Effect of Ultrasonic Impact Treatment on Structural Phase Transformations in Ti-6Al-4V Titanium Alloy

Ultrasonically treated Ti-6Al-4V alloy samples were examined by X-ray diffraction analysis and transmission electron microscopy, which revealed the formation of a 1-μm-thick layer with the nanocrystalline structure of titanium oxides TiO 2 (brookite and srilankite) and α phase on their surface. Beneath the oxide layer there is a nanocrystalline structure of the α + β phases. At a depth of 2 to 40 μm from the surface, the grain structure changes from nanocrystalline to microcrystalline. At a depth of 75 μm, the grain size is equal to that of the initial untreated alloy. The surface layers of the titanium alloy experience macroscopic elastic compressive stresses of 0.8 GPa. Their generation is accompanied by microdeformation of the α-phase lattice. The change in the parameters of the α-Ti solid solution after ultrasonic treatment indicates an increase in the concentration of vanadium and oxygen in the solid solution. Ultrasonic treatment causes the local β → α″ martensitic transformation in the β phase of grain-boundary layers, due to which these layers become two-phase (β + α″). The use of a carbon lubricant during ultrasonic treatment leads to a decrease in the volume fraction of titanium oxides in the surface layers.