For example if an Boron forms an alloy with Vanadium is it an interstitial or an intermetallic alloy? And why?
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$\begingroup$ Well, there are 6 known intermediate phases in the B-V binary phase diagram. Most have narrow (at best) phase fields, so I personally would not describe any of them as an interstitial alloy. For clarity, I consider something like TiN to be an interstitial alloy - the Ti sublattice is filled, and the N atoms occupy interstitial positions of the Ti sublattice, and the N occupancy can vary widely resulting in a large phase field. $\endgroup$– Jon CusterCommented Oct 16 at 15:37
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Interstitial compounds are formed when lighter atoms (H, B, C, N) occupy interstitial positions in the crystal sub-structure formed by metallic atoms. For example, some amounts of N can be dissolved into Nb forming a solid solution with the same crystal structure of pure Nb (body-centered cubic structure). If you increase the amount of N, new compounds form with peculiar crystal structures, such as Nb2N (hexagonal) and NbN (face-centered cubic).
Intermetallic alloys are formed between metallic elements; in this case, the atomic sizes of metal atoms are comparable. It follows that in no alloy or intermetallic compound is it possible to obtain a sub-structure capable of having interstitial sites sufficiently voluminous to accommodate other metallic elements.
Phases belonging to the V-B system are interstitial compounds (not alloys).
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$\begingroup$ Do you have a reference for this terminology? $\endgroup$ Commented Oct 17 at 4:04
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$\begingroup$ If you're asking for a specific scientific publication, I would say [Goldschmidt, Interstitial Alloys (1967)] and [Toth, Transition Metal Carbides and Nitrides, Academic, New York (1971)]. A more recent publication can be found here [researchgate.net/profile/Walter-Lengauer/publication/… $\endgroup$– gryphysCommented Oct 17 at 8:48
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1$\begingroup$ Just to be clear, my question comes after wrestling with more than a thousand binary phase diagrams over the years, so I've seen more of the mess behind the curtain than many. Lets take Ti-N. Both hcp and bcc (the stable pure phases of Ti) show substantial solubility of N (hcp more than bcc) and they indeed occupy interstitial sites which allows the broad phase field. Similarly TiN has an fcc Ti structure with again interstitial N with wide solubility. Cool. Then there is Ti2N, a line compound, which is inverse rutile where the N really can't be thought of as interstitial. $\endgroup$ Commented Oct 17 at 12:41
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$\begingroup$ To be clear too, my answer comes from years of preparation and study of transition metal nitrides. You asked for references and I put some quite authoritative ones in my previous answer. When you’re referring to Ti2N, I guess you mean the epsilon phase (it’s the only one with stoichiometry). Actually, this is not a line compound, as you see here [Lengauer, Acta Metall. Mater. 39 (1991) 2985; see fig. 8-9, for example]. $\endgroup$– gryphysCommented Oct 18 at 14:54
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$\begingroup$ However, I can't follow your reasoning (or rather, I follow it up to a certain point) when you write “…is inverse rutile where the N really can't be thought of as interstitial”. Please explain why you state this. $\endgroup$– gryphysCommented Oct 18 at 14:55