Definition: A metal is an element or alloy with metallic bonding (delocalised electrons), typically crystalline, electrically/thermally conductive, and deformable (ductile/malleable).
Term | Definition |
|---|---|
Annealing | A heat treatment (heat → hold → cool, often slowly) that allows diffusion to relieve internal stresses, reduce dislocation density, and modify microstructure |
Grain | A single-crystal region within a polycrystalline metal; neighbouring grains differ in lattice orientation and are separated by grain boundaries (defect-rich interfaces) |
Melt | The liquid state of a metal or alloy above its melting point; also used to refer to the molten material itself during casting |
Wrought | Describes metal shaped by plastic deformation (rolling, forging, drawing, extrusion) rather than by casting; grains become elongated/fibrous, often producing anisotropic properties |
Metals used in dentistry are crystalline: their atoms repeat in regular lattices. In practice, a casting does not freeze as one perfect crystal but as many small crystals, or grains, because numerous nuclei form throughout the melt as it cools. If nuclei arise spontaneously from the liquid (tiny clusters that become stable unit cells) the process is called homogeneous nucleation and requires very controlled conditions to grow a single crystal from an entire melt. Far more common is heterogeneous nucleation, where impurities or deliberately added particles provide surfaces on which atoms deposit as the temperature drops below the melting point. Grains grow until they impinge; the misfit where they meet is a grain boundary which is an orderly structure interrupted by a defect-rich interface.
A fine grain size generally raises yield stress by grain-boundary strengthening. Dentistry exploits this: rapid solidification, such as casting into an investment held well below the alloy’s melting temperature, produces finer grains; adding grain refiners like iridium to dental gold alloys seeds many nuclei and further refines the structure.
Alloys
An alloy is a deliberate mixture of two or more elements (usually metals) melted together and solidified. Instead of each element forming its own separate chunk, the atoms can mix on the atomic scale, form new ordered compounds, or split into multiple solid phases. Which of these happens depends mainly on atomic size, crystal structure, valency/electronegativity, composition, and temperature. Because the mix of phases determines strength, ductility, corrosion resistance, casting behaviour, and heat-treat response, dentistry almost always uses alloys rather than pure metals. Alloys can be subdivided into solid solutions and intermetallic compounds.
Solid solutions (single solid phase)
A solid solution is a single, uniform solid phase in which different atoms share the same crystal lattice. Think of it as a metal lattice with “guest” atoms either taking the place of host atoms or squeezing between them. Solid solutions are generally tough and workable, and they’re the foundation for many dental casting and wrought alloys.