Modification of the jogged-screw model for creep of gamma-TiAl

During the high-temperature creep of the gamma-phase (L1(0) structure) of a "near-gamma" Ti-48Al microstructure, observations using transmission elect ron microscopy indicate that a/2(110] or "unit" dislocation activity is a d ominant deformation mode. These unit dislocations tend to be elongated alon g the screw orientation, and exhibit a large number of localized pinning po ints. Tilting experiments demonstrate that these pinning points are associa ted with jogs on the screw dislocations, suggesting that the jogged-screw m odel for creep should be appropriate in this case. However, it is shown tha t in its conventional formulation, the jogged-screw model is not capable of reproducing the measured creep response (i.e. stress exponents or absolute creep rates). Microscopic observations also demonstrate that a spectrum of jog heights are present, with some as large as 40 nm, based on present obs ervations. A modification of the jogged-screw model is proposed in which th e average jog height is assumed to depend on stress. This modified model re sults in good agreement between predicted and measured creep rates while us ing reasonable model parameters. Additional implications of the model and r equired experiments to further validate the model are also discussed. (C) 1 999 Published by Elsevier Science Ltd on behalf of Acta Metallurgica Inc. A ll rights reserved.