Grain Refinement
Grain refinement, also known as inoculation, is the set of techniques used to implement grain boundary strengthening in metallurgy. The specific techniques and corresponding mechanisms will vary based on what materials are being considered.
One method for controlling grain size in aluminum alloys is by introducing particles to serve as nucleants, such as Al–5%Ti. Grains will grow via heterogeneous nucleation; that is, for a given degree of undercooling beneath the melting temperature, aluminum particles in the melt will nucleate on the surface of the added particles. Grains will grow in the form of dendrites growing radially away from the surface of the nucleant. Solute particles can then be added (called grain refiners) which limit the growth of dendrites, leading to grain refinement. TiB2 is a common grain refiner for Al alloys; however, novel refiners such as Al3Sc have been suggested.
One common technique is to induce a very small fraction of the melt to solidify at a much higher temperature than the rest; this will generate seed crystals that act as a template when the rest of the material falls to its (lower) melting temperature and begins to solidify. Since a huge number of minuscule seed crystals are present, a nearly equal number of crystallites result, and the size of any one grain is limited.
| Metal | Inoculant |
|---|---|
| Cast iron | FeSi, SiCa, graphite |
| Mg alloys | Zr, C |
| Cu alloys | Fe, Co, Zr |
| Al–Si alloys | P, Ti, B |
| Pb alloys | As, Te |
| Zn alloys | Ti |
| Ti alloys | Al–Ti intermetallics |
Read more about this topic: Grain Boundary Strengthening
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—Chinese proverb.
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