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 high-pressure torsion (HPT) on phase transformations and structure formation in near-β titanium alloy Ti-15Mo (wt.%) as well as with the dependence of the elastic modulus E and mechanical properties of the nanostructured alloy in the temperature range of 250–600°C. It is revealed that room-temperature nanostructuring of the β-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 tensile strength (UTS) of the Ti-15Mo alloy (UTS = 1550 MPa, El. = 7%) compared to that of the β-quenched alloy. It is shown that, after aging of the quenched and deformed Ti-15Mo alloy, the metastable β solid solution undergoes isothermal decomposition, resulting in the formation of the ω- and α-phases. The high defect density of the nanostructured alloy shifts the temperature range of the α-phase precipitation to lower temperatures (by 120°C on average) and has a significant effect on the volume fraction and morphology of α-phase precipitates. The latter have an equiaxed shape compared to the needle-like α-phase that 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 the embrittlement of the alloy. The formation of an equiaxed α + β structure during HPT and aging at 550°C contributes to a balance between strength and ductility (UTS = 1270 MPa, El. = 10%). Changes in the structural-phase composition and phase ratios result in a nonmonotonic behavior of the elastic properties of the Ti-15Mo alloy. Studies of biological activity showed that both coarse-grained and nanostructured states of the Ti-15Mo alloy do not exhibit in vitro cytotoxicity towards blood leukocytes, indicating that these specimens are biocompatible. However, the nanostructured specimens demonstrated a pronounced inhibition of surface adhesion of S. aureus bacteria, which may potentially reduce the risk of postsurgical infectious complications following implantation of orthopedic metal devices based on the Ti-15Mo alloy in this structural state.