NUMERICAL-MODELS OF CREEP AND BOUNDARY SLIDING MECHANISMS IN SINGLE-PHASE, DUAL-PHASE, AND FULLY LAMELLAR TITANIUM ALUMINIDE

Finite element simulations of the high-temperature behavior of single- phase gamma, dual-phase alpha(2) + gamma, and fully lamellar (FL) alph a(2) + gamma TiA1 intermetallic alloy microstructures have been perfor med. Nonlinear viscous primary creep deformation is modeled in each ph ase based on published creep data. Models were also developed that inc orporate grain boundary and lath boundary sliding in addition to the d islocation creep flow within each phase. Overall strain rates are comp ared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the micr ostructures considered. Facet stress enhancement factors were also det ermined for the transverse grain facets in each model to examine the r elative susceptibility to creep damage. The results indicate that a me chanism for unrestricted sliding of gamma lath boundaries theorized by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater creep strength obs erved experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary sliding (GBS) in this microstructur e compared to relatively unimpeded GBS in the equiaxed microstructures . The serrated nature of the former grain boundaries generally observe d for lamellar TiAl alloys is consistent with this finding.