853 Alloy Classes

Titanium alloys are categorized into one of four groups: alpha (a), alpha-beta (a-P), beta alloys (P), and the intermetallics (TixAl, where x = 1 or 3). Titanium alloys for aerospace application contain a- and P-stabilizing elements to achieve required mechanical properties such as tensile strength, creep, fatigue, fatigue crack propagation resistance, fracture toughness, stress-corrosion cracking, and resistance to oxidation [12]. Once the chemistry is selected, optimization of mechanical properties is achieved by working (deformation) to control the size, shape, and dispersion of first the P phase and later the a phase.

Beta-isomorphous alloying elements (e.g., Mo, V, Nb), which do not form intermetallic compounds, have traditionally been preferred to "eutectoid-type" elements (e.g., Cr, Cu, Ni). However, some P-eutectoid-type compound formers are added to a-P or P alloys to improve hardenability and increase the response to heat treatment.

a Alloys The a alloys contain predominantly a phase at temperatures up to well above 540°C (1000°F). A major class of a alloys is the unalloyed titanium family of alloys that differ in the amount of oxygen and iron in each alloy. Alloys with higher interstitial content are higher in strength, hardness, and transformation temperature compared to high purity alloys. Approximately every 0.01 wt% oxygen gives a 10.5-MPa (1.5-ksi) increase in strength level [12]. Other a alloys contain additions such as Al and Sn (e.g., Ti-5Al-2.5Sn and Ti-6Al-2Sn-4Zr-2Mo).

Generally, a-rich alloys are more resistant to high-temperature creep than a-P or P alloys, and a alloys exhibit little strengthening by heat treatment. These alloys are usually annealed or recrystallized to remove stresses from cold working, and they have good weldability and generally inferior forgeability in comparison to a-P or P alloys.

a-P Alloys a-P Alloys contain one or more of the a and P stabilizers. These alloys retain more P after final heat treatment than the near a alloys and can be strengthened by solution treating and aging, although they are generally used in the annealed condition. Solution treatment is usually performed high in the a-P phase field followed by aging at lower temperature to precipitate a, giving a mixture of fine a in an a-P matrix. The solution treating and aging can increase the strength of these alloys by up to 80% [12]. Alloys with low amounts of P stabilizer (e.g., Ti-6Al-4V) have poor hardenability and must be rapidly quenched for subsequent strengthening. A water quench of Ti-6Al-4V will adequately harden sections only less than 25 mm (1 in.).

P Alloys P Alloys have more P-stabilizer content and less a stabilizer than a-P alloys. These alloys have high hardenability with the P phase retained completely during air cooling of thin sections, and water quenching of thick sections. P Alloys have good forgeability and good cold formability in the solution-treated condition. After solution treatment, aging is performed to transform some P phase to a. The strength level of these alloys is greater than a-P alloys, a result of the finely dispersed a particles in the P phase. These alloys have relatively higher densities and generally lower creep strengths than the a-P alloys. The fracture toughness of aged P alloys at a given strength level is generally higher than that of an aged a-P alloy, although crack growth rates can be faster [12].

Titanium Aluminides To increase the efficiency of gas turbine engines, higher operating temperatures are necessary, requiring alloys with enhanced mechanical properties at elevated temperatures. The family of titanium alloys showing potential for applications at temperatures as high as 900°C (1650°F) are the titanium aluminide intermetallic compounds Ti3Al(a2) and TiAl(y) [13-15]. The major disadvantage of this alloy group is low ambient temperature ductility. However, it has been found that niobium, or niobium with other P-stabilizing elements, in combination with microstructure control, can increase room temperature ductility in the Ti3Al alloys up to as much as 26% elongation. Recently, by careful control of the microstructure the ambient temperature ductility of two-phase TiAl (y + a2) has been raised to levels as high as 5% elongation. The TiAl compositions (e.g., Ti-48Al-2Cr-2Nb) have now reached a stage of maturity where they are serious contenders for use in advanced gas turbine engines and automobiles [3-7].

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