There are two types of solid solutions: 1. Substitutional Solid Solution 2. Interstitial Solid Solutions.
Solute is the minor element that is added to the solvent, and solvent is the major element of solution. When a particular crystal structure of the solvent is maintained during alloying the alloy is called a solid solution. The amount of solute that may be dissolved by the solvent is generally a function of temperature (with pressure constant) and usually increase with temperature. There are three possible condition for solution i.e., unsaturated, saturated and supersaturated.
Type # 1. Substitutional Solid Solution:
If the size of the solute atom is similar to that of the solvent atom, the solute atoms can replace solvent atoms to form a substitutional solid solution.
Example:
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Brass, in which zinc (solute atom) is introduced into the lattice of copper (Solvent).
Two conditions are generally required to from complete substitutional solid solution:
(i) Two elements must have similar crystal structure.
(ii) The difference in their atomic radii should be less than 15%.
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Example:
In the Aluminium nickel alloy system, both metal are FCC. The relative size factor is 14%. However, nickel is lower in valence then Al and as per relative valence factor solid nickel dissolve 5% Al but higher valence Al, dissolve only 0.04 % Ni.
Type # 2. Interstitial Solid Solutions:
If the size of the solute atom is much smaller than that of the solvent atom the solute atom can occupy an interstitial position forming interstitial solid solution. Since the space of the lattice structure are restricted in size, only atoms with atomic radii less than 1 angstrom are generally form interstitial solid solution.
These are hydrogen (0.46), boron (0.97), Carbon (0.77), Nitrogen (0.71) and Oxygen (0.60). More solute atoms may be dissolved interstitially until the solution become saturated at that temperature. Interstitial solid solutions normally have limited solubility and generally are of a little importance.
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Carbon in iron is a notable exception and forms the basis for hardening steel. Carbon dissolves in iron interstitially. The maximum solubility of Carbon in iron (F.C.C.) is 2% at 1150°F while maximum solubility of carbon in α iron (B.C.C.) is only 0.025% at 727°C.
Two necessary conditions for forming ISS are:
(i) The solvent atom must have more than one valence.
(ii) The atomic radius of solute atom must be less than 59% of the atomic radii of solvent atom.
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Example:
Steel, where carbon atoms are present in interstitial positions between iron atoms with maximum percentage of 2. Atomic radius of carbon is 0.071 nm which is less than 59% of 0.125 nm radius of iron atom.
Intermediate Alloy Phase of Compound:
The intermediate alloy phase are compound whose chemical composition are intermediate between the two pure metals and generally have crystal structures different from those of pure metals.
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These are of three types:
(i) Intermetallic Compound or Valency Compound:
These are generally formed between chemical dissimilar metals and are combined by following the results of chemical valence. They have strong bonding their properties are essentially non-metallic. These are formed where metals are far away in periodic table.
i.e., poor ductility, poor electrical conductivity.
e.g., MgPb2, Mg2Sn, Cu2Se, Ti3A/, Ni3A/ etc.
(ii) Interstitial Compound:
These compounds formed between the transition metal such as Scandium (SC), titanium (Ti), Ta, W, Fe etc. with hydrogen, oxygen, carbon, boron & nitrogen.
These compound are metallic having high melting point and are extremely hard, e.g. TiC, TaC, Fe4N, W2C, CrN, TiH etc.
(iii) Electron Compound:
These are formed by Copper, gold, silver, iron, and nickel with the metal like cadmium, magnesium, tin, zinc and aluminium.
These compound have definite ratio of valence electrons to atom and therefore called electron compound. Many electron compounds have properties resembling to solid solution.