Metal atoms sit in repeating patterns, and the pattern decides how easily the metal deforms and what it can dissolve. Iron is body-centred cubic at room temperature and face-centred cubic once heated into the austenite range, while copper, aluminium and nickel are face-centred cubic at every temperature and so stay ductile. Hot face-centred iron can hold far more carbon than cold iron can, which is the entire basis of hardening steel by quenching.
Part of Materials Science Principles
Crystal structure is the repeating three-dimensional arrangement of atoms in a solid, and it determines density, how planes of atoms slip past each other, and how much of another element can dissolve.
Three Patterns Cover Almost Every Common Metal
A metal crystal is a stack of identical cells. Three cells matter in practice:
- Face-centred cubic (FCC): an atom at each cube corner and one in the middle of each face. Copper, aluminium, nickel, gold, silver, lead, and iron at high temperature (austenite).
- Body-centred cubic (BCC): an atom at each corner and one in the centre of the cube. Iron at room temperature (ferrite), chromium, tungsten.
- Hexagonal close-packed (HCP): layers stacked in a hexagonal pattern. Zinc, magnesium, titanium.
FCC Is Denser, BCC Leaves Less Room, And The Arithmetic Shows It
The fraction of space actually filled by atoms (treated as touching spheres of radius r) can be worked out from the geometry of one cell.
FCC: 4 atoms per cell. Atoms touch along the face diagonal, so the diagonal is 4r and the cell edge is a = 4r / √2 = 2√2 r. Cell volume = a³ = 16√2 r³. Atom volume = 4 × (4/3)πr³ = (16/3)πr³. Fraction = (16π/3) / (16√2) = π / (3√2) = 3.1416 / 4.2426 = 0.7405.
BCC: 2 atoms per cell. Atoms touch along the cube’s body diagonal, which is 4r, so a = 4r / √3. Cell volume = a³ = 64r³ / (3√3). Atom volume = 2 × (4/3)πr³ = (8/3)πr³. Fraction = (8π/3) × (3√3 / 64) = π√3 / 8 = 5.4414 / 8 = 0.6802.
So FCC fills about 74% of space and BCC about 68%. The surprise is that the denser structure, face-centred iron, can dissolve more carbon. The reason is the shape of the gaps. FCC has fewer but larger holes between atoms, while BCC’s gaps are smaller and awkwardly shaped.
Hot Iron Dissolves Carbon And Cold Iron Cannot
The existing articles state the working consequence without the reason. Heat Treatment says the crystal structure transforms to austenite above a critical temperature of around 725-850 °C and that austenite dissolves carbon uniformly, and Carburization says carbon can squeeze in above about 727 °C. The structural reason is the FCC cell above. Austenite can hold roughly 2% carbon, and room-temperature ferrite holds only a few hundredths of a percent. The corpus’s own steel table in Steel Production puts the boundary between steel and cast iron at 2.0% carbon, which is the same limit seen from the other side.
When the austenite is cooled slowly, the carbon has time to leave as cementite and the lattice reverts to soft BCC ferrite plus pearlite. When it is quenched, the lattice tries to revert to BCC but the carbon is stuck in it, and the distorted result is martensite. That is the hard, brittle structure that Hardening produces and Tempering then relaxes.
Structure Explains Ductility, Magnetism And Cold Toughness
- Ductility. Metals deform by planes of atoms slipping, and FCC has many close-packed planes and directions to slip along. That is why copper, aluminium and austenitic stainless are easy to draw and bend. HCP metals have fewer slip systems and are harder to form cold. BCC metals slip readily when warm but become sluggish in the cold.
- Cold brittleness. Ferritic, BCC steels show a transition from tough to brittle as temperature falls, while FCC metals do not. This is why Austenitic Steel stays tough where Ferritic Steel does not.
- Magnetism. BCC iron is ferromagnetic up to its Curie point. The corpus’s magnet test in Heat Treatment relies on this: steel stops attracting a magnet at about 770 °C, close to where it becomes austenite. FCC iron is not ferromagnetic, which is why nickel-stabilised austenitic steel is non-magnetic at room temperature.
- Ceramics. Ionic and covalent crystals have strong directional bonds and almost no plane slip. They are strong and hard, but they crack instead of bending, which is the reason Advanced Ceramics fail in tension at a fraction of their compressive strength. Glass has no repeating crystal at all, and the corpus’s Glass Properties for Blowing notes it has no single melting point.
Common Mistakes
- Confusing crystal with grain — a grain is one crystal, and a piece of metal contains millions of them joined at boundaries. See Grain Boundaries.
- Assuming the structure is fixed — iron changes pattern with temperature, and that change is what a smith exploits. Other alloys change little on heating, which is why they cannot be hardened by quenching.
- Reading density as strength — close packing helps ductility, not strength. Strength comes from blocking slip.
What’s Next
- Learn next: Grain Boundaries — what happens where crystals meet
- Combine with: Phase Diagrams — which structure exists at which temperature and composition