Titanium service temperature

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

The most common matrices are the low-density metals, such as aluminum and aluminum alloys, and magnesium and its alloys. Some work has been carried out on lead alloys, mainly for bearing applications, and there is interest in the reinforcement, for example, of titanium-, nickel- and iron-base alloys for higher-temperature performance. However, the problems encountered in achieving the thermodynamic stability of fibers in intimate contact with metals become more severe as the potential service temperature is raised, and the bulk of development work at present rests with the light alloys. 

One candidate is a titanium alioy that is reinforced with iarge diameter SiC/C filaments (see Chapter 4) and is fabricated by superplastic forming/ diffusion bonding. This MMC is suited to the fabrication of bladed compressor rings, shafts, ducks, fan components or structural rods for jet engines. Their use for parts submitted to still higher temperatures is limited by tiber/matrix reaction and environmental considerations. Titanium aluminide TisAI (or y-TiAl) matrices could permit an increase in the service temperature of the composites. 

It should be noted that titanium aUo5 are generally not susceptible to sulfide stress cracking (SSC) in HjS-rich, sulfides, and/or sulfur containing environments (e.g., sour gas/oil well fluids). This inherent SSC resistance stems from the fact that formation of titanium sulfide corrosion products is not thermodynamically favored, such that stability of titanium s protective oxide surface film wiU prevail even at higher service temperatures. In these hot sour brine service environments, resistance to chloride-induced SCC is a more relevemt issue for titanium alloys. 

H. M. Clearfield joined Martin Marietta Laboratories in January, 1985. Since then, he has primarily investigated surface and interfacial phenomena in adhesive bonding, including surface preparation of titanium alloys for structural applications at high service temperatures, mechanisms of bond failures that occur at high temperatures, and bonding of the thermal protection system to the space shuttle external tank. Additionally, he has investigated dopant depth distributions in ion-implanted and laser-annealed silicon. Dr, Clearfield is supervisor of surface analysis facilities at Martin Marietta Laboratories. Recently, Dr. Clearfield joined IBM s T. J. Watson Research Center. 

Whereas at low temperatures, elements in substitutional solid solution are supposed to contribute to the athermal component of flow stress, and interstitial elements to the thermal barriers, as the temperature rises all alloying species become more or less mobile and associate themselves with atmosphere effects to extents that depend on solute-atom dif-fusivities [Ros73]. Chemical effects such as oxidation and hot-salt stress corrosion may limit the service temperature of a titanium alloy in some applications in others, mechanical degradation such as high-temperatrue creep will limit the service-temperatiue range. 

Oxidation. A blue oxide film typically forms in about 6 to 10 h in exposures not exceeding 540 °C (1000 °F). Degradation of mechanical properties fi-om oxidation at longer times and usual service temperatures has not been observed. In a strong oxidizing environment, resistance is probably comparable to grade 2 titanium or Ti-6A1-4V. 

The engine platforms are constructed from titanium pillars and titanium sheeting. The metal is prelreated with an etchant acid mixture and then bonded at 175°C with Redux 319, to obtain the required higher service temperature. 

Aqueous media and marine corrosion. Zirconium has excellent corrosion resistance to seawater, fresh water, brackish water, and other polluted water streams and is a material of choice for heat exchangers, condensers, and other equipment handling these media, where it can replace titanium-palladium alloys. Unlike titanium and its alloys, zirconium is highly resistant to crevice corrosion. With their high corrosion resistance to pressurized water and steam, low neutron absorption (with low hafiiium content), good mechanical strength, and ductility, at nuclear reactor service temperatures, and their ability to remain stable even after extensive radiation, zirconium alloys are used extensively in fuel cladding, fuel channels, and pressure tubes for... 

Dry chlorine has a great affinity for absorbing moisture, and wet chlorine is extremely corrosive, attacking most common materials except HasteUoy C, titanium, and tantalum. These metals are protected from attack by the acids formed by chlorine hydrolysis because of surface oxide films on the metal. Tantalum is the preferred constmction material for service with wet and dry chlorine. Wet chlorine gas is handled under pressure using fiberglass-reinforced plastics. Rubber-lined steel is suitable for wet chlorine gas handling up to 100°C. At low pressures and low temperatures PVC, chlorinated PVC, and reinforced polyester resins are also used. Polytetrafluoroethylene (PTFE), poly(vinyhdene fluoride) (PVDE), and... 

Titanium is resistant to nitric acid from 65 to 90 wt % and ddute acid below 10 wt %. It is subject to stress—corrosion cracking for concentrations above 90 wt % and, because of the potential for a pyrophoric reaction, is not used in red filming acid service. Tantalum exhibits good corrosion resistance to nitric acid over a wide range of concentrations and temperatures. It is expensive and typically not used in conditions where other materials provide acceptable service. Tantalum is most commonly used in appHcations where the nitric acid is close to or above its normal boiling point. 

Types 321 and 347 have additions of titanium and niobium, respectively, and are used in welding appHcations and high temperature service under corrosive conditions. Type 304L may be used as an alternative for Types 321 and 347 in welding (qv) and stress-reHeving appHcations below 426°C. 

In general, it is fair to state that one of the major difficulties in interpreting, and consequently in establishing definitive tests of, corrosion phenomena in fused metal or salt environments is the large influence of very small, and therefore not easily controlled, variations in solubility, impurity concentration, temperature gradient, etc. . For example, the solubility of iron in liquid mercury is of the order of 5 x 10 at 649°C, and static tests show iron and steel to be practically unaltered by exposure to mercury. Nevertheless, in mercury boiler service, severe operating difficulties were encountered owing to the mass transfer of iron from the hot to the cold portions of the unit. Another minute variation was found substantially to alleviate the problem the presence of 10 ppm of titanium in the mercury reduced the rate of attack to an inappreciable value at 650°C as little as 1 ppm of titanium was similarly effective at 454°C . 

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