Because the finer the grain, the more grain boundaries, and the irregular arrangement of atoms on the grain boundaries will lead to lattice distortion and energy state increase. In fact, it is an unstable high-energy state, and with the increase of temperature, it tends to spontaneously transform into a more stable low-energy state. Therefore, the parts working at high temperature are generally not as fine as possible, but choose relatively stable and large grains.

Extended data:

By measuring the cleavage strength of different grain sizes at low temperature, Petch accurately perfected the mathematical relationship proposed by Hall, which was named holpage relationship after them.

σy represents the yield limit of the material, that is, the yield stress σ0.2 when the material is deformed by 0.2%, which can usually be expressed by microhardness Hv.

σ0 represents the lattice friction resistance caused by moving a single dislocation.

Ky constant is related to the kind, properties and particle size of materials.

Average particle size.

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