Saturday, February 10, 2024

Intermediate Phases

 If an alloying element is added in excess of the limit of solid solubility, a second phase appears along with the primary solid solution.

INTERMEDIATE PHASES

If an alloying element is added in excess of the limit of solid solubility, a second phase appears along with the primary solid solution. This second phase may be a primary solid solution of base metal in the alloying element, or an 'intermediate' phase (which differs in both crystal structure and properties from primary solid solution).

This intermediate phases may have either narrow or wide ranges of homogeneity, and may or may not include a composition having a simple chemical formula.

Intermediate phases may range between an ideal solid solution and an ideal chemical compound.

1. Intermetallic Compounds

The compounds formed by two or more metals in an apparently stoichiometric proportion is called intermetallic compounds.

Intermetallic compounds are intermediate phases only when they have (i) a narrow range of homogeneity; (ii) simple stoichiometric proportions i.e., chemically correct proportions; and (iii) atoms of identical crystal structures.

Thus a intermetallic compound has a characteristic structure which is generally more complicated than structures of pure metals or solid solutions.

These intermetallic compounds and solid solutions are used to obtain many properties in industrial alloys. Some of the intermetallic compounds are given below.

1. Interstitial compounds

Interstitial compounds are a special case of intermetallic compounds. They form between transition elements upon exceeding the solubility limit of the interstitial element.

Difference between interstitial solutions and interstitial compounds: In interstitial solutions, the solute atoms are not in regular patterns but are randomly distributed throughout the solvent. In interstitial compounds, there is a regular pattern (crystal structure) characteristic of the specific compound.

2. Valency compounds

Intermediate phases which tend to obey valence laws are known as valency compounds.

Examples: These phases are most often in the form of oxides, fluorides, hydrides and carbides.

3. Electron compounds

If two metals consist of atoms of more or less similar size but different valency, then the compounds formed are called electron compounds.

Examples: Cu3Al, CuZn, NiAl, Cu3Sn, etc., are some of the electron compounds.

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