The failure mechanism at adiabatic shear bands of titanium alloy: High-precision survey using precession electron diffraction and geometrically necessary dislocation density calculation

The difficulty of obtaining key information, such as the crystal orientation and geometrically necessary dislocation (GND) density distribution, from a large deformation region in an adiabatic shear band (ASB) has hindered further study of the ASB failure mechanism in titanium alloys. In this work, the crystal orientation information of the failure position and surrounding region in ASS of a Ti-5Al-2.5Cr-0.5Fe-4.5Mo-1Sn-2Zr-3Zn alloy was obtained via transmission electron microscopy (TEM) and precession electron diffraction (PED) with a high spatial resolution. The GND density distribution was calculated and the adiabatic shear failure mechanism was revealed from the collected data. ASBs (original width: similar to 3-4 mu m) formed in the cylinder sample during dynamic compression (strain rate: similar to 3000 s(-1)). In the transition region at the edge of ASB, the grains were severely elongated along the direction parallel to the ASB boundaries with an average grain size on the order of pm and an average GND density of 5.5374 x 10(15)/m(2) in the a phase. Relatively strong < 0001 > textures and weak < -12-10 > textures were observed. However, significant dynamic-recrystallization-dominated grain refinement in ASB, especially at the crack tip, resulted in many ultrafine equiaxed recrystallized grains (10 nm level) and weakened textures. The highest average GND density (8.6242 x 10(15)/m(2), in alpha phase) occurred in the crack tip region. Moreover, the vicinity of the primary microcracks in the main crack extension line was characterized by combinations of an extremely high GND density work hardening region and a group of low GND density recrystallized grains. This indicates that cracks in the ASB were initiated by the deformation incompatibility between the antecedent recrystallization region and the surrounding high work hardening region.