Microstructural evolution during creep of a duplex near-γ TiAl-Alloy

The present work studies the evolution of dislocation density in γ-grains during creep of a duplex near-γ TiAl-alloy, with 80 vol.% of γ-grains and 20 vol.% of lamellar grains. The paper briefly reviews earlier results on mechanical twinning and on dynamic recrystallisation, which both were shown to start contributing to high temperature deformation early on in the creep process. It is found that the density of non-c-component-dislocations in γ-grains steadily increases with strain during primary creep and reaches a saturation value at high strains under creep conditions (σ=255 MPa, T=750°C). In contrast, the density of c-component-dislocations remains low throughout the creep process. Overall strain compatibility requires microscopic deformation in the c-direction of the L10-lattice. The results of the present study and of earlier work briefly reviewed in this paper suggest that creep is governed by the coupling of three elementary deformation processes: glide and climb of non-c-component-dislocations, mechanical twinning and continuous dynamic recrystallisation. In this scenario, mechanical twinning and dynamic recrystallisation provide coarse- and fine-tuning with respect to overall strain compatibility.