CHARACTERIZING FAULTED DIPOLES IN DEFORMED GAMMA-TIAL

The faulted dipoles that are associated with the low temperature defor mation of gamma TiAl have been characterized with conventional high re solution and in situ heating transmission electron microscopy. The res ults of this analysis indicate that the formation of these faulted dip oles involves the localized pinning of a superdislocation, the bypassi ng of that pinning point and the drawing out of a dipole whose energy is subsequently reduced by the passage of partial dislocations and the formation of an extrinsic stacking fault. The extrinsic nature of the stacking fault was determined through comparisons with computer gener ated image simulations and verified by atomic resolution lattice image s. These faults are bounded by either single or double Shockley partia l dislocations, but the latter is a rare occurrence and the former was identified to be most relevant to the mechanical behaviour of this al loy. The low temperature stability of this defect has been attributed to the ordered structure of TiAl, and the high temperature instability of the faulted dipoles has been related to a localized reordering of the crystal structure by diffusive processes.