Microstructural evolution in adiabatic shear band in the ultrafine-grained austenitic stainless steel processed by multi-axial compression

Adiabatic shear bands play the most important role in the deformation and failure of high strength AISI 201 austenitic stainless steel. The microstructure and microtexture of the adiabatic shear band in ultrafine-grained stainless steel are investigated by means of optical microscopy, TEM and EBSD. The shear bands can be generated at about 54 μs after the true flow stress reaches the peak value of about 1135 MPa. The width of the shear bands is about 9 μm, and the grains sizes 50–200 nm in the boundary of the shear band are elongated along the shear direction; the grain subdivision on approaching the shear band can be observed, and the core of the shear band consists of recrystallized equiaxed grains (sizes 30–80 nm) with new microtextures and the ultrafine grains with deformed microtextures and with high dislocation density. The grain boundaries in the adiabatic shear band are geometrical necessary boundaries with high-angles. The calculated temperature in the shear band is estimated to reach 0.58 Tm (968 K). It takes 7 μs for the shear band to cool down from 968 K to the room temperature when the shear localization ceases, and the cooling strain rate is calculated as high as 9.6×107 K/s. Kinetic calculations indicate that during the deformation process, the recrystallized nanosized grains can be formed in the shear band by way of the subgrain boundaries rotation when the subgrains׳ sizes are lower than 80 nm or the temperature in the shear band is higher than 0.5 Tm (834 K), and they do not undergo significant growth by grain boundary migration at the cooling stage. These results indicate that the microstructure development within shear band is controlled by both dynamic recovery and rotational dynamic recrystallization.