Creep behaviour of a cast γ-TiAl based alloy for gas turbine applications

The creep behaviour of the gamma-TiAl based alloy Ti-48A1-2W-0.5Si (at.-%) has been investigated in the stress range 80 - 325 MPa at temperatures from 700 to 850 degrees C. The material was produced by casting, followed by hot isostatic pressing and two successive heat treatments. Variations in the nominal compositions allowed both nearly lamellar (gamma + alpha(2)) as well as duplex (gamma + lamellar) microstructures to develop. The nearly lamellar microstructure exhibits better creep resistance than the duplex material at lower temperatures/higher stresses, whereas both microstructures have similar creep behaviour at higher temperatures/lower stresses. For all conditions the creep rate in regime II passes a minimum at fairly small creep strains followed by gradually increasing creep rates until the onset of tertiary creep in regime III. The tertiary creep takes up a relatively large part of the whole creep life-time. The creep behaviour preceding final fracture depends on nucleation and growth of cavities as well as on microstructural instability.