Dendrite core grain refining and interdendritic coarsening behaviour in W-containing γ-TiAl based alloys

Two W-containing γ-TiAl based alloys were studied in as-cast ingot condition. Pronounced partitioning of heavy-metals–light-metals were found to occur, causing segregation of W and Nb into dendrite core and Al in interdendritic region. The average Al content in α2 + γ lamellae was close to 46% in interdendritic region, and below 44% in dendrite cores, regardless of the nominal Al content of the alloy. About 15% coarse colonies formed in alloy 44Al and 60% in alloy 46Al. Effective grain refinement can be achieved in the dendrite core where both the Al depletion and Nb, W and Ti enrichment ensured a L → β solidification route. In contrast, the solidification in interdendritic regions involves a peritectic reaction L + β → αp. As a result, unusually large αp grains (α2 + γ lamellar colonies) formed in the interdendritic regions.

Geometric features observed in the grain refined regions of Nb, W-containing TiAl alloys: (a) a two dimensional equilateral triangle based on the (11¯1) basal plane of the Thompson reference tetrahedron. (b) BSE micrograph of alloy 44Al showing the similar geometric features in a group of lamellar colonies.Figure optionsDownload as PowerPoint slideHighlights
► Two W-containing γ-TiAl based alloys were studied in as-cast ingot condition.
► Segregation of W, Nb in dendrite core and Al in interdendritic region were found.
► The large αp grains (α2 + γ lamellar colonies) formed in the interdendritic regions.