| Titanium aluminides are of interest as a candidate material for aerospace turbine applications due to their high strength to weight ratio. γ-TiAl + α2-Ti3Al alloys have recently been incorporated in the low pressure turbine region but their loss of strength near 750C limits their high temperature use. Additions of Nb have been shown to have several beneficial effects in γ+α2 alloys, including enhancements in strength and ductility of the γ-phase, along with the stabilization of the cubic BCC β-phase at forging temperatures allowing for thermomechanical processing.;In the ternary Ti-Al-Nb system at high Nb-contents above approximately 10at%, there exists a two-phase γ-TiAl + σ-Nb2Al region at and above current service temperature for the target application. Limited research has been conducted on the mechanical properties of alloys with this microstructure, though they have demonstrated excellent high temperature strength, superior to that of γ+α2 alloys. Because the σ-phase does not deform at room temperature, high volume fractions of this phase result in poor toughness and no tensile elongation. Controlling the microstructural morphology by disconnecting the brittle matrix through heat treatments has improved the toughness at room temperature.;In this study, attempts to further improve the mechanical properties of these alloys were undertaken by reducing the volume fraction of the σ-phase and controlling the scale of the γ+σ microstructure through the aging of a meta-stable parent phase, the β- phase, that was quenched-in to room temperature. Additions of β-stabilizing elements, Cr and Mo, were needed in order to quench-in the β-phase. The room temperature mechanical properties were evaluated by compression, Vickers’ indentation and single edge notch bend tests at room temperature. The formation of the large γ-laths at prior β- phase grain boundaries was found to be detrimental to ductility due to strain localization in this coarsened region of the microstructure. Furthermore, samples aged from β- phase single crystals proved to have excellent combinations of strength and ductility under compression. In the single crystals, microcracking did not develop until much larger plastic strains were reached. Lowering the volume fraction of the σ-phase proved to enhance the fracture toughness in these alloys. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html). |
