Microstructure evolution during annealing of an amorphous TiAl sheet

An amorphous, 150 μm thick freestanding sheet of a TiAl-based alloy was produced by physical vapor deposition (PVD). The following phase transformations were observed after different stages of crystallization of the amorphous sheet and analyzed using differential thermal analysis, X-ray diffraction, and transmission electron microscopy: amorphous→body centered cubic (β)→hexagonal close-packed (α)?→tetragonal (γ)+ordered α2. The β phase was formed as near-spherical particles that were evenly distributed in the amorphous phase and the size of these particles was approximately 90 nm. Formation of the α phase by decomposition of β and the remaining amorphous phases led to a very fine feathery-like microstructure arranged in colonies of approximately 100 nm in size. Interface boundaries between the α phase particles were poorly defined. The transformation of the metastable α phase into a mixture of the γ and α2 phases led to formation of an equiaxed γ-grain structure with the grain size of approximately 150 nm.