A MICROSTRUCTURAL CHARACTERIZATION OF AS-CAST AND RAPIDLY SOLIDIFIED TITANIUM-SILICON ALLOYS (SILICIDE, MELT-SPINNING, PRECIPITATION, METASTABLE, OXYGEN)

Rapid Solidification (RS) studies have been carried out on a number of Titanium-Silicon alloys. Two techniques have been employed to produce rapidly solidified material, namely laser surface melting and melt-spinning. Prior to RS, a careful microstructural characterization of the as-cast material was undertaken. This also involved a study of an homogenized and heat treated base alloy (Ti-2 wt%Si) to determine the types of precipitation reactions present in conventionally processed material. In the base alloy the equilibrium silicide in the temperature range 773 K - 1023 K was found to be Ti(,5)Si(,3) which would precipitate out of solid solution in accordance with two different orientation relationships. A tetragonal phase of chemical composition Ti(,3)Si was observed in the as-cast hypoeutectic alloys; however, it proved to be metastable and dissolved upon heat treatment (773-1023 K), resulting in an equilibrium microstructure consisting of (alpha)-Ti and Ti(,5)Si(,3). The as-cast microstructures of an eutectic and an hypereutectic alloy were also examined. Upon arc-melting both of these alloys exhibited an eutectic microstructure of Ti(,5)Si(,3) rods within an (alpha)-Ti matrix.;Melt-spinning produced very different microstructures from those observed after arc-melting or laser surface melting. This could be accounted for in terms of undercooling achieved prior to nucleation. Large amounts of Si could be retained in solution, and the RS microstructures were seen to change from (alpha)' to (beta) to an amorphous structure as the amount of Si was increased. These microstructures have been related to metastable features of the Ti-Si phase diagram.;Oxygen contamination of the melt during melt-spinning was also seen to play an important role in microstructural development during RS. High levels of oxygen contamination resulted in the formation of a microcrystalline structure consisting of (alpha)-Ti, Ti(,5)Si(,3) and a third previously unreported phase (designated X) in all the alloys investigated.;The RS microstructures produced by laser surface melting did not vary greatly due to the low degree of undercooling reached prior to nucleation, and could therefore be explained by an observation of the equilibrium phase diagram. The main differences noted to exist between the as-cast and RS microstructures were (1) transformation of the (alpha) (seen in the as-cast hypoeutectic alloys) to (alpha)'; (2) the absence of the metastable phase Ti(,3)Si in the RS alloys, and (3) an overall refinement of the as-cast microstructure.