Alloy titanium

Titanium is only about half as dense as steel but is stronger than many steels even in its commercially pure (unalloyed) form. It is allotropic with the hep a-phase stable at ambient temperatures and the fee (3-phase stable above 882°C. The most common 

By combining various combinations of a and P alloys, it is possible to optimize the desired properties. Ti-6Al-4 V is used for prosthetic devices because of its strength, corrosion resistance, and biocompatibility. Ti-10 V-2Fe-3Al has the best combination of strength and toughness. Its yield strength of 1150 MPa and ductility of 10% elongation is tougher than any steel. 

Commercially pure titanium consists of several grades with increasing amount of oxygen and increase in strength. The versions most used are grades 2 and 3 for tubing, vessels and pipe. 

Titanium-palladium alloys (grades 7 and 11) have significantly improved corrosion resistance and are used where grades 2 and 3 are inadequate. 

Ti-6 aluminum-4 vanadium (grade 5) is the most widely used alloy for pumps, shafts, valves and high strength parts. 

Ti-5 aluminum-2.5 tin (grade 6) is used for high temperature strength in process equipment and also as hardware to handle cryogenic liquified gases. 

Addition of rare earths to Ti-base alloys results in a profound effect on recrystallized grain size and other microstructural features as shown in Table 12.6. 

MODERN ASPECTS OF RARE EARTHS AND THEIR COMPLEXES 

Effects of RE additions on the microstructure and mechanical properties of Ti alloys. 

Alloy RE addition (wt%) Recrystallized grain size (pm) Room temperature Y.S. % elongation (MPa) Stress rupture life (h) (500°C/55 kg/mm2) Remarks 

Ti-6Al-tV (as-cast) 0.03 Y — 769 14 - a-colony size, while prior beta 

TABLE 25.1. Corrosion Rates of Commercial Titanium in Alkaline-Peroxide Solutions [8] 

On exposure of the metal for Ih or more, stressed or unstressed, to fuming HNO3 containing 2.5-28% NO2 and not more than 1.25% H2O, a dark substance forms over the surface (97.5% Ti metal by X-ray analysis) [9]. In the dry state, this surface film is pyrophoric when scratched, and it explodes when abraded in contact with concentrated HNO3. The 8% Mn-Ti ahoy is especially sensitive in this regard. 

Titanium, presumably as sponge, in contact with hquid oxygen is reported to be sensitive to detonation by impact [10]. 

TABLE 25.2. Some Commonly Used Titanium Alloys [2] 

Common Alloy Designation UNS Number ASTM Grade Nominal Composition (%) 

General description and properties. There are several reasons for using titanium alloys according to their properties and characteristics. The chief advantages of titanium alloys that are important to design engineers are, in order of importance  

Metallurgical classification. The crystallographic structure of titanium exhibits a phase transformation from a low-temperature close-packed hexagonal arrangement (i.e., a-hcp, alpha-titanium) to a high-temperature form body-centered cubic crystal lattice (i.e., 6-bcc, beta-titanium) at 882°C. This transformation can be considerably modified by the addition of alloying elements (Table 4.52) to produce at room temperature alloys that have all alpha, all beta, or alpha-beta structures. 

Therefore, the basic properties of titanium and its alloys strongly depend on their basic metallurgical structure and the way in which this is manipulated in their mechanical and thermal treatment during manufacture. Four main types of titanium alloy have been developed, and hence titanium alloys fall into the four categories alpha, near alpha, alpha plus beta, and beta. 

Alpha-titanium alloys. These alloys range in yield tensile strength from 173 to 483 MPa. Variations are generally achieved by alloy selection and not heat treatment. They usually contain alpha stabilizers and have the lowest strengths. However, they are formable and weldable. Some contain beta stabilizers to improve strength. Alpha-titanium alloys are generally in the annealed or stress-relieved condition. They are considered fully annealed after 

Chemical equivalents. In practice, empirical parameters, called the aluminum, oxygen, and molybdenum equivalents, are utilized to assess the type and quantity of phases that are formed during processing. These chemical equivalents for titanium alloys are defined as follows  

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Fig. 5. Radioactivity after shutdown per watt of thermal power for A, a Hquid-metal fast breeder reactor, and for a D—T fusion reactor made of various stmctural materials B, HT-9 ferritic steel C, V-15Cr-5Ti vanadium—chromium—titanium alloy and D, siUcon carbide, SiC, showing the million-fold advantage of SiC over steel a day after shutdown. The radioactivity level after shutdown is also given for E, a SiC fusion reactor using the neutron reduced...

