On an extraordinarily high strength of a two-phase VT8-based titanium alloy heavy alloyed with zirconium

Abstract

A novel two-phase titanium alloy based on VT8 (Ti-6.5Al-3.3Mo-0.3Si) having the composition VT8-20Zr-0.1B (wt. %) has been considered in the present paper. It has been established that modifying with boron leads to refinement of prior β grains and α / β colonies by a factor of about ten and alloying with zirconium results in some additional colony refinement. Alloying with 20 wt. % of zirconium leads to decreasing the α + β → β transformation temperature by about 100°C that along with refinement of the as-cast structure improved the hot workability of the alloy. After multidirectional hot forging in the α + β phase field (T = 800°С) and hardening heat treatment the strength of the ВТ8-20Zr-0.1B alloy at 20-500°C was found to be higher by 30-40 % as compared with that of the VT8 alloy after the same treatment while retaining similar ductility of the alloys. This allows one to characterize the VT8-20Zr-0.1B alloy as an ultrastrong titanium alloy. For the VT8-20Zr-0.1B alloy the following tensile properties have been attained: σ UTS = 1560 MPa and δ = 4 % at room temperature, σ UTS = 1230 MPa and δ = 14 % at T = 500°С. The specific strength reached at T = 500°С (σ UTS / ρ ≈ 248 MPa / g × cm −3) is the highest even obtained at this temperature for currently known titanium alloys. It was revealed that water quenching from the β phase field led to the martensitic β → α" transformation with retained β phase and subsequent ageing resulted in formation of ultrafine lamellar structure with a nanosized lamellae thickness that promoted achieving significant strengthening.