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

Discontinuous coarsening (DC) of the primary lamellar structure (PLS) occur ring 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) terna ry alloys was systematically investigated by using optical microscopy and s canning and transmission electron microscopy. The compositions of the alpha (2) and gamma phases in the primary lamellar structure were estimated base d on the weight fractions of the two phases, determined by X-ray diffractio n. When the solution-treated Ti-(40 to 45) at. pet Al binary alloys were su bsequently soaked at 1000 degreesC, the primary lamellae in the Ti-40 at. p et 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 th e reduction of the chemical free energy (i.e., out-of-equilibrium chemical composition) and the interfacial energy of primary lamellae, whereas the co arsening resistance is mainly from the increase of the elastic strain energ y 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 thi s improvement is uncertain now. Based on this study, a proposal is finally addressed to improve the thermal stability of the primary lamellar structur e of titanium aluminides.