The coverage of titanium and its alloys in this chapter has been divided into four sections for systematic presentation. The system takes into account unalloyed titanium and three groups of alloys based on metallurgical differences which in turn result in differences in fabrication and property characteristics. The sections and the individual alloys covered under each are shown in Table 5.1.

The material properties of titanium and its alloys are determined mainly by their alloy content and heat treatment, both of which are influential in determining the allotropic forms in which this material will be bound. Under equilibrium conditions, pure titanium has an “alpha” structure up to 1620°F, above which it transforms to a “beta” structure. The inherent properties of these two structures are quite different. Through alloying and heat treatment, one or the other or a combination of these two structures can be made to exist at service temperatures, and the properties of the material vary accordingly. References 5.1.2(a) and (b) provide general discussion of titanium microstructures and associated metallography.

Titanium and titanium alloys of the alpha and alpha-beta type exhibit crystallographic textures in sheet form in which certain crystallographic planes or directions are closely aligned with the direction of prior working. The presence of textures in these materials lead to anisotropy with respect to many mechanical and physical properties. Poisson’s ratio and Young’s modulus are among those properties strongly affected by texture. Wide variations experienced in these properties both within and between sheets of titanium alloys have been qualitatively related to variations of texture. In general, the degree of texturing, and hence the variation of Young’s modulus and Poisson’s ratio, that is developed for alpha-beta alloys tends to be less than that developed in all alpha titanium alloys. Rolling temperature has a pronounced effect on the texturing of titanium alloys which may not in general be affected by subsequent thermal treatments. The degree of applicability of the effect of textural variations discussed above on the mechanical properties of products other than sheet is unknown at present. The values of Young’s modulus and Poisson’s ratio listed in this document represent the usual values obtained on products resulting from standard mill practices. References 5.1.2(c) and (d) provide further information on texturing in titanium alloys.

Comments relating to formability, weldability, and final heat treatment are presented under individual alloys. These comments are necessarily brief and are intended only to aid the designer in the selection of an alloy for a specific application. In practice, departures from recommended practices are very common and are based largely on in-plant experience. Springback is nearly always a factor in hot or cold forming.

Final heat treatments that are indicated as “specified” heat treatments do not necessarily coincide with the producers’ recommended heat treatments. Rather, these treatments, along with the specified room-temperature minimum tensile properties, are contained in the heat treating-capability requirements of applicable specifications, for example, MIL-H-81200. Departures from the specified aging cycles are often necessary to account for aging that may take place during hot working or hot sizing or to obtain more desirable mechanical properties, for example, improved fracture toughness. More detailed recommendations for specific applications are generally available from the material producers.

Comments relating to temperature limitations in the application of titanium and titanium alloys are presented under the individual alloys.

Below about 300°F, as well as above about 700°F, creep deformation of titanium alloys can be expected at stresses below the yield strength. Available data indicate that room-temperature creep of unalloyed titanium may be significant (exceed 0.2 percent creep-strain in 1,000 hours) at stresses that exceed approximately 50 percent F_ty, room-temperature creep of Ti-5Al-1.5Sn ELI may be significant at stresses above approximately 60 percent F_ty, and room-temperature creep of the standard grades of titanium alloys may be significant at stresses above approximately 75 percent F_ty. References 5.1.4(a) through (c) provide some limited data regarding room-temperature creep of titanium alloys.

The use of titanium and its alloys in contact with either liquid oxygen or gaseous oxygen at cryogenic temperatures should be avoided, since either the presentation of a fresh surface (such as produced by tensile rupture) or impact may initiate a violent reaction [Reference 5.1.4(d)]. Impact of the surface in contact with liquid oxygen will result in a reaction at energy levels as low as 10 ft-lb. In gaseous oxygen, a partial pressure of about 50 psi is sufficient to ignite a fresh titanium surface over the temperature range from −250°F to room temperature or higher.

