Effect of microstructure on

Good candidates to replace the conventional titanium alloys and nickel-based superalloys currently employed in gas turbine engines are the titanium aluminides. These ordered structures are attractive because of their good modulus retention at high temperatures, low densities, and improved creep resistance. The purpose of this research was to investigate how the micro- structure of the titanium aluminide, Ti-24Al-11Nb (at. pct), influences the creep rate. Study of the stress dependence of the steady-state creep rate has shown that microstructures with a coarse α₂ lath size, in general, display improved creep resistance over fine-lathed structures. Specimens with an equiaxed α₂-phase microstructure exhibit much greater creep rates at all stresses and temperatures, implying that dislocation slip length, proportional to α₂ phase dimension, does not always control creep in these materials and that texture may adversely affect creep resis- tance. When microstructures and dislocation substructures were investigated by transmission electron microscopy (TEM), observations of crept specimens indicated that the majority of dis- locations have a-type Burgers vectors and are located in the α₂ phase. Formerly Graduate Student, Carnegie Mellon University