Almost all metals found in nature are polycrystalline so that there must be grain boundaries. My understanding is that individual grains are tiny defectless crystals and different grains are rotated w.r.t each other at the grain boundaries where the periodicity is interrupted. Are the grain boundaries same as line defects or in particular, same as dislocations? Even if they are not same, does the polycrystalline nature of metals i.e., the existence of grain boundaries imply the existence of line defects? The answer here tends to suggest that.
Grain boundaries are 2-dimensional crystallographic defects. Dislocations are 1-dimensional crystallographic defects. Small angle grain boundaries (<15°) can be considered to be composed of an array of step dislocations. Large angle grain boundaries are more complicated.