Question 1

What is an interstitial compound?

Accepted Answer

An interstitial compound is formed when a small atom such as hydrogen, carbon, nitrogen or boron occupies a tetrahedral or octahedral interstitial site in a transition-metal lattice. The parent metal lattice remains present, and examples include Fe3H, Fe4N, TiC and Fe3C.

Question 2

How are interstitial compounds different from alloys?

Accepted Answer

An interstitial compound uses small atoms in lattice voids and is usually non-stoichiometric. An alloy is also a homogeneous mixture whose composition can vary. Formulas such as Fe3C are examples, not a rule that every interstitial compound has fixed stoichiometry.

Question 3

What properties are associated with interstitial compounds?

Accepted Answer

They are generally hard, have high melting points, are chemically inert, and retain metallic electrical and thermal conductivity. Toughness is not a universal property; cementite (Fe3C) is brittle.

Question 4

Why are the tetrahedral and octahedral sites shown in the BCC model?

Accepted Answer

They show where small interstitial atoms can sit without replacing host atoms at lattice points. Accounting for boundary sharing, a conventional BCC cell has 6 effective octahedral sites and 12 effective tetrahedral sites.

Question 5

Is brass the same as an interstitial compound?

Accepted Answer

No. Brass is a copper-rich homogeneous alloy of copper and zinc, not an interstitial compound. Bronze is Cu-Sn, stainless steel is Fe-Cr-Ni, and steel is Fe-C with variable composition.

Into the Lattice: Interstitial Compounds & Alloy Formation

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