Magnesium titanium alloys form the hydrides Mg2TiHg [74811-18-0] and MgTi2H [58244-88-5] (17). Traces of a third metal are often added to adjust dissociation pressures and/or temperatures to convenient ranges. 

Peripheral pitting and etching associated with the low current densities arising outside the main machining zone occur when higher current densities of 45-75 A/cm are appHed. This is a recurrent difficulty when high alloy, particularly those containing about 6% molybdenum, titanium alloys are electrochemicaHy machined. 

The occurrence of pitting seems to stem from the differential stabiUty of the passive film that forms on the titanium alloy. This film does not break down uniformly even when the electrolytes are fluoride and bromide based. The pitting can be so severe that special measures are needed to counteract it. 

The abihty of magnesium metal to reduce oxides of other metals can be exploited to produce metals such as zirconium, titanium [7440-32-6] and uranium [7440-61-1] (see ZiRCONiUMAND ZIRCONIUM COMPOUNDS Titaniumand titanium alloys Uraniumand uranium compounds). These reactions are... 

Preparation and Manufacture. Magnesium chloride can be produced in large quantities from (/) camalhte or the end brines of the potash industry (see Potassium compounds) (2) magnesium hydroxide precipitated from seawater (7) by chlorination of magnesium oxide from various sources in the presence of carbon or carbonaceous materials and (4) as a by-product in the manufacture of titanium (see Titaniumand titanium alloys). 

The recovery of vanadium from these slags is of commercial interest because of the depletion of easily accessible ores and the comparatively low concentrations (ranging from less than 100 ppm to 500 ppm) of vanadium in natural deposits (147,148). In the LILCO appHcations the total ash contained up to 36% 20 (147). Vanadium is of value in the manufacture of high strength steels and specialized titanium alloys used in the aerospace industry (148,149). Magnesium vanadates allow the recovery of vanadium as a significant by-product of fuel use by electric utiUties (see Recycling, nonferrous LffiTALS). 

Chlorination. In some instances, the extraction of a pure metal is more easily achieved from the chloride than from the oxide. Oxide ores and concentrates react at high temperature with chlorine gas to produce volatile chlorides of the metal. This reaction can be used for common nonferrous metals, but it is particularly useful for refractory metals like titanium (see Titanium and titanium alloys) and 2irconium (see Zirconium and zirconium compounds), and for reactive metals like aluminum. 

Ion implantation is being used to form a thin haid case on matetials othei than steels. Titanium alloys have been successfully implanted with nitiogen. The process has been appHed to ceramics to modify the surface region. 

MgCl2-Supported Catalysts. Examination of polymerizations with TiCl catalysts has estabUshed that only a small percentage of titanium located on lateral faces, edges, and along crystal defects is active (52) (see Titanium and titanium alloys). This led to the recognition that much of the catalyst mass acted only as a support, promoting considerable activity aimed at finding a support for active titanium that would not be detrimental to polymer properties. 

Titanium alloy, composed of titanium, aluminum, and vanadium, is preferred by some orthopedic surgeons primarily for its low modulus of... 

Nickel—Iron and Cobalt—Iron Alloys. Selenium improves the machinabifity of Ni—Ee and Co—Ee alloys which are used for electrical appfications. Neither sulfur nor tellurium are usefiil additives because these elements cause hot britdeness. The addition of 0.4—0.5% selenium promotes a columnar crystal stmcture on solidification, doubling the coercive force of cobalt—iron-titanium alloy permanent magnets produced with an equiaxial grain stmcture. 

Titanium alloy systems have been extensively studied. A single company evaluated over 3000 compositions in eight years (Rem-Cm sponsored work at BatteUe Memorial Institute). AHoy development has been aimed at elevated-temperature aerospace appHcations, strength for stmctural appHcations, biocompatibiHty, and corrosion resistance. The original effort has been in aerospace appHcations to replace nickel- and cobalt-base alloys in the 250—600°C range. The useful strength and corrosion-resistance temperature limit is ca 550°C. 

Fig. 7. Temperature—pH limits for crevice corrosion of titanium alloys in naturally aerated sodium chloride-rich brines. The shaded areas indicate regions...

Titanium does not stress-crack in environments that cause stress-cracking in other metal alloys, eg, boiling 42% MgCl2, NaOH, sulfides, etc. Some of the aluminum-rich titanium alloys are susceptible to hot-salt stress-cracking. However, this is a laboratory observation and has not been confirmed in service. Titanium stress-cracks in methanol containing acid chlorides or sulfates, red Aiming nitric acid, nitrogen tetroxide, and trichloroethylene. 

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