Ordered ω phases in a 4Zr–4Nb-containing TiAl-based alloy

A considerable amount of B2 phase with a cellular morphology is retained in a 4Zr–4Nb-containing TiAl-based alloy. Heterogeneous precipitation of ordered ω from B2 is found to occur readily after HIPping: B2 → ω with the B82-structure in cell regions and B2 → ω with the D88-structure in cell-wall regions. Congregated D88-ω domains and particles form as a network surrounding the well-developed B82-ω cells. The heterogeneous formation of different ω variants is caused by a heterogeneous distribution of Zr + Nb elements across B2, which plays an important role in stabilizing vacancies and promotes the formation of D88-ω. Fine D88-ω particles are also observed to precipitate from the B82-ω cell matrix after ageing at 700 °C for 1000 h, showing a transformation path of β → B2 → B82-ω → D88-ω for the aged cells. The heterogeneous formation of a D88-ω network and B82-ω cells is found to be detrimental to ductility and fatigue strength. A very brittle fine-grained TiAl alloy is produced as a result.