EFFECT OF THE STRUCTURAL-PHASE COMPOSITION ON THE MECHANICAL PROPERTIES AND BIOCOMPATIBILITY OF NANOSTRUCTURED TI-15MO ALLOY

The paper is concerned with the effect of severe plastic deformation by torsion (SPDT) on phase transformations and structure formation in near-p titanium alloy Ti-15Mo (wt. %) as well as with the dependence of the elastic modulus E and mechanical properties of the nanostructured alloy on the aging temperature in the range 250-600 °C. It is revealed that room-temperature nanostructuring of the p-quenched Ti-15Mo alloy to the von Mises strain ε~200 results in a homogeneous microstructure with a high defect density and the size of structural elements less than 100 nm. Formation of the nanostructure ensures an 80 % increase in the ultimate strength σВ of the Ti-15Mo alloy (σВ = 1550 MPa, 5 = 7 %) compared to that of the P-quenched alloy. It is shown that, after aging of the quenched and deformed Ti-15Mo alloy, the metastable β-solid solution experiences isothermal decomposition with the formation of the ω and a phases. The high defect density of the nanostructured alloy shifts the temperature range of the a-phase precipitation to lower temperatures (by 120 °C on average) and has a significant effect on the volume fraction and morphology of a-phase precipitates. The latter have an equiaxed shape compared to the needle-like a phase, which precipitates during aging of the quenched coarse-grained alloy. After aging at 600 °C, an equiaxed α+β structure with the average size of structural elements 380 nm is formed in the deformed alloy. Analysis of the mechanical properties after aging showed that the precipitation of dispersed ω-phase particles makes a significant contribution to precipitation hardening of Ti-15Mo alloy, significantly increases the microhardness (by 50 %) compared to the quenched and deformed alloy, and can be considered as a macromechanical cause of embrittlement of the alloy. The formation of an equiaxed α+β structure during SPDT and aging at 550 °C contributes to a balance between strength and ductility (σВ = 1270 MPa, δ = 10 %). The change in the structural-phase composition and phase ratio ensures a nonmonotonic behavior of the elastic characteristics of the Ti-15Mo alloy. The study of the biological activity showed that the Ti-15Mo alloy in the coarsegrained and nanostructured states does not exhibit in vitro cytotoxicity to blood leukocytes, which makes these specimens biocompatible. However, the nanostructured specimens demonstrate an intense inhibition of the surface adhesion of S.aureus bacteria, which can potentially reduce the risk of postsurgical infectious complications after implantation of orthopedic metal structures based on the Ti-15Mo alloy in the studied state.