Deformation and Strengthening Mechanisms in AISI 321 Austenitic Stainless Steel Under Both Dynamic and Quasi-Static Loading Conditions

Abstract

The mechanical response of AISI 321 austenitic stainless steel under compressive loads at strain rates of 6600 s−1 and 4.2 × 10−3 s−1 were studied using the split Hopkinson pressure bar and Instron R5500 mechanical testing system respectively. Specimens subjected to quasi-static compression showed lower yield strength and higher strain hardening capacity than the dynamically impacted specimen. High-resolution electron backscattered diffraction (HR-EBSD) study revealed that precipitation of nano-sized carbide and evolution of strain-induced martensite contributed to strengthening while plastic deformation mechanisms occurred in the specimens by slip and mechanical twinning during deformation under both quasi-static and dynamic loading conditions. The strain-induced phase transformation follows the FCC ɣ-austenite → BCC ά-martensite kinetic path with both phases maintaining the Kurdjumov-Sachs’ {(111)ɣ||(110)ά and ɣ|| ά} orientation relationship. A transformed shear band consisting of nano-grains with an average size of 0.28 µm was one of the microstructural features of the dynamically impacted specimen. HR-EBSD analysis revealed that the equiaxed ultra-fine grain structure in the TSB developed by rotational dynamic recrystallization mechanism while dynamic recovery occurred at the interface between the inside and outside of the band. During the deformation under both loading conditions, volume fraction of compression direction (CD)//{110} and CD//{111} increases substantially and slightly, respectively at the expense of CD//{100} fibre texture for the austenitic phase.