Titanium Aluminide Alloys: Part One

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Alloys based on titanium aluminides, such as γ (TiAl) may be made through the use of casting or powder metallurgical processes and heat treatments. The alloys contain titanium, 38 to 46 atom % aluminum, 5 to 10 atom % niobium, with small amounts of Cr, Si and Ni added according to required properties. Titanium aluminide alloys are characterized by a low density, a high rigidity and good corrosion resistance. In the fixed state, they have domains with hexagonal (α), two-phase structures (α+β) and cubically body-centered β phase and/or γ phase.

TiAl alloy is a new alloy based on intermetallic compounds and is characterized by the properties of light weight and high strength. As this alloy is suitable for rotating and reciprocating parts, its applicability is related closely to blades for airplanes and other industrial engines, and exhaust valves and turbochargers for passenger vehicle engines.

Two-phase TiAl alloys have been receiving considerable attentions because of their attractive properties, such as the low density, excellent high-temperature strength, and good oxidation resistance. However, TiAl alloys are quite brittle at room temperature and have relatively low fracture toughnesses.

The mechanical properties of the TiAl-based alloys with lamellar structures depend on three factors: the colony size, interlamellar spacing, and alloying addition. The tensile elongation at room temperature is strongly dependent on the lamellar colony size, showing the increased ductility with decreasing the colony size. The strength at room and elevated temperatures is sensitive to the interlamellar spacing, exhibiting the increased strength with decreasing the interlamellar spacing.

For industrial practice, alloys based on an intermetallic phase γ (TiAl) with a tetragonal structure and containing minority shares of intermetallic phase α₂ (Ti₃Al) with hexagonal structure in addition to the majority phase γ (TiAl) are particularly interesting. These γ titanium aluminide alloys are characterized by properties like low density (3.85-4.2 g/cm³), high elastic modulus, high rigidity and creep resistance up to 700°C., which make them attractive as lightweight construction materials for high-temperature applications. Examples of such applications include turbine buckets in aircraft engines and in stationary gas turbines, and valves for engines and hot gas ventilators.

As mentioned above, Titanium aluminides based on γ (TiAl) are characterized in general by relatively high rigidities, high elastic modulus, good oxidation and creep resistance with simultaneously lower density. Based on these properties, TiAl alloys should be used as high temperature materials.

These types of applications are heavily impaired through the very low plastic malleability and the low fracture toughness. Rigidity and malleability, as with many other materials, behave hereby inversely. The technically interesting high-strength alloys are therefore often particularly brittle. Comprehensive examinations for the optimization of the structure were performed in order to eliminate these disadvantageous properties.

 

Figure 1: Ti-Al-Nb ternary equilibrium phase diagram (1473K isothermal section)

 

The development of this alloy was carried out as follows. First, it was decided to add Nb for improvement of the oxidation resistance and high-temperature strength, and the appropriate Nb concentration in the Ti-Al-Nb ternary system (Figure 1) was investigated. Next, the effects of the fourth element on this ternary alloy were examined and the appropriate additives were selected. The turbine wheels made from the newly developed alloy were used to evaluate their material properties and durability in a realistic environment. Also, the properties of this alloy were compared to those of conventional TiAl alloys and Inconel 713C in current use.