A QUANTITATIVE TEM ANALYSIS OF THE LAMELLAR MICROSTRUCTURE IN TIAL BASED ALLOYS

The microstructure of lamellar TiAl alloys was analysed by transmissio n electron microscopy. Two alloys (a PST crystal and a Ti-54-Al-46 all oy) were investigated. The microstructure is formed by al and gamma la mellae which are very long and very thin with respect to their length. The proportion of gamma lamellae is higher than that of alpha(2) lame llae. The orientation relationships associated with the gamma-gamma in terfaces are examined: they correspond to the order domain, twin and p seudo-twin relationships. Statistical analyses have been performed on the volume fraction of the two phases, on the orientation of the gamma lamellae and on the nature of the gamma-gamma interfaces. These resul ts are used to have a better understanding of the structural transform ation (hexagonal phase - > face centred cubic phase) and the ordering process which are active in the process of formation of the microstruc ture during cooling. (C) 1997 Acta Metallurgica Inc.