Interstitial solid solution

In practical metallurgy, the systems encountered are usually more complex than the simple binary ones depicted above. Chosen metals, or their alloys, usually contain (unavoidably or by design) quite a number of impurities , all of which may affect the properties of the product. Common non-metallic impurities are C Si, S and P, all of which are possible candidates for introducing lattice addition of some kind or other. The larger sizes of Si, S and P compared to those of B, C, N and O would seem to make them less likely than the latter to participate in interstitial solid solutions. Metallic impurities, if not involved in substitutional solid solution, may form new compounds which crystallise at the grain boundaries. 

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When a pure metal A is alloyed with a small amount of element B, the result is ideally a homogeneous random mixture of the two atomic species A and B, which is known as a solid solution of in 4. The solute B atoms may take up either interstitial or substitutional positions with respect to the solvent atoms A, as illustrated in Figs. 20.37a and b, respectively. Interstitial solid solutions are only formed with solute atoms that are much smaller than the solvent atoms, as is obvious from Fig. 20.37a for the purpose of this section only three interstitial solid solutions are of importance, i.e. Fc-C, Fe-N and Fe-H. On the other hand, the solid solutions formed between two metals, as for example in Cu-Ag and Cu-Ni alloys, are always substitutional (Fig. 20.376). Occasionally, substitutional solid solutions are formed in which the... 

Fig. 20.37 (a) Interstitial solid solution, (b) random substitutional solution and (c) an ordered substitutional solid solution forming a superlattice... 

There are a number of differences between interstitial and substitutional solid solutions, one of the most important of which is the mechanism by which diffusion occurs. In substitutional solid solutions diffusion occurs by the vacancy mechanism already discussed. Since the vacancy concentration and the frequency of vacancy jumps are very low at ambient temperatures, diffusion in substitutional solid solutions is usually negligible at room temperature and only becomes appreciable at temperatures above about 0.5T where is the melting point of the solvent metal (K). In interstitial solid solutions, however, diffusion of the solute atoms occurs by jumps between adjacent interstitial positions. This is a much lower energy process which does not involve vacancies and it therefore occurs at much lower temperatures. Thus hydrogen is mobile in steel at room temperature, while carbon diffuses quite rapidly in steel at temperatures above about 370 K. 

Interstitial Solid Solutions Interstitial solid solutions involve occupation of a site by introduced ions or atoms, which is normally empty in the crystal structure, and no ions or atoms are left out. Many metals form interstitial solid solutions in which small atoms (e.g., hydrogen, carbon, boron, nitrogen) enter empty interstitial sites within the host structure of the metal. Palladium metal is well known for its ability to absorb an enormous volume of hydrogen gas, and the product hydride is an interstitial sohd solution of formula PdH, 0 0.7, in which hydrogen atoms occupy... 

When stored under increasing relative humidities (RH), cromolyn sodium absorbed water, resulting in a continuous series of interstitial solid solutions [11]. The amount of water absorbed was proportional to the relative humidity of the environment and could be up to about nine molecules of water per molecule of drug. Such an unusual system was characterized by combining XPD with single-crystal x-ray studies. The unit cell parameters of cromolyn sodium were obtained from single-crystal x-ray studies, and this permitted the authors to index the powder pattern. The b axis spacing was found to increase dramatically as a function of the relative humidity up to 20% RH (Table 2). Above 40% RH, the unit cell dimensions were nearly constant. 

For interstitial solid solutions, too, the criteria used historically for the degree of solid solubility relates to elastic and electronic interactions. Experimentally observed maximum interstitial solubilities of H, B, C and N in Pd are inversely proportional to the sum of the s and p electrons, and hence are controlled by the valence electron concentration. Thus the electronic interactions dominate the... 

Interstitial fluid velocity, 25 290 Interstitial-free steels, 23 263, 296, 299 Interstitial solid solutions, alloying, 73 498 Interstitial vanadium compounds, 25 533-534... 

Substitutional solid solution with a major component. The trace element substitutes for a major element in a regular position of the crystal lattice. interstitial solid solution. Similar to the preceding phenomenon, but here the trace element occupies an interstitial position in the crystal lattice. 

The sub-lattice model is now the predominant model used in most CALPHAD calculations, whether it be to model an interstitial solid solution, an intermetallic compound such as 7-TiAl or an ionic solution. Numerous early papers, often centred around Fe-X-C systems, showed how the Hillert-Staffansson sub-lattice formalism (Hillert and Staffansson 1970) could be applied (see for example Lundberg et al. (1977) on Fe-Cr-C (Fig. 10.8) and Chatfteld and Hillert (1977) on Fe-Mo-C (Fig. 10.9)). Later work on systems such as Cr-Fe (Andersson and Sundman 1987) (Fig. 10.10) showed how a more generalised sub-lattice treatment developed by Sundman and Agren (1981) could be applied to multi-sub-lattice phases such as a. 

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. 

