Shear localization and its related microstructural evolution in the ultrafine grained titanium processed by multi-axial compression

Ultrafine grained titanium has unique mechanical properties and attracts tremendous interest due to its scientific and technological application. Shear localization is frequently denoted as adiabatic shear band, and is one of the most important deformation and failure mechanisms for it used at high rate deformation. Hat shaped specimens are used to induce the formation of an adiabatic shear band under controlled dynamic conditions. Unstable shear deformation of the alloy emerges after the true flow stress reaches about 750 MPa, the first vibration peak during the split Hopkinson pressure bar testing, and the whole deformation process lasts about 50 μs. The microstructure and microtextures in the shear band with width about 16.7 μm in ultrafine grained titanium processed by multi-axial compression are investigated by means of optical microscopy, scanning electron microscopy/electron back-scattered diffraction, and transmission electron microscopy. The results show that the grains in the boundary of the shear band are highly elongated along the shear direction, and the core of the shear band consists of a number of scattered recrystallized equiaxed grains with 50-80 nm in diameters and coalesced grains with 100-150 nm in diameters. Some new microtextures (20°, 4°, 0°), (81°, 0°, 0°), and (55°, 0°, 30°) with recrystallized characteristics are generated in the shear band. The grain boundaries in the core of an adiabatic shear band are high-angle boundaries and geometrical necessary boundaries created with aims of accommodating the imposed shear strain. Calculations of temperature rise suggest that the maximum temperature in the shear band is about 870 K being sufficient for the recrystallization. Finally, the mechanisms of formation of an adiabatic shear band in the ultrafine grained titanium processed by multi-axial compression and its microstructural evolution are proposed.