Question

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

Short answer

Substitutional alloys are formed when elements with similar atomic size and bonding properties replace atoms in the host metal lattice. Interstitial alloys occur when smaller-sized elements occupy spaces between host metal atoms. Conditions favoring substitutional alloy formation include atomic size similarity (less than 15% difference), compatible crystal structures, chemical compatibility, similar valency, and temperatures that allow sufficient diffusion without causing melting or unwanted phase formation.

Step-by-step solution

1. Definition of Substitutional Alloys

Substitutional alloys are formed when one or more elements in the host metal lattice are replaced by another element or elements. The substituting element often has a similar atomic size and comparable bonding properties with the host element, allowing it to occupy positions in the crystal lattice without disturbing the lattice structure too much.

2. Definition of Interstitial Alloys

Interstitial alloys are formed when one or more elements fill the interstitial spaces (small holes) between the atoms of the host metal lattice. These elements usually have significantly smaller atomic sizes compared to the host atoms, allowing them to occupy the spaces between the host atoms without significantly altering the lattice structure.

3. Conditions Favoring Substitutional Alloys

There are specific conditions that favor the formation of substitutional alloys, which include: 1. Size factor: The atomic radii of the substituting and host elements should be similar. A general rule of thumb is that the size difference should be less than 15% to favor substitutional alloy formation. 2. Crystal structure: The host metal and the substituting element should have the same or similar crystal structures. This similarity helps the new element(s) to fit more easily into the host lattice without drastically altering the overall structure. 3. Chemical compatibility: The host and the substituting element(s) should be chemically compatible, without forming intermetallic compounds. This compatibility helps to ensure that the new element can be dispersed uniformly throughout the host matrix. 4. Valency: The valency of the substituting element should not differ too much from the host element. A large difference in valency may lead to charge imbalances that might destabilize the alloy. 5. Temperature: The formation of substitutional alloys may be favored at temperatures that allow for sufficient diffusion of the substituting element(s) into the host lattice, but not so high as to cause complete melting or formation of unwanted phases.