Formation of lamellar microstructures in Al-rich TiAl alloys between 900 and 1100°C

The formation of lamellar microstructures in a Ti–62 at.% Al alloy at temperatures of 900–1100°C was investigated by transmission electron microscopy (TEM). The lamellar structure consists of γ-TiAl+r-Al2Ti in the equilibrium state. h-Al2Ti and Al5Ti3 are observed in the as-cast alloy, but Al5Ti3 transforms into γ-TiAl above 900°C. The phase h-Al2Ti transforms into r-Al2Ti by two alternative processes, i.e. by continuous or discontinuous transformation. While the first process is comparably sluggish, areas generated by the second process enlarge quickly, leading to the formation of a fine lamellar microstructure of γ-TiAl and r-Al2Ti. This fine lamellar microstructure coarsens through lamellar fault or/and grain boundary migration during long time annealing. In the equilibrium state, the fully coarsened lamellar microstructure contains regular misfit dislocations, i.e. two types of [110] edge dislocations in the (001) planes at the interlamellar boundaries.