MICROSTRUCTURAL CHARACTERIZATION OF TWIN-ROLL CAST GAMMA-TITANIUM ALUMINIDE SHEETS

As an effective route for near-net-shaping of sheets of poor workability materials, twin-roll casting is applied to production of gamma titanium aluminide sheets. An expermental process has been established for shaping of flat 100-mm wide sheets with a uniform thickness in a range of 1 to 2 mm. Changes in parameters controlling the rate of solidification in twin-roll casting provide a fairly wide variety of microstructures in a stoichiometric gamma titanium aluminide alloy. The microstructures are characterized with special attention to their through-the-thickness variations for understanding of the path of solidification and solid state transformation in direct sheet casting. In the subsurface layer of cast sheets a primary alpha phase forms from liquid and the alpha dendrites grow along the heat flow to form a sharp preferred orientation with the <0001>-alpha direction parallel to the sheet normal. Subsequently the alpha grains transform into an extensively refined (gamma + alpha-2) structure and develop a transformation texture of <111>-gamma fiber parallel to the sheet normal with an orientation relationship of {0001}-alpha parallel-to {111}-gamma. In the intermediate layer gamma single phase increases in relative fraction and evolves a growth texture of [001]-gamma parallel to the sheet normal. In the dendrite and interdendritic gamma interfaces, gamma grains accomodate thermal stresses generated during cooling. Toward the central zone the growth directions of dendrite deviate from the sheet normal with increasing depth below the surface as a reflection of meniscus profile. In the central zone shrinkage cavities are formed in the absence of liquid metal on solidification. Based on these findings metallurgical guides for further improvements in casting procedure and for controlling of cast structure are discussed.