Poly-crystalline Metals
Let us now look beyond the boundaries of a single crystal. A single
crystal is usually most sought after for Gemstones where lattice structure
extends up to the edge of the material boundary and for specific
engineering applications like Turbine Blades.
In general, during phase change, the crystal solidification begins from
multiple sites, each of which can produce a different orientation.
As a result “polycrystalline” materials are formed consisting of many
small crystals (also known as “grains”) each of which has the same
lattice, but with a change of orientation from grain to grain. Typical grain
structure of polycrystalline steel is shown below:
![](images/image1.jpg)
The distinct boundaries are clearly visible here. Each grain undergoes
compression in certain regions and tension in other locations due to the
interactions between the grains. The question is how the sizes of the
grains affect the material property of the metal. Smaller grain sizes imply
larger surface areas which the dislocations have to travel to initiate
plastic deformation. This, in turn, implies increase in the yield strength of
the metal. The relationship between the yield strength and grain size is
given by the Hall-Petch equation as follows:
σy =σ0+Kd-1/2 Where, σy is the yield strength, d – the grain size, K and σ0 are the
material constants.
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