810 Design Aspects

Designing with titanium and its alloys can conveniently be divided into the two areas discussed in Section 8.9: corrosion-resistant use and structural applications.

8.10.1 Corrosion-Resistant Design

As discussed in Section 8.8 the highly adherent oxide film that forms on the surface of titanium and its alloys offers exceptional resistance to a broad range of acids and alkalis, as well as natural salt and polluted waters. Titanium alloys are especially resistant to corrosion in oxidizing environments, and this behavior can be extended into the reducing regime with the addition of platinum group metals. A summary of corrosion environments where titanium's oxide film provides resistance are shown in Table 8.15.

8.10.2 Structural Design

With its high strength-to-density ratio, excellent fracture-related properties (fracture toughness, fatigue, and fatigue crack growth rate), and superior environmental resistance titanium is the material of choice for many aerospace and terrestrial structural (load-bearing) applications [36].

Selection of titanium for both airframes and engines is based upon its specific properties: weight reduction (due to the high strength-to-density ratio), coupled with exemplary reliability attributable to its outstanding corrosion resistance and general mechanical properties.

Highly efficient gas turbine engines are possible through the use of titanium alloy components such as fan blades, compressor blades, rotors, discs, hubs, and numerous nonrotor parts like inlet guide vanes. Titanium is the most common material for engine parts that operate up to 1100°F (593°C) because of its strength and ability to tolerate the moderate temperatures in the cooler parts of the engine. Other key advantages of titanium-based alloys include low density (which translates to fuel economy) and good resistance to creep and fatigue. The development of titanium aluminides should allow the use of titanium in even hotter sections of a new generation of engines.

Titanium alloys have replaced nickel and steel alloys in nacelles and landing gear components in the Boeing 777. This includes investment cast parts that allow complex shapes to be made at relatively low cost. For example, heat shields that protect wing components from engine exhaust are cast from titanium. Cold hearth melting leads to production of essentially clean metal for structural applications while controlling costs. Superplastic forming/diffusion bonding and powder metallurgy have helped to increase the use of titanium alloys in new airframe designs, by lowering the cost of machining and the amount of waste material produced (revert).

TABLE 8.15 Corrosive Environments Where Titanium Oxide Film Provides Resistance

Chlorine and other halides

Fully resistant to moist chlorine and its compounds.

Fully resistant to solutions of chlorites, hypochlorites, perchlorates, and chlorine dioxide.

Resistance to moist bromine gas, iodine, and their compounds is similar to chlorine resistance.

Water

Immune to corrosion in all natural, sea, brackish, and polluted waters.

Immune to microbiologically influenced corrosion (MIC).

Oxidizing mineral acids

Highly resistant to nitric, chromic, perchloric, and hypochlorous (wet chlorine gas) acids.

Gases

Corrosion-resistant to sulfur dioxide, ammonium, carbon dioxide, carbon monoxide, hydrogen sulfide, and nitrogen.

Information provided as an overview. Before specifying titanium in any aggressive environment, consult corrosion experts. Adapted from James S. Grauman and Brent Willey, "Shedding New Light on Titanium in CPI Construction," Chemical Engineering, August 1998.

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