CREEP DEFORMATION AND CREEP MICROSTRUCTURES OF A NEAR GAMMA-TIAL ALLOY TI-48AL-2CR

The creep deformation and microstructures of a Ti-48Al-2Cr (at.%) alloy consisting of two-phase TiAl/Ti3Al lamellar grains surrounded by single-phase gamma-TiAl grains were investigated at 800-degrees-C. Tension and compression creep curves show no steady state creep rate, rather a minimum creep rate was obtained followed by a steep increase in the creep rate due to dynamic recrystallization of the single-phase gamma grains. The stress exponent of 7.6 obtained in the stress range investigated is quite high in comparison with exponents obtained for dislocation creep mechanisms, and is discussed with respect to the values found in the literature for single- and dual-phase gamma-base alloys, and in terms of the composite-like behaviour of the microstructure investigated here. The microstructures associated with creep show that the gamma lamellae and the single-phase gamma regions are generally characterized by different creep deformation structures. The minimum creep range is associated with the formation of subgrains in the single-phase regions, where the subgrain boundaries consist of arrays of 1/2[110] dislocations in near edge orientation and of long 1/2[110] screw segments, whereas the gamma lamellae deform by twinning across the lamellar boundaries, and exhibit tangles of screw and curved 1/2[110] dislocations. In both structures the contribution of superdislocations to creep seems to be marginal. The loss of creep strength during tertiary creep was found to be associated with dynamic recrystallization of the single-phase gamma regions and spheroidization of the Ti3Al laths.