Microstructure evolution during annealing of an amorphous TiAl sheet

An amorphous, 150 mum 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 (beta) --> hexagonal close-packed (alpha)?--> tetragonal (gamma) + ordered alpha(2). The beta 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 a phase by decomposition of a 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 a phase particles were poorly defined. The transformation of the metastable alpha phase into a mixture of the gamma and alpha(2) phases led to formation of an equiaxed gamma-grain structure with the grain size of approximately 150 nm. (C) 2002 Elsevier Science B.V. All rights reserved.