In Situ Study of γ-TiAl Lamellae Formation in Supersaturated α2-Ti3Al Grains

In situ heating transmission electron microscopy (TEM) was used to investigate the initial stage of gamma-TiAl lamellae formation in an intermetallic Ti45Al7.5Nb alloy (in at.%). The material was heat treated and quenched in a non-equilibrium state to consist mainly of supersaturated, ordered a2-Ti3Al grains. Subsequently, specimens were annealed inside a TEM up to 750 degrees C. The in situ TEM study revealed that ultra-fine gamma-TiAl laths precipitate in the a2-matrix at approximate to 730 degrees C which exhibit the classical Blackburn orientation relationship, i.e. (0001 alpha 2//(111)gamma and [$11{\bar {2}}0$]alpha 2//<110]gamma. The microstructural development observed in the in situ TEM experiment is compared to results from conventional ex situ TEM studies. In order to investigate the precipitation behavior of the ?-phase with a complementary method, in situ high energy X-ray diffraction experiments were performed which confirmed the finding that gamma-laths start to precipitate at approximate to 730 degrees C from the supersaturated a2-matrix.