A Study on Adiabatic Shear Banding of TC4-Titanium Alloy under Planar Shock Compression

Investigations for adiabatic shear banding (ASB) of a TC4-titanium alloy under planar shock compression were performed by using a compressed gas gun of 57 mm bore diameter, and symmetric impact technique. The stresses in the target, from 3 to 15 GPa, were designed above the Hugoniot elastic limit, HEL=2.85 GPa, of the target material. Microstructural features of the recovered sample were examined metallographically by using scanning electron microscope. Results indicated: (1) ASB emerges only if the stress in target is not less than 5.87 GPa (or impact velocity 500 m/s), a stress far above HEL. It implies that the ASB formation, resulted from heterogeneous energy deposition, is merely a stage in the material yielding process. Results from X-ray diffraction analysis indicated that phase transition occurred in ASB, and also in matrix material but with weaker degree and rather dispersive manner; (2) The angle between ASB and shock compression direction is about 45, consistent with the theoretical prediction of maximum shear direction for isotropic medium, which is just the case of TC4-titanium alloy; (3) Spherical and elliptic voids appeared in ASB and increased both in number and in size with increasing shock stress in target, showing the typical features of the dynamic damage for ductile materials; (4) A few ASB branches appeared at stress of 15 GPa, with an angle of about 15 inclined to the main ASB; and (5) microstructure of ASB, characterized by the appearance of tiny and lengthy crystallites with preferred orientation parallel to the bounds of ASB, is significantly different from that of the matrix material. Accordingly, we believe that a melting and recrystallization process happened in ASB during its development. Calculations for ASB temperature and the melting temperature at corresponding shock stress showed that the above arguments are reasonable.