Interpretation of the morphology and crystallography of precipitate γ1-Ti4Nb3Al9 in L10-TiAl(Nb)-based intermetallics by invariant line theory

The coherent precipitate γ1-Ti4Nb3Al9 in the Nb-modified TiAl-based intermetallic compound with a nominal composition of Ti–48 at% Al–10 at% Nb was investigated by transmission electron microscopy. The crystallographic and morphologic characteristics of the precipitating phase have been calculated based on invariant line theory. It is revealed that in the γ-TiAl(Nb)/γ1-Ti4Nb3Al9 couples, both the morphological and crystallographical characteristics of the precipitate γ1-Ti4Nb3Al9 could be predicted by the three-dimensional phase transformation invariant line model. The needle direction of [001] at the early stage for coherent precipitate γ1-Ti4Nb3Al9 and [111] for equilibrium phase γ1-Ti4Nb3Al9 have both been explained well. The characterization of morphology and crystallography of the precipitate reaction in γ-TiAl/γ1-Ti4Nb3Al9 system sustain the postulate that precipitates are bounded by unrotated planes (eigenplanes) when three real eigenvalues exist.