Abstract
The effect of solid-state phase transformations on grain refinement of cast peritectic TiAl-based alloy with nominal composition Ti-47Al-5Nb-0.2B-0.2C (at.%) has been investigated. Three multi-stage heat treatment routes consisting of pretreatment in single α phase field followed by air, oil or water cooling and subsequent annealing in α + γ or single α fields were applied to refine grain structure of the alloy. The microstructure of the as-cast samples consists of coarse fully lamellar α₂(Ti₃Al) + γ(TiAl) grains with a grain size of 1310 µm. The annealing of the pretreated samples consisting of massive γ_M and fine lamellar α₂ + γ grains in the α + γ phase field leads to the formation of a duplex type of microstructure composed of lamellar α₂ + γ and single γ phase grains with a grain size ranging from 14 to 22 µm. The solution annealing in the single α phase field of the pretreated samples aged in the α + γ phase field leads to the formation of fully lamellar α₂ + γ grains with a grain size ranging from 395 to 472 µm.

Key words
titanium aluminides, TiAl, heat treatment, grain refinement, phase transformation, microstructure

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