Grain boundary

The area where grains meet.

It shows up as a line on micrographs but, since grains exist in three dimensions, it is actually a surface where two solid objects meet. The simplest way to visualise grain boundaries is to press two transparent balloons together and you will be able to see the surface where they come together.

As two adjacent crystals or grains solidify, the orientation of their layers of atom s differ. When they meet, there is a misalignment between the grains, which forms the grain boundary just a few atoms thick.