If the solute and solvent are roughly similar in size, the solute atoms will occupy lattice points in the crystal of solvent. This is called substitutional solid solution.
If the solute atoms are much smaller than the solvent atom they occupy interstitial positions in the solvent lattice. eg: C, N, O, H, B etc.
Smaller atoms produce a local tensile stress field in the crystal where as larger atoms produce a local compressive field. In both cases, the stress field of solute atom results in increase in stress required to move dislocation. Hence strength increases and hardness also increases.
Limitation: each alloying element has got its own solubility limit.
Factors favour extensive substitutional solid solubility (Hume rothery rules)
(1) Crystal structure and valency of the both metals should be same.
(2) The size difference between the parent atom and solute atom must be <15%.
(3) The electro negativity difference between two metals must be small.
(1) Crystal structure and valency of the both metals should be same.
(2) The size difference between the parent atom and solute atom must be <15%.
(3) The electro negativity difference between two metals must be small.
Mechanism of solute atom-dislocation interaction
Elastic, modulus, stacking-fault, electrical, short-range order, long-range order.