Initial Stages of Lamellae Formation in High Nb Containing gamma-TiAl Based Alloys

In this study a two-step heat treatment was applied to high Nb containing gamma-TiAl based alloys in order to investigate the initial stage of the lamellae formation in ordered alpha(2)-grains as well as in massively transformed gamma(m) grains. The first heat treatment step, conducted in the single alpha-phase field followed by oil quenching, leads to a microstructure consisting of supersaturated alpha(2)-grains and a small volume fraction of gamma(m)-grains. The second step of the heat treatment was performed below the eutectoid temperature, i.e. within the (alpha(2)+gamma)-phase field region and was again followed by oil quenching. There, the formation of ultra-fine gamma-lamellae takes place in the alpha(2)-grains and (some) fine alpha(2)-lamellae are formed in the gamma(m)-grains. In both cases the lamellae show a Blackburn orientation relationship with the matrix grain. It was found that the precipitation of gamma-laths in the supersaturated alpha(2)-grains is faster than the formation of alpha(2)-laths in gamma(m) grains. The characteristics of the initial stage of formation were investigated by transmission electron microscopy.