Influence of Thermomechanical and Heat Treatments on the Structure, Phase Composition and Mechanical Properties of Biocompatible Ti-(18-20)Nb-(1-1.2) Si Alloys

The Ti-(18-20)Nb-(1-1.2)Si alloys are obtained by electron-beam melting; the sizes of the ingots are as follow: d = 60 mm, l = 650 mm. As shown, the applied smelting method provides a more stable phase composition of alpha + beta + (Ti, Nb)(3)Si in the cast alloys. Hot deformation is carried out at & Tcy; congruent to 1000 degrees & Scy; by means of the rotary forging up to d = 20 mm, followed by thermomechanical treatment (TMT - screw rolling with water cooling) up to d = 12 mm; quenching in water is carried out at 1050 degrees & Scy; with a holding time of 30 min. The structure after deformation is non-equilibrium, consisting of the alpha ( alpha ' )-phase, the residual metastable beta- phase, a small amount of large (Ti, Nb)( 3) Si silicides mainly on the boundaries of the primary beta- grains, as well as dispersed silicides on structural defects, that causes the high strength sigma(B) = 1155 MPa, but low plasticity delta = 3.5 %. During the quenching of the deformed Ti-(18-20)Nb-(1-1.2)Si alloys at 1050 degrees & Scy;, the orthorhombic alpha ''- phase is formed, and the amount of silicides increases. Herewith, the strength is slightly reduced to sigma(B) = 1135 MPa with significant increase in plasticity 8 = 9 %. Two-stage deformation including TMT with final quenching in water at 1050 degrees & Scy; causes the release of a larger amount of dispersed silicides and, as a result, the formation of the alpha ''- phase depleted with alloying elements and the residual beta- phase. The resulting structure provides a better combination of mechanical properties (sigma(B) = 1165 MPa, delta = 12.5 % ) due to dispersion strengthening with silicides and increased plasticity of the solid solution. For deformed experimental Ti-(18-20)Nb-(1-1.2)Si alloys, the quenching temperature & Tcy; = 1080 +/- 10 degrees & Scy; is also determined that allows obtaining the maximum strength sigma(B) = 1190 MPa, while maintaining plasticity at the level of 8 = 9.5 %.