Dynamic shear properties and microstructure evolution of commercially pure titanium with heterogeneous gradient microstructure

Abstract

Heterogeneous gradient microstructure in commercially pure titanium (CP Ti) was fabricated through the explosion hardening (EH) technique followed by annealing treatment (AT). The heterogeneous gradient microstructure was characterized by a microhardness tester, optical microscope and transmission electron microscope. Subsequently, dynamic shear properties and microstructure evolution of the material were systematically studied by split Hopkinson pressure bar apparatus with hat-shaped shear specimens. Experimental results showed that various heterogeneous gradient microstructures were obtained by EH + AT at different annealing temperatures. Enhanced dynamic shear properties with special yield stress-fracture strain combinations were observed, exhibiting better mechanical properties than the other metallic materials. Such excellent mechanical properties originated from finer grain size, higher twin/dislocation density and gradient synergistic effect. During dynamic shear tests of gradient CP Ti, adiabatic shear bands (ASBs) were easy to nucleate, propagate and interact, especially on EH treated specimen surface due to the generation of high-density micro-defects. Formation of twins/dislocations during EH treatment promoted dynamic recovery during the ASBs nucleation stage, whereas AT delayed ASBs nucleation due to a decrease in micro-defects, which contributed to better ductility. During dynamic shear tests of gradient CP Ti, propagation of ASBs and cracks was delayed by the gradient microstructure. The adiabatic temperature rise of 2-EH treated CP Ti was calculated to be 108 °C when ASBs nucleated, which was so low that it was supposed to have no effect on ASBs nucleation.