Correlation of dynamic impact properties with adiabatic shear banding behaviour in Ti-15Mo-5Zr-3Al alloy

A compressive split-Hopkinson pressure bar is used to investigate the impact properties of Ti-15Mo-5Zr-3A1 alloy under strain rates ranging from 800 to 8000s(-1) at temperatures between 25 and 900 degrees C. The morphologies of the deformed microstructures and the fracture features of the adiabatic shear bands on the fracture surfaces are examined using optical and scanning electron microscopes. Based on the macroscopic and microscopic observations, the dynamic impact properties of the alloy are correlated with its adiabatic shear banding behaviour. The experimental results indicate that the strain rate and testing temperature both have a significant effect on the mechanical properties of Ti-15Mo-5Zr-3Al alloy. At a constant temperature, the flow stress increases with increasing strain rate. However, at a given strain rate, the flow stress reduces as the temperature increases. Furthermore, the fracture strain decreases with increasing temperature prior to phase transformation at 785 degrees C, but increases thereafter as the temperature is further increased. The temperature-dependent variation of the fracture strain is thought to be related to the amount of a phase in the deformed microstructure, which increases with increasing temperature prior to phase transformation, but dissolves entirely at higher temperatures and is replaced by a pure P phase. Fractographic analysis reveals that the specimens fracture as a consequence of adiabatic shear band formation. Specimens impacted at higher strain rates and lower temperatures are more likely to form adiabatic shear bands. The width of the shear band decreases with increasing temperature prior to phase transformation, but increases with increasing temperature thereafter. The microhardness within the adiabatic shear band is found to increase slightly in the specimens tested at higher strain rates and temperatures. The fracture surfaces of the impacted specimens exhibit both dimple-like and cleavage-like features. Finally, the splats and knobble-like features observed on the fracture surfaces of specimens deformed at the highest strain rate of 8 x 10(3) s(-1) and temperatures of 300 degrees C or higher, indicate that significant melting takes place during deformation under high strain rate and high temperature loading conditions prior to specimen fracture. (c) 2007 Elsevier B.V. All rights reserved.