Nitrogen in tantalum

Also for the solubility of nitrogen in tantalum different data are given in the literature. Table 1-8 shows the data of Albert (3). 

As for niobium, the solubility of nitrogen in tantalum follows Sievert s square root law. The absorption and desorption of nitrogen depend on the temperature and on the N2-partial pressure. So, the nitrogen content can be reduced to below 0.1 at.% (77 A g/g) at 2000°C and 1.3 10 Pa, whereas at 3000 C the same nitrogen content can already be obtained at 0.67 Pa. 

The nitrogen content of tantalum ranges from 20 to 500 fig/g depending on the applied fusion technique. Samples, zone refined in an electron beam, contain only 2.5 tig/g of oxygen and nitrogen. 

The diffusion of nitrogen is slower than that of oxygen. Variations of hardness by heat treatment in oxygen and in nitrogen at 1200°C, show that nitrogen has a less decisive influence both at the surface and in the bulk of the metal (3). 

The yield point of tantalum is influenced by nitrogen and oxygen. Equa- 

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In 1970 Friedrich et al. (24) reached the following conclusion "The determination of nitrogen in tantalum using the nickel-magnesium-cerium technique is possible if the nickel. -tantalum ratio is above 8 1. It has still not been possible to find a method suitable for the determination of nitrogen in niobium". 

The natural niobites and tantalites axe usually black, and form iso-morphous, prismatic crystals, belonging to the rhombic system. They are easily fusible and very brittle, presenting an uneven fracture. Their density increases from 5-2 to 8 2 with increase in tantalum content.6 When heated to redness in vacuo they evolve small quantities of gas, which consists of carbon dioxide, nitrogen and oxygen.7 Small quantities of helium have also been found occluded in them. 

Tantalum reacts slowly with nitrogen when heated in the gas combination commences at about 900° C. One gram of tantalum wire absorbed 2-2 cc. of nitrogen in one hour between 920° and 1030° C.,a and this was not expelled by heating in vacuo. In an earlier investigation4 thirty-one hours exposure at 1000° C. gave an absorption equal to 17-3 per cent, of the weight of tantalum. 

Friedrich, K., Lassner, E., Paesold, E. Determination of oxygen and nitrogen in niobium and tantalum. J. Less-Common Metals 22, 429 (1970). 

An ordering of dissolved oxygen atoms in solution in a-zirconium and a-titanium (both metallic phases) has been reported by Dagerhamn and Holm-berg (2,7). Dissolved nitrogen atoms in tantalum also can order (14, 18). 

Compared with titanium, the solubility of nitrogen in both metals is low. According to Fromm and Gebhardt (1), the figures at I000°C are 0.3 at.% for niobium and 5 at.% for tantalum. The nitrogen content of industrial metals is usually very low (< 20 Mg/g) and only seldomly reaches values above 100 t g/g. 

In 1968 Friedrich et al. (23), who also tested flux methods (Pt flux or bath, or Ni-Ce-Mg) and used an empty crucible for each sample, were still unable to report any improvement in this situation. Their conclusion was that "since none of the techniques investigated up to the present time has given satisfactory nitrogen results, we must advise against the determination of nitrogen in niobium and tantalum by vacuum fusion at present, unless an empirical factor can be tolerated". 

Because of favourable nuclear properties of these matrices, several nuclear reactions can be used for the instrumental determination of nitrogen in niobium and tantalum. 

The conditions for the determination of nitrogen in niobium and tantalum (42)(45)(46) using the N(p,n) 0 reaction are similar to those described under 2.3 except for the irradiation (12 MeV protons, 0.5 lA for Nb and 3 to 4 /.A for Ta) and measuring conditions (No lead absorber needed, detector with 5 % relative detection efficiency). 

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

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