producing higher strain hardening rates at the low strain rates, i.e. a negative strain rate sensitivity effect. (3) The content of α" martensite increases significantly, the grain size is refined obviously, and the average values of KAM (Kernel average misorientation) and GNDs (Geometrically necessary dislocations) increase dramatically with decreasing the strain rate, indicating that the SIMT increase can promote the accumulation of dislocation density in the deformed sample and is more conducive to the coupling effect of multiple plastic deformation mechanisms. (4) The fracture morphology is mainly tensile fracture mode caused by the aggregation of ductile dimples and voids. The stress on the dimples rapidly develops from unidirectional tension to triaxial tension resulting in sufficient growth of the dimples during the deformation process. Low strain-rate deformation promotes the occurrence of this phenomenon, so the ductile fracture characteristics is dominated.