Reducing titanium alloys

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...

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... 

Other coatings, such as TiAlN (96), TiCN, Zr02, and ZrN (97), and CrN (98) were developed for special appHcations. The last was developed for higher speed machining of titanium alloys. Sometimes a coating is developed not for its wear-resistance but for its heat insulation. The case in point is alumina coating of cBN to reduce the heat conductivity at the surface so that the cBN performance can be enhanced (99). 

Stern, eta obtained potentiostatic polarisation curves for titanium alloys in various solutions of sulphuric acid and showed that the mixed potentials of titanium-noble metal alloys are more positive than the critical potential for the passivity of titanium. This explains the basis for the beneficial effects of small amounts of noble metals on the corrosion resistance of titanium in reducing-type acids. Hoar s review of the work on the effect of noble metals on including anodic protection should also be consulted... 

Gettering. Gettering materials, such as zirconium or titanium alloys, are heated to 400°C. At that temperature, they react with the impurities in the gas stream such as O2, H2O, N2, H2, CO, CO2, and hydrocarbons. Total impurities can be reduced to <100 ppb. 

HP IR cells need to exhibit high mechanical strength and resistance to corrosion by solvents and reagents. They are often fabricated from austenitic steels (e. g. type 316) which are satisfactory for relatively mild temperatures and pressures but can be corroded by acid or form [Fe(CO)5] and [Ni(CO)4] by reaction with CO. Alternative materials for construction include some titanium alloys (which can be vulnerable to primary alcohols at high temperature) and nickel-molybdenum-chromium alloys (e. g. Hastelloy C-276, Hastelloy B2) which are highly resistant to reducing, oxidising and acidic conditions. 

Alloy with Memory. In seeking a way to reduce the brittleness of titanium, U.S. Navy researchers serendipitously discovered a nickel-titanium alloy having an amazing memory. Previously cooled clamps made of the alloy (nitinol) are flexible and can be placed easily in position. When warmed to a given temperature, the alloy hardware then exerts tremendous pressure. Use of conventional clamps for holding bundles of wires or cables in a ship or aircraft structure requires special tools. For this and other applications in industry and medicine, nitinol has been in demand. The alloy, however, is not easy to produce because only minor variations in composition can affect the snap back" temperature by several degrees of temperature. 

Increasingly, titanium alloys are competing with nickel-base alloys on the basis of cost, strength, and corrosion resistance. The alloy Ti-3Al-2.5 V, for example, is finding expanded use in die process industries because of its resistance to mildly reducing chloride en ironments. 

Sheet, thin plate, welded tubing, and small-diameter bar of commercially pure titanium are manufactured into parts by conventional cold-working techniques. The formability of titanium, when worked at room temperature, is like that of cold-rolled stainless steel. At 65°C the formability compares with stainless steel annealed at room temperature. Cold-working maybe difficult for some titanium alloys and heat may be required, especially for severe forming operations. Generally, titanium and its alloys are worked between 200 and 300°C. Lubricants reduce friction and galling. Slow forming speeds at controlled rates improve workability and are recommended for more difficult operations. 

The corrosion resistance of steel can be greatly increased by alloying with chromium to form the stainless steels. Figure 12 shows the effect of increasing chromium content on the corrosion rate of steel. At 12-14% Cr there is a dramatic decrease in corrosion rate. The corrosion resistance is due to the formation of a thin adherent layer of chromium oxide on the steel surface [23]. The steel will remain stainless provided the oxide layer remains intact or can be rapidly repaired, i.e. the steel is exposed to oxidising conditions. The precipitation of chromium carbide at grain boundaries will cause disruption of this oxide film (See Sect. 3.2.5) and hence localised corrosion. Precipitation of chromium carbide can be reduced by alloying with elements which form carbides more readily than chromium, e.g. titanium, niobium, and tantalum. 

Cavitation, which is the source of the main effects of ultrasound, is also the origin of a common problem with probe systems tip erosion, which occurs despite the fact that most probes are made of a titanium alloy. There are two unwanted side effects associated with erosion, namely (a) metal particles eroded from the tip will contaminate the system and (b) physical shortening of the horn reduces efficiency — eventually, the horn will be too short to be efficiently tuned. The latter problem is avoided by... 

The feasibility of FSW of titanium was first demonstrated prior to 1997 at which time a TWI group sponsored project (GSP 5689) was instituted to further the development of FSW in titanium alloys [1]. Proof- of-concept and initial development was performed mainly on Ti-6A1-4V. While some reports of FSW of titanium are available in conference proceedings or have been presented at conferences [2-6] no papers in the archival literature have been published on the subject. Conceptually, FSW of titanium is attractive because it may mitigate some problems associated with fusion welding of titanium alloys. For example, because peak temperatures in FSW are necessarily lower than those encountered in fusion welds, problems such as grain growth in the HAZ and embrittlement due to contamination by interstitial elements (0, N, C) uptake may be reduced. 

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