A TEM study of deformation processes and microstructural changes during long-term tension creep of a two-phase gamma-titanium aluminide alloy

Long-term tension creep tests were performed on a Ti-48 at.% Al-2 at.% Cr a lloy in order to assess the material behaviour under the intended service c onditions for structural parts in turbine engines. Deformation processes an d microstructural changes were investigated by TEM on a specimen loaded to 140 MPa for 5988 h at 700 degrees C. At lamellar boundaries the emission of interfacial dislocations was observed and was thought to contribute signif icantly to the high primary creep rate of the material. Under the applied c reep conditions gliding dislocations apparently become locked by heterogene ous formation of precipitates along their cores. Also substantial microstru ctural changes were observed in lamellar colonies. These are the dissolutio n of at lamellae and the formation of ledges at lamellar interfaces with he ights up to 200 nm. By ledge formation and growth the interfaces migrate, w hich contributes to creep deformation. At ledges recrystallized grains were also formed contributing to the degradation of the lamellar microstructure . (C) 1999 Elsevier Science Ltd. All rights reserved.