Solid solutions in metals

Solid solutions in metallic alloys are normally compositionally very uniform random variations of 5% would be unusual. Also, a two phase layer normally is found between two solid solutions. The sulfur distribution in coal seems not to behave this way. Apparently, the distribution pattern established at some early stage of coal formation is frozen-in and the organic sulfur is bound so tightly to its hydrocarbon sites that it cannot diffuse until the temperature of the coal is raised to 400°C or above (18). 

Hume-Rothery rules - A set of empirical rules for predicting the occurrence of solid solutions in metallic systems. The rules involve size, crystal structure, and electronegativity. 

The likelihood of forming a substitutional solid solution between two metals will depend on a variety of chemical and physical properties, which are discussed in Chapter 6 (see the Hume-Rothery solubility rules in Section 6.1.4). Broadly speaking, substitutional solid solution in metallic systems is more likely when ... 

Eshelby, J. D. (1954). Distortion of a Crystal by Point Imperfections. Journal of Applied Physics, Vol. 25, No. 2, (February 1954), pp. 255-261. ISSN 0021-8979 Eshelby, J. D. (1956), The continuum theory of lattice defects. Solid State Physics Advances in Research and Applications. Frederick Seitz and David Turnbull, (Ed.), Vol. 3, (1956), pp. 79-144, Elsevier, ISBN 978-0-12-374292-6, Amsterdam, the Netherlands Friedel, J. (1954). Electronic structure of primary solid solutions in metals. Advances in Physics, Vol. 3, No. 12, (October 1954), pp. 446-507, ISSN 0001-8732 Fan, G. J. Choo, H. Liaw, P. K. (2007). A new criterion for the glass-forming ability of liquids. Journal of Non-Crystalline Solids, Vol. 353, No.l, (January 2007), pp. 102-107, ISSN 0022-3093... 

Friedel J (1954) Electronic stmcture of primary solid solutions in metals. Adv Phys 3 446-507... 

Questions of the analytic control of maintenance of the bivalent metals cations to their joint presence in materials of diverse fixing always were actual. A simultaneous presence in their composition of two cations with like descriptions makes analysis by sufficiently complicated process. Determination of composition still more complicates, if analyzed object is a solid solution, in which side by side with pair of cations (for example, Mg " -Co ", Mn -Co, Zn -Co ) attends diphosphate anion. Their analysis demands for individual approach to working of methods using to each concrete cations pair. 

G. Foumet, Order-disorder phenomena in solid solutions, in. Phase Stability in Metals and Alloys", P S. 

We have already learned that metals may be deformed easily and we have explained this in terms of the absence of directional character in metallic bonding. In view of this principle, it is not surprising that two-element or three-element metallic crystals exist. In some of these, regular arrangements of two or more types of atoms are found. The composition then is expressed in simple integer ratios, so these are called metallic compounds. In other cases, a fraction of the atoms of the major constituent have been replaced by atoms of one or more other elements. Such a substance is called a solid solution. These metals containing two or more types of atoms are called alloys. 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

Reference has already been made to the dehydration of alums (Sect. 1.2 and Table 10), decomposition of ammonium metal phosphates (Sect. 4.1.5) and the use of KMn04—KCIO4 solid solutions in mechanistic studies of the decomposition of potassium permanganate (Sect. 3.6). 

Elements dissolved in boron influence its crystal structure. Dissolved impurities also influenee the physical and chemical properties of boron, especially the electrical properties, because boron is a semiconductor. Preparation of solid solutions in jS-rh boron requires a careful choice of crucible material. To avoid contamination, boron nitride or a cold, coinage-metal crucible should be used or the levitation or floating-zone melting techniques applied. 

S. Solid Solution of Transition and Inner Transition Metals 6.7.2.S.2. Solid Solutions in 6-Rhomtx>hedral Boron. 

The solubility of rare-earth metals in /3-rh boron is unknown. Rare-earth-boron systems are cubic borides - with an composition (E = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb). The occurrence of this phase excludes extensive solid solutions in... 

