High temperature impact properties and dislocation substructure of Ti–6Al–7Nb biomedical alloy

Abstract

The high temperature deformation and dislocation substructure of Ti–6Al–7Nb biomedical alloy are investigated under high strain rate loading conditions using a split-Hopkinson pressure bar. Impact tests are performed at strain rates ranging from 1×103s−1 to 3×103s−1 and temperatures of 300°C and 700°C, respectively. The experimental results show that the flow stress, work hardening coefficient and strain rate sensitivity all increase with increasing strain rate, but decrease with increasing temperature. Moreover, the fracture observations reveal that the Ti–6Al–7Nb specimens fail predominantly as the result of intensive localised shearing. The fracture surfaces of the deformed specimens contain both cleavage structures and dimple-like structures. Transmission electron microscopy observations reveal that the dislocation density increases with increasing strain rate, but decreases with increasing temperature. A pronounced thermal softening effect is observed in the specimens deformed at 700°C due to a rapid annihilation of the dislocations. However, a work hardening effect occurs at higher strain rates and lower temperatures due to an enhanced degree of dislocation multiplication and tangling. Finally, a linear relationship is observed between the square root of the dislocation density and the flow stress.