Effect of strain rate on microstructure and mechanical properties of martensitic steel

Abstract

Martensite steel is one kind of unique ultra-high strength steel, which is widely used in aerospace, automotive, weaponry, etc. mechanical systems. The strain rate is known to have significant effect on mechanical properties and microstructure of steel materials. This study focused on the mechanical properties and microstructure evolution of martensitic steel under different strain rates. The results showed that the yield strength and tensile strength increase with the increasing strain rate, but the elongation is the opposite. The electron backscatter diffraction results exhibited that as the strain rate decreases, the content of substructures decreases while the content of deformed structure increases. Moreover, the Schmidt factor decreases as the strain rate increases. The geometrically necessary dislocations (GNDs) densities calculated indirectly through the kernel average misorientation diagram reached a maximum value of 18.55×1014 m2. The GNDs can hinder the slip of dislocations and thus improve the strength of the material. The fracture morphology observed by scanning electron microscopy showed that the high strain rate resulted in small and shallow dimples, bringing high strength; while the dimples formed by low strain rate were large and deep, contributing to the optimal plasticity. Our research reveals the relationship between strain rate and structure and properties of martensitic steel, providing guidance for its application on complicated mechanical systems under varying loads.