Abstract
Ti15Mo alloy was subjected to two techniques of intensive plastic deformation, namely high pressure torsion and rotary swaging at room temperature. The imposed strain resulted in the formation of an ultrafine-grained structure in both deformed conditions.
Detailed inspection of the microstructure revealed the presence of grains with a size of around 100 nm in both conditions. The microstructure after rotary swaging also contained elongated grains with a length up to 1 mu m.
Isothermal ageing at 400 degrees C and 500 degrees C up to 16 h was applied to both conditions to investigate the kinetics of precipitation of the alpha phase and the recovery of lattice defects. Positron annihilation spectroscopy indicated that the recovery of lattice defects in the beta matrix had already occurred at 400 degrees C and, in terms of positron trapping, was partly compensated by the precipitation of incoherent alpha particles.
At 500 degrees C the recovery was fully offset by the formation of incoherent alpha/beta interfaces. Contrary to common coarse-grained material, in which the alpha phase precipitates in the form of lamellae, precipitation of small and equiaxed alpha particles occurred in the deformed condition.
A refined two-phase equiaxed microstructure with alpha particles and beta grain sizes below 1 mu m is achievable by simple rotary swaging followed by ageing.