Creep of lamellar TiAl alloys: degradation, stabilization and design of lamellar boundaries

Hard oriented PST crystals of a Ti–48 mol% Al alloy, in which the stress axis is parallel or normal to lamellar boundaries, were creep tested. The lamellar structures contain γ/α2 boundaries and three types of γ/γ boundaries. Changes of lamellar structures in the PST crystals were examined during high-temperature exposure with (creep) or without applied stress (annealing). Coarsening and spheroidization of the lamellar structures are typical degradation processes observed experimentally. γ/γ lamellar boundaries migrate and coalesce with other γ/γ boundaries and α2 lamellae dissolve during the high-temperature exposure. These events result in the coarsening and the spheroidization. Thermal stability of each lamellar boundary depends on its boundary type. A higher density of γ/α2 lamellar boundaries is recommended for retarding the coarsening and the spheroidization, and thereby making a thermally stable lamellar structure. The high density of γ/α2 boundaries is confirmed experimentally to provide good creep resistance.