Discontinuous coarsening of the lamellar structure of γ-TiAl-based intermetallic alloys and its control

Discontinuous coarsening (DC) of the primary lamellar structure (PLS) occurring at lamellar colony boundaries (LCBs) and in surface layers of various Ti-(40 to 45) at. pct Al binary and Ti-46 at. pet Al-X (X = Si and C) ternary alloys was systematically investigated by using optical microscopy and scanning and transmission electron microscopy. The compositions of the alpha (2) and gamma phases in the primary lamellar structure were estimated based on the weight fractions of the two phases, determined by X-ray diffraction. When the solution-treated Ti-(40 to 45) at. pet Al binary alloys were subsequently soaked at 1000 degreesC, the primary lamellae in the Ti-40 at. pet Al alloy were the most stable, while those in the Ti-44 at. pct Al were the most unstable. Both the thermodynamic analysis and experimental results confirm that the driving force of the coarsening is mainly derived from the reduction of the chemical free energy (i.e., out-of-equilibrium chemical composition) and the interfacial energy of primary lamellae, whereas the coarsening resistance is mainly from the increase of the elastic strain energy of lamellar interfaces and the surrounding during coarsening. It is found that Si has an exceptional ability to hinder the coarsening of the primary lamellar structure at high temperatures, but the precise mechanism for this improvement is uncertain now. Based on this study, a proposal is finally addressed to improve the thermal stability of the primary lamellar structure of titanium aluminides.