Dottorato in INGEGNERIA ELETTRICA, DEI MATERIALI E DELLE
NANOTECNOLOGIE

Titolo della tesi: Advanced intermetallic titanium aluminides for high – temperature applications

TiAl alloys are a class of intermetallic compounds that are gaining popularity in the aerospace and automotive industries due to their superior physical and mechanical qualities. Due to their reduced density (approximately 4.0 g/cm3 for - TiAl alloys and 8.0 g/cm3 for Ni-based superalloys), which makes their unique mechanical qualities equivalent to those of nickel-based superalloys, they are specifically thought to be a desirable alternative to nickel-based superalloys. These materials' light weight makes it possible to reduce the total weight of an airplane engine or an automobile engine component. As a result, it is possible to improve component performance while lowering fuel use and emissions. Ingot casting, investment casting, and powder metallurgy processing are the most widely used traditional industrial scale processing techniques for titanium aluminides. Due to the material's low ductility and fracture toughness, processing titanium aluminide using these production techniques can be complicated. The goal of this thesis is to face the issues related to investment casting of TiAl, develop the right process parameters for investment casting, improve the alloy’s mechanical properties for high temperature applications by using the principle of oxide dispersion strengthening and lastly improve the high temperature oxidation resistance of this alloy. The first stage of the research deals with studying the issues related to the investment casting procedure. It was found out that, in the investment casting of TiAl components, the mould ceramic and the casting geometry plays a very important role. After evaluating three different kinds of ceramic, alumina-based ceramics were found to be the most suitable for TiAl investment casting as it did not react with the molten metal and produced the best surfaces. In terms of design, special attention should be given to areas where there can be stress intensification. Although dispersion strengthening is often used for improving alloy alloys mechanical strength, there are few studies on its effect on the strengthening of TiAl intermetallic alloys. In this research various nanometric dispersoids have been evaluated to see their effect on the mechanical properties of TiAl. The results have shown that alumina is the best suited oxide for strengthening TiAl based on the free energy of formation that allows to obtain a homogenous oxide distribution. Moreover, in-situ alumina has been more effective in increasing the alloy mechanical properties as it produces small and well distributed alumina particles that strengthen the alloy. TiAl based alloys have very poor oxidation resistance above 600℃. This research evaluates various methods to improve the oxidation resistance of the alloy. While anodizing the alloy is not an effective way to improve the oxidation resistance, pre-oxidation has some effects on the oxidation behaviour as it reduces the mass gain but does not produce a protective oxide layer. The best method to improve the oxidation resistance was found to be hot dipping the alloy in molten aluminium to produce an Al rich outer layer on the alloy, which produces alumina when oxidized at high temperature. This method is so effective that even at 1000℃, the oxide mass gain is less than 0.62 mg/cm2. Hence, this thesis faces all the various issues and presents a solution related to high temperature application of Titanium Aluminide intermetallics.