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An alloy is a solid solution or mixture of two or more elements, where at least one element is a metal. It retains the features of a metal while showing enhanced physical or chemical properties due to the presence of other elements.

Substitutional alloys are formed when atoms of the solute element (the element that is present in a smaller amount) replace or substitute some of the solvent metal atoms in the metal's crystal lattice. In this case, the solute and solvent atoms should have a similar atomic size and structure, and their difference in atomic radii should not exceed around 15% for proper substitution. This type of alloy exhibits uniform distribution of the solute atoms, resulting in homogeneous properties throughout the material. Example of Substitutional Alloy: Brass is a common example of a substitutional alloy, which is formed by substituting a small percentage of copper atoms in the lattice with zinc atoms (usually in a proportion of 60-80% copper and 20-40% zinc). It has better strength and corrosion resistance than pure copper.

Interstitial alloys are formed when the solute atoms are significantly smaller compared to solvent metal atoms, and they occupy the voids or "interstices" within the crystal lattice of the solvent metal. Due to the size differences between the atoms, the solute atoms distort the lattice structure, creating internal stresses and leading to increased hardness, strength, and other properties. Example of Interstitial Alloy: Steel is a typical example of an interstitial alloy, where small carbon atoms (around 0.2-2% by weight) fill the interstitial spaces in the lattice of iron atoms. The presence of carbon atoms leads to improved strength, hardness, and other properties compared to pure iron.

In summary, the main differences between substitutional and interstitial alloys lie in their structural arrangement of atoms. Substitutional alloys have solute atoms replacing solvent atoms in the lattice, while in interstitial alloys, smaller solute atoms occupy the void spaces in the lattice. Examples of these alloys include brass (substitutional) and steel (interstitial), respectively.