Dislocation Network Density
The total slip line length per unit volume within the crystal lattice, documenting the microstructural deformation history. A high density blocks further slip processes and increases strength.
Dislocation density simply means: how many lattice defects are in the structure. The more there are, the harder and stronger the material becomes, but also more brittle and susceptible to stress. When welding, this means: if you weld cold-worked or hardened material, the heat-affected zone can crack quickly because dislocations rearrange and build up stress. So before welding, you must ensure the material is stress-relieved – either by preheating or by prior annealing. And afterwards let it cool slowly, don't quench with water, otherwise the weld will crack.