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any one of the alloys based on titanium. The lightness, high strength in the range of temperatures from the cryogenic (–250°C) to the moderately high (300°–600”C), and excellent corrosion resistance make titanium alloys promising structural materials in many areas, in particular, the aerospace industry and the branches of machine building dealing with transportation.
Titanium alloys are obtained by alloying titanium with the following elements (the numbers in parentheses representing the maximum percentage concentration by weight for industrial alloys): Al (8), V (16), Mo (30), Mn (8), Sn (13), Zr (10), Cr (10), Cu (3), Fe (5), W (5), Ni (32), and Si (0.5); the use of Nb (2) and Ta (5) is less common. Small amounts of Pd (0.2) and B (0.01) are added in order to, respectively, improve corrosion resistance and reduce grain size. The alloying additives have different solubilities in the α and β forms of titanium, and the additives alter the temperature of the α/β transition. Aluminum, as well as oxygen and nitrogen, dissolves more readily in α titanium and raises the transition temperature commensurately with its concentration. This ability leads to an expansion of the domain of existence of the α phase, and such elements are called α stabilizers. Sn and Zr dissolve well in both allotropic forms of titanium and have very little effect on the temperature of the α/β transition; they are classed as neutral strengtheners. All other additives used in industrial titanium alloys dissolve more readily in β titanium; these additives, referred to as β stabilizers, lower the transition temperature. Since the additives’ solubilities in the α and β forms of titanium change with temperature, alloys containing these elements can be strengthened by hardening and aging.
As a consequence of titanium’s polymorphism and ability to form solid solutions and chemical compounds with many elements, the phase diagrams of titanium alloys are highly varied. However, in industrial titanium alloys, the concentration of alloying elements, as a rule, does not extend beyond the limits of solid solutions based on α titanium and β titanium; intermetallic-compound phases are usually not observed.
In unalloyed titanium, as well as in alloys of titanium containing α stabilizers and neutral strengthened, the high-temperature β phase cannot be maintained by hardening owing to the existence of a martensitic transformation, resulting in the formation of a secondary, acicular α phase. On the other hand, in alloys containing β stabilizers, it is possible, depending on concentration, to maintain any amount of the β phase up to 100 percent. Binary alloys containing at least 4 percent Fe, 7 percent Mn, 7 percent Cr, 10 percent Mo, 14 percent V, 35 percent Nb, or 50 percent Ta can be hardened so as to have a uniform β structure. The above concentrations are called critical. In hardened alloys with subcritical or critical compositions, the β phase is unstable, and upon subsequent low-temperature treatment (aging), this phase decomposes with the formation of dispersed segregations of a secondary α phase, thereby producing a strengthening effect. In alloys with supercritical composition, for example, Ti with 30 percent Mo, a stable β phase is formed, and a strengthening effect is not observed.
Industrial titanium alloys are commonly divided into three groups, according to the type of crystal structure. Alloys based on the α structure include those with Al, Sn, and Zr, as well as alloys containing small amounts of β stabilizers (0.5–2 percent). In view of the insignificant amount or absence of the β phase in their structure, these alloys are strengthened hardly at all by heat treatment and thus are considered medium-strength alloys (σb = 700–950 meganewtons/m2, or 70–95 kilograms-force/mm2). Sheets of these alloys can only be stamped in a hot state. Among the advantages of the α alloys are excellent weldability and good creep resistance; furthermore, the alloys do not need to be heat treated, and they have excellent casting properties, an important feature when shaped castings are desired. Low-alloy α alloys, a category including commercially produced titanium, have strengths of less than 700 meganewtons/m2, or 70 kilogramsforce/mm2, and they have good cold stamping.
Diphase α + β alloys comprise the largest group of industrial titanium alloys. These alloys have better workability than α alloys; in addition, they can be heat-treated to very high strengths (σb = 1,500–1,800 meganewtons/m2, or 150–180 kilograms-force/ mm2), and they can have good heat resistance. Drawbacks of diphase alloys include a somewhat poorer weldability relative to α alloys owing to the formation in the heat-affected zone of brittle regions and cracks, the prevention of which requires special heat treatment after welding.