Titanium is susceptible to stress-corrosion cracking in certain anhydrous chemicals including methyl alcohol and nitrogen tetroxide. Traces of water tend to inhibit the reaction in either environment. However, in N₂O₄, NO is preferred and inhibited N₂O₄ contains 0.4 to 0.8 percent NO. Red fuming nitric acid with less than 1.5 percent water and 10 to 20 percent NO₂ can crack the metal and result in a pyrophoric reaction.

Titanium alloys are also susceptible to stress corrosion by dry sodium chloride at elevated temperatures. This problem has been observed largely in laboratory tests at 450 to 500°F and higher and occasionally in fabrication shops. However, there have been no reported failures of titanium components in service by hot salt stress corrosion. Cleaning with a nonchlorinated solvent (to remove salt deposits, including fingerprints) of parts used above 450°F is recommended.

In laboratory tests, with a fatigue crack present in the specimen, certain titanium alloys show an increased crack propagation rate in the presence of water or salt water as compared with the rate in air. These alloys also may show reduced sustained load-carrying ability in aqueous environments in the presence of fatigue cracks. Crack growth rates in salt water are a function of sheet or section thickness. These alloys are not susceptible in the form of thin-gauge sheet, but become susceptible as thickness increases. The thickness at which susceptibility occurs varies over a visual range with the alloy and processing. Alloys of titanium found susceptible to this effect include some from alpha, alpha-beta, and beta-type microstructures. In some cases, special processing techniques and heat treatments have been developed that minimize this effect. References 5.1.4(e) through (g) present detailed summaries of corrosion and stress corrosion of titanium alloys.

Under certain conditions, titanium, when in contact with cadmium, silver, mercury, or certain of their compounds, may become embrittled. Refer to MIL-HDBK-1568 for restrictions concerning applications with titanium in contact with these metals or their compounds.

• 5.1.2(a)Jaffe, R. I., “The Physical Metallurgy of Titanium Alloys”, Progress in Metal Physics, Vol. 7, Pergammon Press, Oxford, England, pp 65-167 (1958).
• 5.1.2(b)“Aircraft Designer's Handbook for Titanium and Titanium Alloys”, AFML-TR-67-142 (March 1967).
• 5.1.2(c)Larson, F. R., “Anisotropy in Titanium Sheet in Uniaxial Tension”, ASM Transactions, 57, pp 620-631 (1964).
• 5.1.2(d)Larson, F. R., “Textures in Titanium Sheet and Its Effects on Plastic Flow Properties”, Army Materials Research Agency, AMRA-TR-65-24 (October 1965).
• 5.1.4(a)VanEcho, J. A., “Low Temperature Creep Characteristics of Ti-5Al-2.4Sn and Ti-6Al-4V Alloys”, DMIC Technical Note, Defense Metals Information Center, Battelle Memorial Institute, Columbus, Ohio (June 8, 1964).
• 5.1.4(b)Broadwell, R. G., Hatch, A. J., Partridge, J. M., “The Room Temperature Creep and Fatigue Properties of Titanium Alloys”, Journal of Materials, 2, (1), pp 111-119 (March 1967).
• 5.1.4(c)Reimann, W. H., “Room Temperature Creep in Ti-6Al-4V”, AFML-TR-68-171 (June 1968).
• 5.1.4(d)White, E. L., and Ward, J. J., “Ignition of Metals in Oxygen”, DMIC Report 224, Defense Metals Information Center, Battelle Memorial Institute, Columbus, Ohio (February 1, 1966).
• 5.1.4(e)Jackson, J. D., and Boyd, W. K., “Corrosion of Titanium”, DMIC Memorandum 218, Defense Metals Information Center, Battelle Memorial Institute, Columbus, Ohio (September 1, 1966).
• 5.1.4(f)“Accelerated Crack Propagation of Titanium by Methanol, Halogenated Hydrocarbons, and Other Solutions”, DMIC Memorandum 228, Defense Metals Information Center, Battelle Memorial Institute, Columbus, Ohio (March 6, 1967).
• 5.1.4(g)Lectures from AICE Materials Conference, “Titanium for the Chemical Engineer”, DMIC Memorandum 234, Defense Metals Information Center, Battelle Memorial Institute, Columbus, Ohio (April 1, 1968).