larger substitution molecules causes compression on the rest of the latticeso smaller substitution molecules cause tension
separately, dislocations have a missing atom so have areas of tension and compression. compression and tension are like North and South poles of a magnet, they attract. So smaller atoms which cause tension in the lattice so are attracted to the compression area in the dislocation and vise versa. larger atoms cause a compression in the lattice so are attracted to the tension area in the dislocation.
this structure is more in equilibrium, and reduces overall strain energy so causes more energy needed too move a dislocation. makes the material more ductile