b Chemical affinity When two metals have a high affinity for one another the

B chemical affinity when two metals have a high

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(b) Chemical affinity When two metals have a high affinity for one another the tendency is for solid solubility to be severely restricted and intermetallic compounds to be formed instead. This occurs when one element is electronegative and the other is electropositive. (c) Relative valency If a metal of one valency is added to a metal of another valency the number of valency electrons per atom, the electron ration, will be altered. Crystal structures are very sensitive to a decrease in the electron ratio. Consequently, a metal of high valency can dissolve very little of a metal of low valency, although a metal of low valency might be able to dissolve an appreciable amount of a high-valency metal. (d) Crystal type If two metals are of the same crystal lattice type and all other factors are favourable, it is possible for complete solid solubility to occur over the whole composition range. (It is also necessary that the relative size of atoms differ by not more than 7% for complete solid solubility.) Some examples of binary systems in which a continuous range of solid solutions is formed are Cu- Ni, Cu-Au, Al 2 O 3 -Cr 2 O 3 and MgO-NiO. (a) Substitution Solid Solutions Based on Hume- Rothery’s rule on relative size, if the atoms of the solvent metal and solute element are of similar sizes (not more than 15% difference), they form substitution solid solution , where part of the solvent atoms are substituted by atoms of the alloying element (see the picture below).
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~ Page 63 of 79 ~ Generally, the arrangement of solute in a substitutional solid solution is random or unordered, but in some instances solid solutions of an ordered type may be formed. An ordered solid solution (also known as superlattice ) can exist only at either one fixed composition or over a narrow range of composition. (b) Interstitial Solid Solutions If the atoms of the alloying elements are considerably smaller, than the atoms of the matrix metal, interstitial solid solution forms, where the matrix solute atoms are located in the spaces between large solvent atoms (see the picture below). When the solubility of a solute element in interstitial solution is exceeded, a phase of intermediate compound forms. These compounds (TiN, WC, Fe 3 C, etc.) play important role in strengthening - steels and other alloys. Some substitution solid solutions may form ordered phase where ratio between concentration of matrix atoms and concentration of alloying atoms is close to simple numbers like AuCu 3 and AuCu. 2. EUTECTICS A eutectic system is a mixture of chemical compounds or elements which are insoluble in the solid state, and has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature .
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