Correlation between macroscale tensile properties and small-scale intrinsic mechanical behavior of Mo-added Fe–Mn–Al–C lightweight steels

Research and development for increasing the strength of steel materials has been in progress for a long time, but as the strength increases, the stiffness and impact characteristics are deteriorated, the weight reduction by reducing the thickness of steel is gradually reaching its limit. Therefore, lightweight steels having a low density by adding aluminum, which is a substituted element having a smaller atomic weight than iron atoms, has been proposed. These steels have exhibited favorable mechanical properties of high strength and good ductility, making it suitable for the automotive and defense industries. In this study, the effects of Mo addition on the microstructural evolution and mechanical properties of austenitic lightweight steel with Fe-1.1 wt% C-30 wt% Mn-10.5 wt% Al were investigated using a macroscopic tensile test and small-scale nanoindentation. The yield strength decreased as the Mo content increased to 3 wt%. This phenomenon was explained by the precipitation of κ-carbide. In contrast, the yield strength increased as the Mo content significantly increased to 4 wt%. This is due to grain refinement caused by Mo-enriched precipitation such as M₆C and the formation of ferrite and ordered phase D0₃. In addition, the intrinsic mechanical properties inside grain and at the grain boundary were statistically evaluated using nanoindentation data. From the maximum shear stress at the elastic–plastic transition of the grain boundary during nanoindentation, we could determine the grain boundary strengthening due to M₆C precipitation. From the grain boundary strengthening and the grain refinement, the effects of Mo on the microstructure evolution and mechanical properties of lightweight steel could be described.