Niobium alloying element

Niobium is important as an alloy addition in steels (see Steel). This use consumes over 90% of the niobium produced. Niobium is also vital as an alloying element in superalloys for aircraft turbine engines. Other uses, mainly in aerospace appHcations, take advantage of its heat resistance when alloyed singly or with groups of elements such as titanium, tirconium, hafnium, or tungsten. Niobium alloyed with titanium or with tin is also important in the superconductor industry (see High temperature alloys Refractories). 

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.)... 

Elements, which usually originate from alloyed steel scrap and non-ferrous metals in the steel charge, of the type which can be added to steels as alloying elements. They include nickel, chromium, molybdenum, copper, niobium and vanadium. Residual elements... 

Each fuel assembly consists of 18 fnel pins. A fuel pin is a tube of zirconium/2V2 niobium alloy with an external diameter of 13-6 mm and a minimum thickness of 0-825 mm, filled with pellets of uranium dioxide. The fuel pellets are 11-52 mm in diameter and 15 mm high with cavities in the end faces. The inner space of the fuel pin is filled with an argon/ helium gas mixture. Top and bottom terminal grids hold the fuel pins and are positioned above and below the level of the core. Two fnel assemblies are combined in the element giving an active length of 7 m. The nranium feed enrichment is 2-0%. 

Bra] Brandis, H., Preisendanz, H., Sehueler, R, The Influenee of Alloying Elements Titanium, Vanadium and Niobium on the Aetivity of Carbon in Fe-X-C Alloys in the Temperature Range from 900 to 1100°C (in German), Thyssen Edelst. Techn. Ber., 7(1), 92—104 (1981) (Experimental, Thermodyn., 32)... 

The stabilised austenitic stainless steels for cladding contain an alloying element (niobium), which forms stable grain boundary carbides. This prevents chromium depletion along the grain boundaries and makes the material immune to stress corrosion cracking. Non-stabilised material was used for the first layer because the thermal expansion coefficient of the material is closer to that of the low-alloy pressure vessel material. The presence of niobium in the second layer allows performance of so-called retrospective dosimetry in the RPV inner surface by machining out some scraps for further chemical separation and activity measurement to determine real neutron fluence on the RPV inner surface. 

Most LWR fuel rod cladding is made of Zircaloy (and its derivatives), which is an alloy of primarily zirconium and tin. Other alloying elements include niobium, iron, chromium, and nickel. Zircaloy was chosen because it has a very low cross section for thermal neutrons. Naturally occurring zirconium contains about l%-5% hafnium. The hafnium must be removed because it has a very high thermal neutron cross section and is often used in making control rods for reactors. The separation process used in the United States is a liquid-liquid extraction process. It is based on the difference in solubility of the metal thiocyanates in methyl isobutyl ketone. In Europe, a process known as extractive distillation is used to purify zirconium. This method employs a separation solvent that interacts differently with the zirconium and hafnium, causing their relative volatilities to change. This enables them to be separated by a normal distillation process. The separated zirconium is then alloyed with the required constituents. 

Various TisAl-based alloys have been developed with niobium as a major alloying element and further components for obtaining an optimised balance of strength, formability, toughness, and oxidation resistance. The alloys are two-phase or three-phase. Current TIbAI-based alloys with engineering significance are listed in Table 3.1-23. 

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. 

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