A Microstructural Characterization of as-Cast and Rapidly Solidified Titanium-Silicon Alloys (Silicide, Melt-Spinning, Precipitation, Metastable, Oxygen)
Chumbley, Leonard Scott
This item is only available for download by members of the University of Illinois community. Students, faculty, and staff at the U of I may log in with your NetID and password to view the item. If you are trying to access an Illinois-restricted dissertation or thesis, you can request a copy through your library's Inter-Library Loan office or purchase a copy directly from ProQuest.
Permalink
https://hdl.handle.net/2142/71831
Description
Title
A Microstructural Characterization of as-Cast and Rapidly Solidified Titanium-Silicon Alloys (Silicide, Melt-Spinning, Precipitation, Metastable, Oxygen)
Author(s)
Chumbley, Leonard Scott
Issue Date
1986
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Abstract
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.
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.
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.
Use this login method if you
don't
have an
@illinois.edu
email address.
(Oops, I do have one)
IDEALS migrated to a new platform on June 23, 2022. If you created
your account prior to this date, you will have to reset your password
using the forgot-password link below.