When two metals A and B are melted together and the liquid mixture is then slowly cooled, different equilibrium phases appear as a function of composition and temperature. These equilibrium phases are summarized in a condensed phase diagram. The solid region of a binary phase diagram usually contains one or more intermediate phases, in addition to terminal solid solutions. In solid solutions, the solute atoms may occupy random substitution positions in the host lattice, preserving the crystal structure of the host. Interstitial soHd solutions also exist wherein the significantly smaller atoms occupy interstitial sites... 

Now, suppose that we have a solid solution of two (2) elemental solids. Would the point defects be the same, or not An easy way to visualize such point defects is shown in the following diagram, given as 3.1.3. on the next page. It is well to note here that homogeneous lattices usually involve metals or solid solutions of metals (alloys) in contrast to heterogeneous lattices which involve compounds such as ZnS. 

For a range of simple substitutional solid solutions to form, certain requirements must be met. First, the ions that replace each other must be isovalent. If this were not the case, other structural changes (e.g., vacancies or interstitials) would be required to maintain electroneutrality. Second, the ions that replace each other must be fairly similar in size. From a review of the experimental results on metal alloy formation, it has been suggested that 15% size difference can be tolerated for the formation of a substantial range of substitutional solid solutions. For solid solutions in nomnetal-lic systems, the limiting difference in size appears to be somewhat larger than 15%, although it is very difficult to quantify this. To a certain extent, this is because it is difficult to quantify the sizes of the ions themselves, but also because solid solution formation is very temperature dependent. 

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... 

Interaction with water is too violent to permit of safe use of the hydride as a drying agent. When dispersed as a solid solution in a metal halide, it can be used as a drying or reducing agent. 

Zinc, Cu and Ni have similar ionic radii and electron configurations (Table 5.6). Due to the similarity of the ionic radii of these three metals with Fe and Mg, Zn, Cu and Ni are capable of isomorphous substitution of Fe2+ and Mg2+ in the layer silicates. Due to differences in the electronegativity, however, isomorphous substitution of Cu2+ in silicates may be limited by the greater Pauling electronegativity of Cu2+ (2.0), whereas Zn2+ (1.6) and Ni2+ (1.8) are relatively more readily substituted for Fe2+ (1.8) or Mg2+ (1.3) (McBride, 1981). The three metals also readily coprecipitate with and form solid solutions in iron oxides (Lindsay, 1979 Table 5.7). 

Alloys of copper and zinc can be obtained by combining the molten metals. However, zinc is soluble in copper up to only about 40% (of the total). When the content of a copper/zinc alloy contains less than 40% zinc, cooling the liquid mixture results in the formation of a solid solution in which Zn and Cu atoms are uniformly distributed in an fee lattice. When the mixture contains more than 40% zinc, cooling the liquid mixture results in the formation of a compound having the composition CuZn. The solid alloy consists of two phases, one of which is the compound CuZn and the other is a solid solution that contains Cu with approximately 40% Zn dissolved in it. This type of alloy is known as a two-phase alloy, but many alloys contain more than three phases (multiple-phase alloys). 

The formation of solid solutions of metals is one way to change the properties (generally to increase strength) of the metals. Strengthening metals in this way is known as solid solution strengthening. The ability of two metals to form a solid solution can be predicted by a set of rules known as the Hume-Rothery rules, which can be stated as follows ... 

The same requirement extends to the minerals considered in the calculation. Minerals in nature occur as solid solutions in which elements substitute for one another in the mineral s crystal structure, but thermodynamic datasets generally contain data for pure minerals of fixed composition. A special danger arises in considering the chemistry of trace metals. In nature, these would be likely to occur as ions substituted into the framework of common minerals or sorbed onto mineral or organic surfaces, but the chemical model would consider only the possibility that the species occur as dissolved species or as the minerals of these elements that are seldom observed in nature. 

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