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SOLID SOLUTIONS

   A solid solution forms when, as the solute atoms are added to the host material, the crystal structure is maintained, and no new structures are formed.

Fig. 4-2 :

  Impurity point defects are found in solid solutions, of which there are two types: substitutional and interstitial. For substitutional, solute or impurity atoms replace or substitute for the host atoms . There are several features of the solute and solvent atoms that determine the degree to which the former dissolves in the latter; these are as follows:

1. Atomic size factor. Appreciable quantities of a solute may be         accommodated in this type of solid solution only when the              difference in atomic radii between the two atom types is less than    about_+15%. Otherwise the solute atoms will create substantial         lattice distortions and a new phase will form.

2. Crystal structure. For appreciable solid solubility the crystal           structures for metals of both atom types must be the same.

3. Electronegativity. The more electropositive one element and the      more electronegative the other, the greater is the likelihood that      they will form an intermetallic compound instead of a                     substitutional solid solution.

4. Valences: Other factors being equal, a metal will have more of a     tendency to dissolve another metal of higher valency than one of     a lower valency.

  An example of a substitutional solid solution is found for copper and nickel. These two elements are completely soluble in one another at all proportions.

1-The atomic radii for Cu and Ni are 0.128 and 0.125 nm,

2- Both have the FCC crystal structure,

3- Their electronegativities are 1.9 and 1.8

4 -The most common valences are +1 for Cu (sometimes can be          +2) and +2 for Ni .

For interstitial solid solutions, impurity atoms fill the voids or interstices among the host atoms . For metallic materials that have relatively high atomic packing factors, these interstitial positions are relatively small. Consequently, the atomic diameter of an interstitial impurity must be substantially smaller than that of the host atoms. Normally, the maximum allowable concentration of interstitial impurity atoms is low (less than 10%). Even very small impurity atoms are ordinarily larger than the interstitial sites, and as a consequence they introduce some lattice strains on the adjacent host atoms.

    Carbon forms an interstitial solid solution when added to iron; the maximum concentration of carbon is about 2%. The atomic radius of the carbon atom is much less than that for iron: 0.071 nm versus 0.124 nm.