Alloys based on the β structure have excellent workability and good cold stamping; after aging, they acquire high strength. These alloys readily fuse, but welded joints subjected to strengthening heat treatment become brittle, which limits the use of this type of alloy. Another disadvantage of β alloys is the relatively low ceiling of working temperatures—approximately 300°C; at high temperatures, most of the alloys in this group become brittle.
The chemical compositions of the industrial titanium alloys produced in the USSR are presented in Table 1, which also has a breakdown according to structural type. The alloys can be grouped according to use and according to the type of semifinished shape produced. One group includes weldable alloys used mainly to produce sheets (VT5–1, OT4–0, OT4–1, OT4, VT20, VT6S, VT14, and VT15); a second group encompasses high-strength alloys intended for stampings (VT5, VT6, VT14, VT16, and VT22); and a third group comprises alloys with good heat resistance that also are intended for stampings (VT3–1, VT8, VT9, and VT18). The alloys VT6S is especially well suited for high-pressure vessels, and all the alloys with good heat resistance are recommended for the disks, blades, and other parts of the compressors of gas turbines. The alloy VT22 is recommended for heavy-duty massive stampings while the alloy VT16 is recommended for bolts. If necessary, for example, in stamped and welded structural components, all the sheet alloys can be subjected to press operations.
The mechanical properties of titanium alloys in the annealed and thermally strengthened state are presented in Table 2. In addition to ordinary heat treatment involving hardening and aging, various modes of annealing and thermomechanical treatment are employed, for example, hardening and aging after subjecting the metal to pressure. Another technique used is isothermal deformation (slow operations in presses that have been heated to the temperature of deformation). Here, excellent mechanical properties and uniformity of properties can be obtained. Titanium and its alloys may be subjected to forging, rolling, extrusion, and drawing and to press operations while in either sheet or bulk form. The same semifinished shapes may be obtained from these alloys as from other structural metals so long as the marked tendency of titanium to undergo oxidation upon heating is taken into account. The use of protective enamel coatings, which also serve as lubricants upon pressure treatment, is recommended. Also, heat treatment should be conducted in furnaces that are evacuated or that have a neutral atmosphere.
Most industrial titanium alloys have a rather narrow crystallization range and thus have satisfactory casting properties. The α alloys, which in addition to having good casting properties permit a correction of defects, are preferred for manufacturing shaped castings. The most widely used titanium casting alloy in the USSR is the alloy VT5L. The alloys VT6L, VT9L, and VT20L are used for parts requiring high strength. Special ceramic and graphite mixtures, as well as steel chills, are used in making the molds.
|Table 1. Chemical composition of industrial titanium alloys produced in the USSR|
|Type of alloy||Alloy||Chemical composition, % (in percent, the remainder being titanium)|
|α + β||VT6S||5.0–6.5||3.5–4.5||–||–||–||–||–|
|VT3–1||5.5–7.0||–||2.0–3.0||–||1.0–2.5||0.15–0.4 0||0.2–0.7 Fe|
|Table 2. Typical mechanical properties of titanium alloys|
|Alloy||Type of semifinished shape||Dimensions (diameter of bars or thickness of sheets, mm)||Type of heat treatment||Ultimate strength (meganewtons/m2)||Elongation1 (%)|
|1 Where two values are given, the first value is tor the minimum thickness and the second for the maximum thickness|
|Hardening and aging||1,050||8|
|Hardening and aging||1,100||6|
|Hardening and aging||1,200||6|
|Hardening and aging||1,200||6|
|Hardening and aging||1,200||6|
|Hardening and aging||1,100–1,200||6–4|
|Hardening and aging||1,300||4|
Still in the development stage in industry are high-alloy Ni-Ti alloys, which have the composition of almost pure intermetallic compounds of Ni and Ti. Alloys of this type, known collectively as Nitinol, have the capacity under certain conditions to reassume their original shape after plastic deformation (“memory effect”), a property used in, for example, the automatic relays of the devices in fire-protection systems.
Among the drawbacks of titanium alloys are poor antifriction properties, necessitating the application of coatings and lubricants to working surfaces.
S. G. GLAZUNOV