Let's use Ggen to explore high fraction Fe + Nitrogen and Carbon.
Like α″-Fe₁₆(N,C)₂.

Short version: steel isn't a crystal, it's a family of them. "Steel" = iron with up to ~2.1 wt% carbon, and which crystal you get depends on composition and thermal history. But your instinct is exactly right for one of them: ferrite really is just BCC iron, and martensite really is BCC iron that has been strained into tetragonality by the carbon trapped inside it.

I built the cells and supercells for all four principal players:

Steel cells and supercells: ferrite, austenite, martensite, cementite

Image

Six-panel figure of steel crystallography: BCC ferrite cell + supercell, FCC austenite with carbon in the octahedral site, BCT martensite with carbon on z-oriented octahedra (cell + 4×4×3 supercell), and measured Pnma cementite Fe₃C.

@hermes

#materials-scienceAug 25

α-ferrite (room temperature, low carbon): plain BCC iron, a = 2.866 Å. This is "just iron" almost exactly, because carbon barely dissolves in it at all, about 0.02 wt% maximum. The BCC octahedral holes are too small and too distorted (two close axial Fe neighbors) to hold carbon, so it gets expelled.

γ-austenite (high temperature): FCC iron, and this is where carbon integrates. The FCC octahedral hole at (½,½,½) is symmetric and roomy, so austenite dissolves up to ~2.1 wt% C. The carbon sits between the iron atoms — interstitial, never substituting for Fe. It's a solid solution: one crystal, one lattice, guest atoms in the holes.

Martensite (quenched austenite): this answers your "strained BCC" question most precisely. When you quench austenite, the FCC→BCC transformation doesn't have time to expel the carbon, so it gets trapped. The carbon atoms all sit on one orientation of octahedral hole (the z-set at (½,½,0)-type positions), and because those sites want to stretch the cell along z only, the BCC cell becomes body-centered tetragonal. That's the Bain distortion: BCC iron pulled along one axis. The c/a ratio rises linearly with carbon content, reaching ~1.045 at 1 wt% C. So martensite is a genuine distinct crystal (BCT, space group I4/mmm for the ordered arrangement), but it is crystallographically descended from strained BCC — it's not merely strained, the carbon ordering breaks the cubic symmetry. The 4×4×3 supercell shows the carbon sublattice ordering that produces the tetragonality.

Fe₃C cementite: when carbon exceeds what the iron lattice can hold and cooling is slow, it forms this instead — orthorhombic Pnma, a true line compound with 12 Fe and 4 C per cell. The carbon sits in trigonal prisms of six iron (C–Fe distances 1.90–2.11 Å). No amount of straining BCC iron gets you here; this is a different crystal with its own bonding.

So: the iron host never becomes a new "steel crystal." The carbon is integrated either as isolated interstitials (dilute solid solution in ferrite/austenite), as an ordered interstitial sublattice that breaks cubic symmetry (martensite), or as a separate compound (cementite). Most real steels are microstructures — grains and mixtures of these phases, whose proportions are what the phase diagram and heat treatment control.

All the CIFs are linked so you can spin them yourself: ferrite, austenite + C

Done. Two runs, both linked below.

Broad scan — GGen explore on C-Fe-N, restricted to ≥80 at% Fe with all three elements required: 240 trials across 24 stoichiometries in 12.9 min, 204 structures kept, 20 new compositions within 150 meV/atom of the hull. The leading ternaries are CFe₃₃N₃ (48 meV/atom), CFe₃₄N₂ (50), and CFe₂₄N (50) — all P1.

Targeted α″ run — exact Fe₁₆NC stoichiometry with I4/mmm (#139) requested, 50 trials. It symmetry-broke to a distorted P-1 cell instead.

I pulled the top CIFs apart before believing any of it, and the local physics is actually right even where the symmetry isn't:

• C and N both sit in octahedral interstices, coordinated by 6 Fe at 1.81–2.00 Å — exactly the martensite/α″ motif, with C and N behaving interchangeably as interstitials.

• The Fe sublattice stays bcc-like: Fe–Fe near-neighbors at ~2.4 Å and 12.2 ų/atom per Fe, expanded from pure bcc (2.48 Å, 11.8 ų) the way martensite expands when interstitials force the cell.

What GGen did not find is the ordered α″ picture: the P1 cells are disordered interstitial arrangements on a bcc host, and no candidate picked up the tetragonal c/a ordering that makes α″-Fe₁₆N₂ what it is. Every mixed N+C composition sits ~50 meV/atom above the hull, while the hull table confirms the ordered binaries nearby: MP's Fe₈N reference (which is α″ itself) at 18 meV/atom, GGen's Fe₁₂N in I4/mmm at 16, and a Fe₁₆N₂ Cmmm polymorph at 22. Nothing ternary lands on the hull.

That metastability is honest and arguably matches experiment — α″-Fe₁₆(N,C)₂ martensites are metastable phases that decompose on annealing, so a generator that places mixed-anion phases consistently above the line between bcc Fe, Fe-N, and Fe₃C-family endpoints isn't obviously wrong.

Two caveats before anyone gets excited: the hull here is computed against GGen's shared database (MP + Alexandria + prior generated structures), not a full DFT-verified ternary hull, and P1 output from generative runs has historically been a signature of the model failing to hold symmetry rather than a real disordered ground state. These cells passed basic geometry checks (no overlaps, sane bond lengths), so they're plausible disordered martensites, but the symmetry verdict deserves a DFT-level look before treating any of them as a candidate phase.

If you want to push on this, the natural next step is seeding from the real α″ structure and substituting C onto the N sublattice explicitly rather than asking GGen to rediscover the ordering.

Explore run: View run · Targeted Fe₁₆NC run: View run