In interstitial compounds, however, the nonmetal is conveniently regarded as neutral atoms inserted into the interstices of the expanded lattice of the elemental metal. Obviously, this is an oversimplification, as the electrons of the nonmetal atoms must interact with the modified valence and conduction bands of the metal host, but this crude picture is adequate for our purposes. On this basis, Hagg made the empirical observation that insertion is possible when the atomic radius of the nonmetal is not greater than 0.59 times the atomic radius of the host metal—there is no simple geometrical justification for this, however, as the metal lattice is concomitantly expanded by an unknown amount. These interstitial compounds are sometimes called Hagg compounds.9,10 They are, in effect, interstitial solid solutions of the nonmetal in the metal (as distinct from substitutional solid solutions, in which actual lattice atoms are replaced, as in the case of gold-copper and other alloys Section 4.3). 

In the Encyclopedia of Twentieth Century Physics, R.W. Cahn describes A.H. Cottrell and B.A. Bilby s result that strain aging in an interstitial solid solution increases with time as t2/3 as the coming of age of the science of quantitative metallurgy [25]. Strain aging is a phenomenon that occurs when interstitial atoms diffuse to dislocations in a material and adhere to their cores and cause them to be immobilized. Especially remarkable is that the t2/3 relation was derived even before dislocations had been observed. 

The alloys just considered are substitutional solid solutions. Interstitial solid solutions are alloys with small atoms, for example, H, C, N, and O, in the interstitial sites, usually O and T sites. Some alloys have random distribution (disordered) if the melt is quenched but become ordered if heated and annealed or if cooled slowly. An example is the 1 1 alloy CuAu. The disordered structure is ccp, and the ordered structure is also ccp, except alternate layers parallel to a cell face contain Cu or Au. 

Discuss the possibility of order in interstitial solid solutions. 

Diffusion in interstitial solid solutions occurs by interstitially dissolved atoms jumping from one interstitial site to another. For an atom to move from one interstitial site to another, it must pass through a position where its potential energy is a maximum. The difference between the potential energy in this position and that in the normal interstitial site is the activation energy for diffusion and must be provided by thermal fluctuations. The overall diffusion rate is governed by an Arrhenius-type rate equation,... 

There are two kinds of solid solutions, namely, substitutional solid solutions and interstitial solid solutions. In substitutional solid solutions, the solute element occupies a position of one of the solvent elements in the solvent crystal. In interstitial solid solutions, on the other hand, the solute element occupies on of the vacant spaces between solvent elements in the solvent crystal lattice without displacing a solvent element. 

Keywords interstitial solid solutions, crystal structure, phase transformation, order-disorder, isotopic effect, antiphase domains, neutron diffraction, TiN026Hoi5, TiN026Doi5, TiN0.MH0.075D0.075 ... 

Abstract. One more method of study of the short-range order kinetics of H-atoms over tetrahedral interstices in lutetium (Lu) is proposed. It can be realized by the using of available data of measurements of heat capacity for h.c.p.-Lu-H interstitial solid solutions during the isothermal annealing. Comparison of estimated-parameters data from heat capacity and residual electrical-resistivity measurements is performed. It is shown that kinetics of heat capacity and residual resistivity at low temperatures is caused by the unique nature (short-range order relaxation) and can be described by two relaxation times at least. 

Tatarenko, V.A. and Radchenko, T.M. (2002) Kinetics of the hydrogen-isotope short-range ordering in interstitial solid solutions h.c.p.-T -H(D, T), in T.N. Veziroglu et al. (eds.), Hydrogen Materials Science and Chemistry of Metal... 

Peculiarity of the fullerene molecule formation also reveals itself in a fullerite crystal structure. Cubic crystal lattices of fullerites and hydrofullerites behave like those of different metals and alloys. Fullerene molecules are distributed in the lattice sites while atoms of elements are distributed in the octa- and tetrahedral interstitial sites forming the interstitial solid solutions. Fullerene molecules substitute each other in the sites of lattice and form the substitution solid solutions. Forming exo- and endocompounds, fullerene molecules that are in the lattice sites can change considerably the properties of crystal, whereas its crystalline structure remain unchanged. 

The transition metal hydrides exhibit such wide variations from stoichiometric compositions that they have often been considered interstitial solid solutions of hydrogen in the metal. This implies that the metal lattice has the same structure in the hydride phase as in the pure metal. That this is not the case can be seen in Table I, where of 28 hydrides formed by direct reaction of metal and hydrogen, only three (Ce, Ac, Pd) do not change structure on hydride formation. Even in these three cases, there is a large discontinuous increase in lattice parameter. The change in structure on addition of hydrogen plus the high heats of formation (20 to 50 kcal. per mole) (27) indicates that the transition metal hydrides should be considered definite chemical compounds rather than interstitial solid solutions,... 

Interstitial solid solutions are treated similarly. The structure is again approximated with a two-sublattice model, but where one sublattice is occupied by substimtional elements, and one by the smaller interstitial elements (e.g. C, N, H) and vacancies. It follows, from Eq. 11.27, that for a binary interstitial solution, the Gibbs energy is given by ... 

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