Tantalum alloying element

The important (3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tantalum, and niobium of the P-isomorphous type and manganese, iron, chromium, cobalt, nickel, copper, and siUcon of the P-eutectoid type. The P eutectoid elements, arranged in order of increasing tendency to form compounds, are shown in Table 7. The elements copper, siUcon, nickel, and cobalt are termed active eutectoid formers because of a rapid decomposition of P to a and a compound. The other elements in Table 7 are sluggish in their eutectoid reactions and thus it is possible to avoid compound formation by careful control of heat treatment and composition. The relative P-stabilizing effects of these elements can be expressed in the form of a molybdenum equivalency. Mo (29) ... 

The corrosion behaviour of amorphous alloys has received particular attention since the extraordinarily high corrosion resistance of amorphous iron-chromium-metalloid alloys was reported. The majority of amorphous ferrous alloys contain large amounts of metalloids. The corrosion rate of amorphous iron-metalloid alloys decreases with the addition of most second metallic elements such as titanium, zirconium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, cobalt, nickel, copper, ruthenium, rhodium, palladium, iridium and platinum . The addition of chromium is particularly effective. For instance amorphous Fe-8Cr-13P-7C alloy passivates spontaneously even in 2 N HCl at ambient temperature ". (The number denoting the concentration of an alloy element in the amorphous alloy formulae is the atomic percent unless otherwise stated.)... 

Tantalum is used as an alloying element up to 12 wL% in specialty superalloys for particular aircraft apphcations (e.g., turbine blades). Tantalum provides solid-solution strengthening, reacts with interstitial carbon to form stable carbides, and improves the thermal stability of intermetallic compounds. 

Passive metals and alloys. Usually alloys such as stainless steel or nickel-chromium can be used unprotected in innocuous environments and in a certain range of aggressive environments such as seawater or mild acids, depending on the content of alloying elements. Superpassive metals—such as tantalum, which resists strong hydrochloric acid—also exist but are considerably more expensive. The main issue with passive metals is their propensity for localized—rather than uniform—co rrosion. 

Niobium is similar in nature to the other psissivating reactive-refractory metals (titanium, zirconium, and tantalum) and has an inherent resistance to a wide range of chemicals. In general, compared to Zr and Ti, Nb has better corrosion properties in acids with small amounts of metal or organic contaminants. Niobium alloys with alloying elements such as Zr and Ti have been evaluated surd have shown increased reactive tendencies in rough proportion to their compositional content as might be expected with solid solution alloys. 

A major issue, for the passivation and corrosion resistance of aluminum alloys, is the existence or not of second phase inter-metallic particles resulting from alloying with elements that have low solubility in aluminum (Rynders et al., 1994 Kowal et al., 1996). These particles are detrimental to the resistance of the passive film to breakdown (the first stage of a localized corrosion process). In contrast to stainless steels, this factor often overwhelms the beneficial alloying effects. However, it must be pointed out that alloying elements such as copper in solid solution are beneficial (Muller and Galvele, 1977). Other elements, such as chromium, molybdenum, titanium, tantalum, and niobium, seem to improve the corrosion resistance of aluminum, but their solubility is too low for them to be used in conventional alloy processes, and they require the use of rapid quenching processes or some sort of nonequilibrium surface deposition. 

Density is a particularly important characteristic of alloys used in rotating machinery, because centrifugal stresses increase with density. Densities of the various metals in Table 1 range from 6.1 to 19.3 g/cm. Those of iron, nickel, and cobalt-base superaHoys fall in the range 7-8.5 g/cm. Those alloys which contain the heavier elements, ie, molybdenum, tantalum, or tungsten, have correspondingly high densities. 

Tungsten with the addition of as much as 5% thoria is used for thermionic emission cathode wires and as filaments for vibration-resistant incandescent lamps. Tungsten—rhenium alloys are employed as heating elements and thermocouples. Tantalum and niobium form continuous soHd solutions with tungsten. Iron and nickel are used as ahoy agents for specialized appHcations. 

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