A thesis submitted to the Faculty of Engineering and the Built Environment, University of the
Witwatersrand, in fulfillment of the requirements for the degree of Doctor of Philosophy,
Johannesburg, August 2017 / Titanium aluminides Ti3Al (α2), γ-TiAl and TiAl3 have received much attention for potential
applications where light weight for energy saving, room temperature corrosion resistance in aqueous
solutions, high-temperature oxidation resistance, or where combinations of the above are needed.
Gamma-TiAl of composition Ti-47.5 at.% Al with additions of platinum group metals (PGMs: Pt,
Pd, Ru and Ir) was investigated for microstructure, hardness, room temperature aqueous corrosion,
high-temperature oxidation resistance, mechanical alloying and consolidation by spark plasma
sintering, and coating on titanium Grade 2 and Ti-6Al-4V substrates. Gamma-TiAl of Ti-47.5 at.%
Al produced by melting and casting gave a microstructure consisting of γ grains and lamellar grains
with alternating of α2 and γ phase lamellae. Additions of 0.2, 1.0, 1.5, and 2.0 at.% PGMs introduced
new phases of high PGM contents. The γ and lamellar phases were still present.
The additions of PGMs significantly improved the aqueous corrosion properties at room
temperature, by improving the pitting corrosion resistance of the γ-TiAl alloy by modifying its
hydrogen evolution of the cathodic reaction. The presence of PGMs also influenced the oxidation
behaviour of γ-TiAl at 950°by forming the Z-phase which stabilized a continuous protective Al2O3
phase. However, Ti-47.5 at.% Al, being a two-phase alloy (α2+γ), PGMs could not sustain a stable
Z-phase, as it transformed into an oxygen supersaturated Ti3Al, which subsequently led to the
formation of TiO2+Al2O3, a non-protective oxide mixture. The optimal PGM addition to γ-TiAl was
0.5 at.%, with iridium giving the best room temperature corrosion and high-temperature oxidation
resistance.
Mechanical alloying of Ti and Al pure powders with PGM additions gave powders where α2 and γ
were only identified after heat treatment. Consolidation of the mechanically alloyed powders by spark
plasma sintering gave different microstructures from the cast alloys, with continuous α2 and γ phases
and evenly distributed nanometer-sized alumina, and much higher hardnesses.
Cold spraying the mechanically alloyed powders on to titanium Grade 2 and Ti-6Al-4V substrates
gave coatings of irregular thickness, dense near the substrates with porosity at the top, giving poor
oxidation protection. / CK2018
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/25176 |
| Date | January 2017 |
| Creators | Mwamba, Ilunga Alain |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (xxv, 274 leaves), application/pdf, application/pdf |
Page generated in 0.0017 seconds