Finite element models of creep deformation and cavitation in TiAl intermetallic alloys

TiAl intermetallics are finding growing applications in the aerospace, power generation and automotive industry due to their superior mechanical properties over the conventional superalloys. Among all the forms of TiAl alloys, single phase {dollar}gamma{dollar} and dual phase ({dollar}alphasb2+gamma{dollar}) fully lamellar forms possess attractive high temperature mechanical properties. Creep deformation and cavitation damage in single phase {dollar}gamma{dollar}, dual phase ({dollar}alphasb2+gamma{dollar}) equiaxed and fully lamellar microstructures have been simulated and analyzed in the present work using finite element techniques. Different models of these microstructures have been developed depending on the different deformation mechanisms active in the microstructures such as grain boundary sliding and phase boundary sliding. Nonlinear viscous primary creep deformation has been modeled in each phase based on published creep data. Overall strain rates are compared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered. Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative susceptibility to creep damage.; Creep constrained grain boundary cavitation is also analyzed for these microstructures. The cavitation in the models is based on the modified equations of Rice and Needleman. It was found that grain boundary sliding strongly enhances the cavity growth in all of the models analyzed. Comparisons of the results for different lath configurations suggest that lath width has little effect on the constraints that govern cavity growth. A study of effect of cavitating facet density on cavitation damage has been studied for the fully lamellar microstructures. Approximate techniques of Hutchinson and Riedel and Eggeler for strain rate enhancement during creep cavitation due to increase in cavitating facet density have been compared using the finite element results as the reference. Interaction between cavitating facets in the fully lamellar TiAl has also been studied. It was found that when the cavitating facets are not on neighboring grain boundaries, a negative interaction effect takes place in the presence of grain boundary sliding.; The present results suggest that the longer creep life observed experimentally for the fully lamellar structure is primarily due to inhibited former grain boundary sliding in this microstructure compared to relatively unimpeded grain boundary sliding in the equiaxed microstructures. The serrated nature of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding.