Mechanical properties and corrosion behaviour of nanocrystalline Ti-5Ta-1.8Nb alloy produced by cryo-rolling

This paper presents a study of microstructure, mechanical properties and corrosion behaviour of a near alpha Ti-5Ta-1.8Nb alloy subjected to cryo-rolling. Grain size decreased with increase in cryo-rolling strain (epsilon(cr)), from about 5 mu m in the annealed sample down to similar to 20 nm at a epsilon(cr)=2.3. Dislocation density initially increased steeply up to epsilon(cr)=0.69 and then decreased with increasing strain up to 2.3. The decrease in the dislocation density was attributed to the formation and interaction of pile-ups resulting in annihilation of dislocations. Hall-Petch analysis showed two distinct slopes, region I, with a rapid increase in stress and region II with a negligible increase in stress with decrease in grain size. This was explained in terms of changes in dislocation density and grain size. Fractography studies revealed dimples and microvoids in the coarse grained alloy but the nanocrystalline alloy revealed a mixture of shear bands at the surface and smaller ductile dimples at the centre. Evaluation of the corrosion behaviour in boiling and concentrated nitric acid in three phases (liquid, condensate and vapour) of the nanocrystalline samples revealed almost no deterioration of the corrosion resistance of the Ti alloy when compared to the solution annealed alloy. (C) 2014 Elsevier B.V. All rights reserved.