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Types Of Alloys And Compounds

 After solidification the alloying elements may exist in the base metal atoms in three general ways ,

1. Alloying Atoms may be relatively soluble in the base metal atoms forming a solid solution.

2. Alloying atoms may form a compound with the base metal atoms producing an intermetallic compound.

3. Alloying atoms may be relatively insoluble in the base metal atoms forming a mechanical mixture.




SOLID SOLUTIONS :

When elements combine to form alloys by completely dissolving in each other they are said to be in solid solutions. In a Solid solution, the different types of atoms of the components in the alloys by completely dissolving in each other they are said to be in solid solutions. In a solid solution, the different types of atoms of the components in the alloy form a common crystal lattice. The component, whose lattice is retained, is called solvent.

Conditions under which metallic solid solutions are formed are : 

(a) the components have the same crystal lattice and 

(b) There is no large difference in the lattice form of the alloying components.

The two most typical kinds of solid solutions are substitutional and interstitial.

In substitutional solid solutions, a part of atoms at the points of the solvent crystal lattice has been replaced by solute atoms.

In Interstitial solid solutions, the atoms of the solute metal occupy the vacant positions or interstices between the atoms of the solvent.

The nature of the solid solutions has important effects on many alloy properties. For example, strength and hardness increase with the amount of solute present , but ductility usually decreases. 

Intermetallic compounds, Compounds are obtained upon chemical interaction of  certain compounds of alloys. They are commonly formed between metals and non metals. Chemical compounds in the alloy not only have a definite chemical composition but also possess distinct physical and mechanical properties.

In an intermetallic compounds, the lattice form is the characteristic of the compound. It differs from those of its components, and usually is more complex. The lower order of symmetry in the complex lattices results in low electrical conductivity, and greater resistance to slip. Consequently , such compounds are very hard and brittle. To be of real value , they must be well dispersed through the structure. The softer base metal will then strengthen the latter.

Mechanical Mixtures, In some cases when a liquid solution is cooled, crystals of all components separate simultaneously from the melt and form a close mechanical mixture. This is also known as eutectic mixture. In mechanical mixtures, metals retain their same structural identity and properties. 

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