Microstructural characterization of adiabatic shear bands in a titanium-stabilized interstitial free steel

The present work-reports the main results of the textural and microstructural characterization performed in shock-induced adiabatic shear bands in a titanium-stabilized interstitial-free steel. The material was dynamically compressed in a split Hopkinson bar at high strain rates. Cylindrical hot-shaped specimens were strained at strain rates varying from 1.0X10(4) to 7.9 x 104 s(-1). The tests were carried out at temperatures of -196 degrees C, -50 degrees C, and 25 degrees C. Light optical microscopy, (LOM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to perform the microestrutural characterization. Orientation Imaging Microscopy (OIM) maps of the. shear regions were obtained with aid of conventional and high-resolution Electron Backscatter Diffraction (EBSD) techniques. Vickers microhardness testing was done in the vicinity and within shear bands and in the starting material. Results show a weak and diffuse (random) texture in the IF-steel plate (initial state). A detailed investigation on the morphology of the adiabatic shear bands was conducted. The width of the shear bands varied from 40 mu m to about 270 mu m. In most of the cases, the microstructure of the adiabatic shear bands consists of a fine lamellar structure. The thickness of the lamellae is about 0.2-0.5 mu m. Furthermore, the local texture within bands.; and in the neighboring grains was determined Deformation twins were observed in the vicinity of the shear bands. Microstructural evidence suggests mechanical twinning occurs before the flow associated to shear banding indicating a well-defined hierarchy of the events during straining. Independent of the test temperature gamma- and alpha-fiber texture components are found within shear bands. Dynamic rotational recrystallization is a plausible mechanism to explain the development of a new structure consisting of ultrafine grains (0.1-0.5 mu m.) within shears bands. The presence of subgrains and grains perfectly aligned within regions resembling a former lamellar structure within bands supports the occurrence of such a mechanism.