Strain rate dependence of strengthening mechanisms in ultrahigh strength lath martensite

The strain rate dependent mechanical properties of lath martensite are important for developing ultrahigh strength steels for automobile applications. Here, taken 2 GPa grade press hardening steel as a model material, we explored the strain rate sensitivity of lath martensite and the underlying physics. Uniaxial tensile tests were carried out over a wide range of strain rates from 10-3 to 1450 s-1 to determine the rate dependent mechanical properties. Strain rate exerts minor effect on mechanical properties at strain rates below 102 s-1. In contrast, both yield strength and work-hardening rate substantially increase with strain rate during high-strain-rate deformation. Microstructural evolution was characterized using electron backscatter diffraction and transmission electron microscopy. Particularly, synchrotron X-ray diffraction was used to measure dislocation density. The respective strengthening contributions from the friction stress, dislocations and high-angle block boundaries in lath martensite were evaluated on the basis of the measured microstructural parameters as well as related phenomenological models for predicting their strengthening effects. It is found that the enhanced yield strength during high-strain-rate deformation is due to a larger lattice friction for dislocation slip. The higher work-hardening rate is attributed to the enhanced mechanical heterogeneity within the current lath martensite microstructure at higher strain rates, which leads to larger strain gradient and thus promoted generation of geometrically-necessary dislocations.