Mechanical Behavior and Superplasticity of a Severe Plastic Deformation Processed Nanocrystalline Ti-6Al-4V Alloy

Mechanical behavior of a severe plastic deformation (SePD) processed nanocrystalline Ti-6Al-4V alloy has been studied in the temperature range 25-675°C. Compared with the microcrystalline state, the nanocrystalline state material had higher strength up to 400°C and comparable strength above that. The ductility was significantly higher for the nanocrystalline state above 500°C, including superplasticity above 600°C. Transmission electron microscopy showed considerable grain growth and dislocation activity during superplastic deformation. A comparison of the superplastic data across the nanocrystalline and microcrystalline range showed an interesting discrepancy in the kinetics of superplastic deformation. Contrary to the general expectation, the kinetics of superplastic deformation was slower in ultrafine grained materials after normalizing the data for grain size and temperature dependence. The slower superplastic deformation kinetics in the nanocrystalline materials is discussed in terms of the difficulty associated with slip accommodation of grain boundary sliding