Structure and stability of a new ternary hexagonal phase in Al₃Ti-Based Al-Ti-V alloys

A novel hexagonal phase (designated H₍₂₎) has been detected as a major constituent of both Al₆₂Ti₁₀V₂₈ and Al₅₅Ti₁₀V₃₅ alloys, following chill casting and after homogenization at 1523 K. The phase H₂ has an ordered hexagonal crystal structure (space group P6₃/mmc, α = 0.558 ± 0.001 and c = 0.450 ± 0.001 nm), similar to that of α₂-Ti₃Al, and an atomic composition of 54 ± 1 Al - 11 ± 1 Ti - 35 ± 1 V in the chill-cast Al₅₅Ti₁₀V₃₅ alloy. A fine-scale, duplex lamellar structure, developed within the ordered H₂ phase in the solid state, was composed of parallel-sided multivariants of ζ-Ti₅Al₁₁ phase, formed parallel to (0001)_H2. The orientation relationship between constituent phases was of the form (111)_ξ ∥ (0001)_H2 and <110>_ξ ∥ [112̄0]_H2. Following homogenization (1 hour at 1523 K) and isothermal heat treatment at temperatures in the range from 1073 and 1273 K, the Al₆₂Ti₁₀V₂₈ and Al₅₅Ti₁₀V₃₅ alloys were observed to develop two-phase microstructures comprising β-(Ti,V)/δ-Al₃(Ti,V) and β/ξ-Ti₅Al₁₁, respectively. In both alloys, the β phase could be retained as a stable phase to room temperature, regardless of cooling rate, whereas the H₂ phase was unstable at temperature below ∼1273 K and was consumed by the formation of ξ phase at lower temperatures.