Polycrystals vs. Single Crystal

In general, most of the engineering materials are polycrystalline consisting of many small crystals of variable sizes – also known as ‘Grains’. The figure below shows the grained structure of lowcarbon steel. There are wide variations of sizes of grains observed – from a few nm to cm. The random orientations of grains generate ‘average’ mechanical properties in ‘crystals’, preventing anisotropy. For example, for a BCC Iron, Modulus of Elasticity along the diagonal direction is around 270 GPa whereas the same along it’s edge is only 125 GPa. Again, in high performance applications like Turbine blades, single crystal metallic alloys are specially developed, exploiting the anisotropy.

Polycrystals of Low-Carbon Steel