Transmission electron microscopy studies of microstructure of Ti-Nb and Ti-Ta alloys after ball milling and hot consolidation

Two Ti alloys with compositions Ti-10Ta and Ti-10Nb (at.%) were milled in a high-energy mill for a maximum of 80 h in an argon atmosphere. A nanocrystalline structure of alpha-Ti(X) (X = Ta or Nb) solid solution was formed in both investigated alloys after milling, as shown by X-ray diffraction. Transmission electron microscopy observations of powders milled for 80 h revealed chemical inhomogeneity of particles in nanometre-scale regions and an average crystallite size of about 10 nm. The pulse plasma sintering method was applied for hot consolidation of milled powders. The mean density of pulse plasma sintering compacts of Ti-Nb alloy was about 99.5% of the theoretical value, whereas the density of the Ti-10Ta sample was lower, close to 92% of the theoretical value. Transmission electron microscopy observation of compacted samples showed that the sintering process caused the formation of a two-phase alpha + beta structure in both investigated alloys, with a mean grain size of 220 nm. The chemical inhomogeneity and high degree of deformation in nanometre-scale regions of milled powders led to a martensitic transformation, resulting in formation of a 9R martensite structure.