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 tu from B2 is found to occur readily after HIPping: B2 -> omega with the B8(2)-structure in cell regions and B2 -> omega with the D8(8)-structure in cell-wall regions. Congregated D8(8)-omega) domains and particles form as a network surrounding the well-developed B8(2)-omega) cells. The heterogeneous formation of different co 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 D8(8)-omega. Fine D8(8)-omega particles are also observed to precipitate from the B8(2)-omega cell matrix after ageing at 700 degrees C for 1000 h, showing a transformation path of beta -> B2 -> B8(2)-omega -> D8(8)-omega for the aged cells. The heterogeneous formation of a D8(8)-omega network and B8(2)-omega cells is found to be detrimental to ductility and fatigue strength. A very brittle fine-grained TiAl alloy is produced as a result. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.