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.