The effect of molybdenum on the physical and mechanical metallurgy of advanced titanium-aluminide alloys and metal matrix composites

This dissertation represents a systematic study of microstructure-mechanical property relationships of titanium-aluminum-niobium-molybdenum (Ti-Al-Nb-Mo) alloys and metal matrix composites (MMCs). The aspects investigated were the microstructures, elevated-temperature creep behavior, room-temperature and elevated-temperature tensile behavior, and the out-of-phase thermomechanical fatigue behavior. The specific alloy compositions investigated were: Ti-24Al-17Nb-0.66Mo (at.%) and Ti-24Al-17Nb-2.3Mo (at.%). The MMCs were reinforced with Ultra SCS-6 fibers and the specific compositions of the matrices were: Ti-24Al-17Nb-0.66Mo (at.%), Ti-24Al-17Nb-1.1Mo (at.%), and Ti-24Al-17Nb-2.3Mo (at.%). All of the materials were fabricated using a powder-metallurgy, tape casting technique. A subtransus heat-treatment produced microstructures containing a hexagonal close-packed a2 phase, orthorhombic (O) phase, and a body-centered cubic (BCC) phase. The higher Mo contents were shown to stabilize the BCC phase and result in an increase the O+BCC phase volume percent and a subsequent decrease in the a2 phase volume percent. The creep deformation behavior of the alloys and MMCs was the main focus of this dissertation. Creep experimentation was performed to understand the deformation mechanisms as a function of stress, temperature, and strain rate. Higher Mo contents significantly increased the creep resistance of the alloys, which was attributed to the decrease in the number of a2/a2 grain boundaries, increased O+BCC colony size, and Mo solid solution strengthening. This was one of the major findings of the work. In-situ tensile-creep experiments indicated that grain boundaries were the locus of deformation and cracking in each of the alloys investigated. MMC creep experimentation was performed with the fibers aligned perpendicular to the loading direction. Similar to alloy creep results, higher Mo contents increased the creep resistance of the MMCs. However, the creep resistance of the MMCs was significantly less than that of their respective alloy compositions. An effort was made to model the creep behavior of the MMCs based on the creep behavior of the alloys and fiber/matrix bond strength. The model predicted the secondary creep rates of the MMCs well for a condition assuming no bond strength between the fiber and matrix. The model predicts that the MMC will exhibit a secondary creep rate lower than that for the alloy when the applied creep stress is less than the fiber/matrix bond strength. However, no such transition was observed in the experimental data. Experimental testing and finite element modeling revealed that the interfacial bond strength between the matrix and the fiber was indeed very small, suggesting that the MMC creep resistance would not be greater than the matrix alloy under practical loading applications. Overall, the work performed in this dissertation helped fill the knowledge gap which exists for the physical and mechanical metallurgy effects of varying Mo additions in titanium aluminides.