What is occurring on the atom level that causes grain boundaries, twin boundaries, and phase boundaries to exist?

In a poly-crystalline substance, the interface between the two grains is known as a grain boundary. In the crystal structure, they are defined  as 2D defects and decreases the conductivity of the substance. The grain boundaries are responsible for disruption in motion through out the substance resulting reduction in size and increase in mechanical strength. According to the extent of disorientation in between the two grains, these boundaries are classified as low-angle grain or sub-grain boundaries (disorientation less than 15 degrees) and high-angle grain boundaries (disorientation greater than 15 degrees).  Therefore, grain boundaries occur due to disorientation between the grains in poly-crystalline substances.

If some of the same crystal lattice points are symmetrically shared between two crystals, it results in crystal twinning. This causes inter growth of two different crystals resulting variety of specific configuration. In twinned crystals, the shared surface with the lattice points is known as twin boundaries. For permanent change in shape of the crystal, twinning is very important. Therefore, twin boundaries occur due to sharing of crystal lattice points between the crystal structures.

In a physical matter, at equilibrium (thermal) phase whether it is solid, liquid or gas can change into other at transitional point this is known as phase boundary.This change in phase is due to immiscibility of two matters present in opposite sides of the boundary, the difference in any one of the physical properties results in this immiscibility. Therefore, phase boundaries occur due to phase difference in physical matter at transitional point.

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