Microstructure, processing and mechanical properties of a titanium alloy Ti-20Zr-6.5Al-3.3Mo-0.3Si-0.1B

A novel two-phase titanium alloy based on Ti-6.5Al-3.3Mo-0.3Si (VT8) (wt%) having the composition VT8-20Zr-0.1B has been studied in the present paper. Modifying with boron and alloying with zirconium led to refinement of prior β grains and α/β colonies by a factor of about ten. Alloying with 20 wt% of zirconium resulted in significant decreasing the β-transus temperature that along with the refinement of the as-cast structure facilitated ingot breakdown and formation of fine grained structure during forging. After multidirectional hot forging and hardening heat treatment the strength of the VT8-20Zr-0.1B alloy at 20-500 °C was found to be higher by 30-40% as compared with that of the VT8 alloy after similar treatment while retaining near the same ductility of the alloys. For the VT8-20Zr-0.1B alloy with the duplex structure the following tensile properties have been attained: σUTS/σ0.2=1560/1400 MPa and δ=4.2% at room temperature, σUTS/σ0.2=1230/1090 MPa and δ=14% at Т=500 °С. The density normalized strength reached at Т=500 °С is the highest even obtained at this temperature for currently known titanium alloys. The novel alloy in the fine lamellar condition showed quite reasonable creep resistance at temperatures near the maximum service temperature. Solution treatment in the β or the α+β temperature field followed by water quenching was revealed to result in the martensitic β→α′′ transformation with retained β phase. Subsequent ageing led to formation of fine lamellar or duplex structure with nanosized lamellar spacings that in turn promoted achieving extraordinary high strength properties.