| Within the last half-century, advances in Ti and Ti alloys have increased their popularity in the aerospace industry as well as in commercial products. Some Ti alloys have even replaced steels and Ni-base alloys due to their high strength and superior corrosion resistance. Of the various Ti alloys, near-beta and metastable beta alloys have become more common since their first large-scale use in the SR-71 Blackbird. In particular, TIMET's Ti-5Al-5V-5Mo-3Cr (Timetal Ti555, Ti-5553) gained high attainable strengths, excellent forging characteristics, and increased sensitivity to heat treatments compared to other beta-Ti alloys. Ti-5553 has become widely known for its desirable attributes and has since become the baseline for the next generation of metastable beta and near-beta Ti alloys.;However, as well known as Ti-5553 is in the aerospace and Ti industry, its responses to welding have, for the most part, gone uncharacterized. The work presented in this dissertation investigates the influence of electron beam welding and post weld heat treatments on the microstructural, mechanical, and fracture responses of Ti-5553. In this study, Ti-5553 was electron beam welded and heat-treated in accordance to three predetermined heat treatments: 700°C for 4 hours followed by air cooling to room temperature, 804°C for 1 hour followed by air cooling to room temperature, and 804°C for 1 hour followed by air cooling to room temperature then aging at 600°C for 4 hours followed by air cooling to room temperature. Subsequently, the mechanical properties, microstructure, solute partitioning, precipitate identities, and fracture characteristics were evaluated.;With the use of traditional techniques and new technology it was shown that electron beam welded Ti-5553 in the as-welded condition and three post weld heat treatment conditions exhibited varying properties, distinctive to each of the corresponding microstructures. It was also found that the o-phase played a large role in the evolution of the resultant microstructures and mechanical properties in each of the four post-welding conditions. In general, o-phase imparted brittle characteristics and acted as nucleation sites for fine scale, uniform alpha-phase precipitation. This investigation also reports the first atomic resolution high angle annular dark field scanning transmission electron microscopy images of o-phase in a commercial metastable beta-Ti alloy containing multiple alloying elements. The atomic resolution images of o revealed unexpected atomic ordering. Typically ordering of "o-like" phases is limited to titanium aluminides, making this the first report of observed ordering in Ti-5553. This ordering likely accounts for the increased brittle behavior of the weld zone compared to bulk Ti-5553. |
