Metallurgy and Alloys: Interstitial vs substitutional

[dunkleosteus]

@excavating_for_a_mind

Just a few comments. I have never known the terms “substitutional” or “interstitial” applied to alloys. It usually applies to elements or atoms. You are correct in your usage, in that substitutional atom will replace an atom in the crystal structure, while an interstitial atom fits in the spaces between the lattice sites.

A carbon steel would not have 1.5% carbon. Maybe 0.15% C. Many low-alloy steels will have both interstitial and substitutional elements, say carbon as interstitial ann chromium and molybdenum as substitional.

In addition, to make it more complicated, in the case of carbon in steel, the amount of interstitial carbon you can have is dependent on temperature. At lower temperatures, you form iron carbide as a second-phase structure, but at higher temperatures, the crystal structure changes to one that will tolerate more carbon, a feature that allows steels to be heat treated to control mechanical properties.

Huh, I found a range of 0.8-2% as the range for carbon steel, with 0.15% being closer to a wrought iron