High temperature microstructural evolution of 304L stainless steel as function of pre-strain and strain rate

304L stainless steel specimens are pre-strained to 0.15 or 0.5 and are then deformed at strain rates ranging from 2000s^-1 to 6000s^-1 at temperatures of 300°C, 500°C and 800°C using a compressive split-Hopkinson pressure bar. The results show that for both values of the pre-strain, the flow stress increases with increasing strain rate, but reduces with increasing temperature. At deformation temperatures of 300°C or 500°C, the flow stress in the 0.5 pre-strained specimen is higher than that in the specimen pre-strained to 0.15. However, at a temperature of 800°C, the two specimens exhibit a similar level of flow stress. Transmission electron microscopy (TEM) observations reveal that the strengthening effect observed in the specimens deformed at 300°C or 500°C is the combined result of dislocations, mechanical twins and martensite transformation. However, at a deformation temperature of 800°C, the strengthening effect is the result primarily of dislocation multiplication. The volume fraction of martensite transformation decreases with increasing strain rate and temperature. In addition, both the dislocation density and the twin density increase with increasing strain rate, but decrease with increasing temperature. Finally, the quantitative analysis results indicate that the flow stress varies with the square root of the dislocation density, the twin density and the volume fraction of martensite, respectively.