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:

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:

σ_y =σ₀+Kd^-1/2

Where, σ_y is the yield strength, d – the grain size, K and σ₀ are the material constants.