MODIFICATION OF THE JOGGED-SCREW MODEL FOR CREEP OF γ-TiAl

摘要

During the high-temperature creep of the γ-phase (Ll_0 structure) of a “near-gamma” Ti-48Al microstructure, observations using transmission electron microscopy indicate that a/2〈110] or “unit” dislo- cation activity is a dominant deformation mode. These unit dislocations tend to be elongated along the screw orientation, and exhibit a large number of localized pinning points. Tilting experiments demonstrate that these pinning points are associated with jogs on the screw dislocations, suggesting that the jogged- screw model for creep should be appropriate in this case. However, it is shown that in its conventional for- mulation, 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 observations. A modification of the jogged-screw model is proposed in which the average jog height is assumed to depend on stress. This modi- fied model results in good agreement between predicted and measured creep rates while using reasonable model parameters. Additional implications of the model and required experiments to further validate the model are also discussed.