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Abstract

The understanding of creep related microstructural deformation mechanisms and their interactions depending on different microstructures plays an important role for the science and technology of gamma titanium aluminides. In the present study, we characterize the creep behavior of two modern high Nb containing gamma titanium alloys (TNB) in terms of apparent activation energies and stress exponents. Two alloys of composition Ti45Al8Nb0.2C and Ti45Al5Nb0.2B0.2C with nearly lamellar and finegrained duplex microstructures, respectively, were deformed by constant load tensile creep experiments at different nominal stresses in the 700900°C temperature range. The initial microstructures and several crept conditions were analyzed using scanning electron microscopy, electron back scatter diffraction measurements, and transmission electron microscopy (TEM). For both alloys, the TEM analysis after creep reveals that dislocation creep and twin activity (classical creep mechanisms in TiAl) are prominent deformation mechanisms in the gamma phase of lamellar colonies. In contrast, only few deformation substructures are found in the equiaxial gamma grains of the duplex alloy. These results, and additional surface observations of tracer lines produced by a complementary preparation technique utilizing focused ion beams, indicate that the fine equiaxial gamma grains (grain size of about 2.5 μm) deform by grain boundary sliding. The overall deformation mechanism under creep conditions represents a combination of classical mechanisms in the lamellar colonies and grain boundary sliding in finegrained gamma regions.