Date

2014

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Materials Science and Engineering

First Adviser

Misiolek, Wojciech Z.

Other advisers/committee members

DuPont, John N.; Vermaak, Natasha; Watanabe, Masashi

Abstract

Within the last half a century, advances in Ti and Ti alloys have led to their increasing 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-β and metastable β alloys have become more common since their first large-scale use in the SR-71 Blackbird. In particular, is TIMET's Ti-5Al-5V-5Mo-3Cr (Timetal Ti555, Ti-5553) which has high attainable strengths, excellent forging characteristics, and increased sensitivity to heat treatments compared to other β-Ti alloys. Ti-5553 has become widely known for its desirable attributes and has since become the baseline for the next generation of metastable β and near-β 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 microstrucural, 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. Subsequent evaluation of the mechanical properties, microstructure, solute partitioning, precipitate identities, and fracture took place. 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 &omega-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, ω-phase imparted brittle characteristics and acted as nucleation sites for fine scale, uniform α-phase precipitation. This investigation also reports the first atomic resolution high angle annular dark field scanning transmission electron microscopy images of ω-phase in a commercial metastable β-Ti alloy containing multiple alloying elements. Unique to the ω-phase in Ti-5553 is the atomic ordering observed to have occurred, making this the first report of an ordered ω-phase in a metastable β-Ti alloy, where ordering of "ω-like" phases is conventionally limited to titanium aluminides.

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