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

Hard oriented PST crystals of a Ti-48 mol% At alloy, in which the stress axis is parallel or normal to lamellar boundaries, were creep tested. The lamellar structures contain gamma/alpha(2) boundaries and three types of gamma/gamma 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. gamma/gamma lamellar boundaries migrate and coalesce with other gamma/gamma boundaries and alpha(2) 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 gamma/alpha(2) lamellar boundaries is recommended for retarding the coarsening and the spheroidization, and thereby making a thermally stable lamellar structure. The high density of gamma/alpha(2) boundaries is confirmed experimentally to provide good creep resistance. (C) 2004 Elsevier B.V. All rights reserved.