Development and microstructure of the Al-depleted layer of oxidized TiAl

The microstructure and development of the scale of oxidized gamma-TiAl were studied in cross section by electron microscopy and metallographic techniques. Ti-50(at.%)Al samples were oxidized at 900 degrees C in air for various times and especially the evolution of the scale/metal interface was investigated. The formation of the aluminium depleted subsurface layer could be observed after 1 h and develops from a single phase to a two phase region with longer exposure times The single phase region is visible up to 100 h and consists of a simple cubic phase with lattice parameter a = 0.692 nm. After an oxidation time of 140 h the formation of a two phase region consisting of alpha(2)-Ti3Al and the cubic phase occurred. Both phases are depleted in aluminium compared to the base metal and contain some amount of oxygen whereby the cubic phase contains more Al and O than alpha(2)-Ti3Al. The metallographic cross sections suggest that the alpha(2)-Ti3Al phase forms at the gamma-TiAl/cubic-phase interface and grows into the cubic phase.