Microstructure Evolution and Mechanical Behavior of Cold-Sprayed, Bulk Nanostructured Titanium

To provide insight into the microstructural evolution and mechanical behavior of bulk nanostructured Ti, we used cold gas dynamic spraying of Ti particles to synthesize thick coatings (e.g., >10 mm in thickness). Accordingly, the grain size, lattice parameter, lattice strain, residual stress, porosity, microhardness, tensile, and compressive behavior of the bulk Ti deposits before and after annealing were comparatively analyzed. Our results show that the microstructure of the as-sprayed bulk Ti was characterized by a grain size of ~60 nm, lattice expansion (~2 pct for \( a \) and ~3 pct for \( c \)), lattice strain (~1.65 × 10⁻⁵), and residual compressive stress (~53 MPa). Moreover, annealing of the as-deposited bulk Ti led to a significant decrease in lattice expansion, lattice strain, and residual stress, whereas porosity remained unchanged (~11 pct). The mechanisms of grain growth, as well as the evolution of particle interfaces during annealing, were also investigated. In terms of mechanical behavior, the as-deposited bulk Ti exhibited a very low modulus (52 GPa) with relatively high tensile and compressive strength values (180 and 850 MPa, respectively). Annealing in the temperature range of 1023 K to 1173 K (750 °C to 900 °C) led to a significant increase of tensile and compressive strength (to 380 MPa and more than 1200 MPa, respectively). Finally, annealing resulted in a slight increase of elastic modulus, which was rationalized on the basis of changes in pore geometry in the bulk Ti deposits.