Creep deformation in two-phase titanium aluminide alloys

The paper presents an electron microscope study of diffusion assisted creep processes in a two-phase (alpha(2) + gamma) titanium aluminide alloy, which had been subjected to long-term creep. The results imply that the high primary creep rate of lamellar TiAl alloys is associated with the relaxation of mismatch structures and coherency stresses present at the interfaces. Long-term creep leads to spheroidization and coarsening of the lamellar morphology, which involve phase transformations and recrystallization. The mechanisms of these morphological changes are closely related to the atomic structure of the alpha(2)'gamma phase boundaries and probably driven by a non-equilibrium of the phase composition leading to the dissolution of the alpha(2) phase. After long-term creep the formation of precipitates was observed, which has been attributed to the alpha(2) --> gamma transformation, because alpha(2) has a significantly higher solubility for interstitial impurities than the gamma phase. (C) 2002 Elsevier Science B.V. All rights reserved.