Powder Metallurgical Processing of Titanium Alloys and Aluminides

Abstract

In this thesis, the economical powder metallurgical approach was investigated to produce two Ti binary alloys (i.e. Ti-Ni and Ti-Sn) and a Ti-48Al based intermetallic alloy. In the first part of the work, Ni and Sn (up to 10 wt-%) were alloyed to Ti to facilitate the diffusion process via liquid phase sintering mechanisms. For Ti-Ni alloys, the persistent liquid clearly improved densification, and density close to 99% was achieved. Conversely, for the Ti-Sn alloys, the transient liquid was not as effective, despite some density gain. Transitions in the size and the size distribution of pores in the Ti binary alloys were also studied and the Ostwald ripening phenomenon was revealed. In the case of manufacturing Ti-48Al, the press-and-sinter PM approach was initially investigated. The Ti-Al elemental compact experienced significant volume expansion after being sintered, and the sintered product was a porous structure. To control the volume expansion due to TiAl3 formation, the effects of sintering temperature, low temperature annealing and powder size were examined. A pre-alloyed Ti-48Al powder was further produced to avoid the swelling problem. Combined with higher sintering temperature (1350°C) or longer duration of isothermal hold (4h), density close to 90% was achieved. The novel spark plasma sintering process was also applied to produce TiAl alloys. This process demonstrated great efficiency, and close to fully dense material was produced with merely 3 minutes of isothermal hold time. The influences of sintering temperature, hold time and particle size were all examined. A reaction model was proposed to explain the layered structure of the ‘in-house’ produced powder, and its formation sequences of the targeted phases Ti3Al, TiAl, and the intermediate phases TiAl2 and TiAl3. Fine and coarse lamellar structures developed within or around the Ti3Al grains were observed. With increased sintering temperature, the Ti:Al ratio gradually approached 60:40 in the lamellar regions, suggesting a 3TiAl:1Ti3Al phase combination. Ostwald ripening and migration of the porosity were also observed. The TiAl alloy was further reinforced with B4C particulates. Promising preliminary results in terms of sintered density and hardness are reported and the compositions in the phases were analysed.