Effect of rare earth additions the microstructure of TiAl alloys

The rare earth elements Ce and Gd were added to binary Ti-44 at% Al alloys by are melting. The elements were added individually in the range of 0.05 to 0.3 atomic per cent. It was observed that the as-cast structures of the ingots consisted of the usual three zones of fine equiaxed grains near the mould wall, columnar grains growing away from the mould wall and coarse equiaxed grains near the centre. The ingots were subsequently heat treated to obtain well-defined fully lamellar structures. The materials containing no rare earth displayed large equiaxed lamellar colonies of average size of >1400 mu m; the materials with additions of the rare earth elements consisted of lamellar colonies of as fine as similar to 350 mu m. In combination with hot pressing, a Ti-44 at% Al-0.15 at% Gd alloy possessed a finer fully lamellar structure of colony sizes as small as similar to 140 mu m.