Metallurgy and Alloys: Interstitial vs substitutional

[shortreply]

Came along this by accident via Google, but I do not like to let the last answer stand: None of what excavating says below is incorrect, but I want to mention that - coming more from chemistry than metallurgy - the terms interstitial alloy and substitutional alloy are completely correct. Different fields, slightly different nomenclature.

I. e. chromium and iron provide a substitutional alloy, i. e. Cr occupies the place of iron in the lattice. Carbon and iron form an interstitial alloy, since Carbon occupies the interstitital spaces (= holes) in the lattice. Both can happen at the same time, i. e. when you mix Carbon, Chromium and Iron.

Iron carbide (or cementite), Fe3C, is an intermetallic compound with a structure different from that of iron. Neither iron nor carbon is here a substitutional or interstitial element… this is a new compound with (relatively) fixed stoichiometry.

@excavating_for_a_mind

Well, I went into a little more depth than an intro to material science, earning a BS in Metallurgical Engineering, and add to that over 30 years in industry getting paid well for my understanding of metals, alloys, and their processing.

It could be that your textbook was a little loose with the terms, to help freshmen gain a basic understanding of how metals can be mixed (alloyed) to control properties. But, I can truthfully say that in the past 35 years dealing with these terms, I had never heard of an “interstitial alloy” before last Thursday. “Interstitial elements” used in producing alloys, yes, but not an “interstitial alloy”. If you look up the definition of the words, you’ll find that “interstitial alloy” doesn’t even make much sense, while “interstitial alloying element” does.