Solid solution composition niobium

Figure 5.4. The niobium-hydrogen system. A small part (from 40 to 50 at.% H) of the diagram is shown in the low- and very-low-temperature region. Notice the complex equilibria and the high number of intermediate solid solution phases. Other phases are formed in the composition ranges not shown in figure the q (11 to 39 at.% H) and 0 (21 to 41 at.% H) solid solutions in the Nb-richer part and the nearly stoichiometric 6 phase ( NbH2, cF12-CaF2-type).

Niobium metal absorbs nitrogen, similar to hydrogen, forming interstitial solid solution. The absorption occurs at 300°C and the solubility of nitrogen in the metal is directly proportional to the square root of the partial pressure of nitrogen. The reaction is exothermic and the composition of such interstitial solid solution varies with the temperature. When the metal is heated with nitrogen at temperatures between 700 to 1,100°C, the product is niobium nitride, Nb2N or (NbNo.s) [12033-43-1]. When heated with ammonia at these temperatures, niobium forms this nitride. Another niobium nitride exists, NbN [24621-21-4], with a face-centered cubic crystalline structure. 

Fig. 5.12. Relationship between hardness H of solid solution crystals and composition of aluminium-niobium and aluminium-tantalum alloys after melting at different temperature, and limited solubility curves for tantalum in aluminium.

Further substitution of niobium results in exceedingly complex structures, and a micrograph of a typical crystal of the niobium end-member of the series, BiuNbzOn, is illustrated in Fig. 11. The unit cell of this material is extremely large, approximately 115 x 80 X 5 5 A, and the x-ray powder diffraction diagram is impossible to interpret. From the micrograph shown, it would appear that the structure is based upon different principles from the one described above, but it can nevertheless be derived from it by repeated overlap of layers on (112) and (113) planes in a very complex sequence. Why such a complex sequence should be employed, and whether either of these phases are true "phases", or merely certain compositions in a quasi-continuous solid solution series, is not yet certain. What is however, demonstrated, is the remarkable ability which these simple layered structures show to variations in stoichiometry. 

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. 

Alloy 625, also known as Inconel alloy 625, is used both for its high strength and aqueous corrosion resistance. The strength of alloy 625 is primarily a solid solution effect from molybdenum and niobium (Columbian). Alloy 625 has excellent weldability. The chemical composition is shown in Table 15.7. 

The process of separating the intermediate products from the purified solutions, in the form of solid complex fluoride salts or hydroxides, is also related to the behavior of tantalum and niobium complexes in solutions of different compositions. The precipitation of complex fluoride compounds must be performed under conditions that prevent hydrolysis, whereas the precipitation of hydroxides is intended to be performed along with hydrolysis in order to reduce contamination of the oxide material by fluorine. 

Agulyansky et al. [492, 493] investigated the complex structure and composition of solid phases precipitated by ammonia solution from experimental and industrial niobium and tantalum strip solutions. Fig. 136 shows isotherms (20°C) of Nb205 content versus pH for solutions prepared by the dissolution of (NH4)3NbOF6 and (NH4)2NbOF5 in water and of Nb metal in... 

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