Distinguish between substitutional and interstitial alloys. What conditions favor formation of substitutional alloys?

A substitutional alloy is formed when atoms of the solute element replace or substitute atoms of the solvent element within the crystal lattice. The solute atoms occupy the regular lattice positions in this type of alloy. An interstitial alloy, on the other hand, is formed when atoms of the solute element are trapped or inserted into the small spaces (interstices) between the solvent atoms in the crystal lattice. The solute atoms do not replace any solvent atoms but instead occupy interstitial sites within the lattice.

There are several primary differences between substitutional and interstitial alloys: 1. Atom arrangement: In substitutional alloys, solute atoms replace solvent atoms, whereas in interstitial alloys, solute atoms are inserted into the spaces between solvent atoms without replacing them. 2. Atomic size: Solute atoms in substitutional alloys are generally similar in size to solvent atoms, while in interstitial alloys, solute atoms are significantly smaller than solvent atoms. 3. Formation: Substitutional alloys can be formed by heating and cooling a mixture of different elements, while interstitial alloys are typically formed by rapid quenching or cooling from high temperatures.

Certain conditions need to be met for the formation of substitutional alloys. These conditions include: 1. Similar atomic size: The atomic radii of solute and solvent atoms should be within 15% of each other for a substitutional alloy to form. This allows solute atoms to substitute effectively for solvent atoms in the crystal lattice without causing significant distortions. 2. Crystal structure compatibility: Both solute and solvent elements should have compatible crystal structures; otherwise, it will be challenging for solute atoms to replace solvent atoms in the lattice. 3. Chemical affinity: The two elements should have some degree of chemical affinity for each other, which helps in the formation of a stable, even mixture of solute and solvent atoms. 4. Temperature: High temperature is often required to form a substitutional alloy, as it allows elements to mix more easily and solute atoms to replace solvent atoms in the crystal lattice. By understanding the differences between substitutional and interstitial alloys and the conditions that favor the formation of substitutional alloys, we can better predict how various elements will combine to form specific alloy types.