Question

Describe the difference between a substitutional solute atom, an interstitial solute atom, and a vacancy. Discuss whether the stress in the material near the defect will be tensile or compressive

Answer 1

Substitutional solute atom - For substitutional solid solution to form the size of the solute atom must nearly be the same size as the solvent atoms since the solute atom directly replaces the solvent atoms. For metal atoms the difference between the size of solute atom and solvent atom must be less than 15%.

Stresses - If the size of the solute atom is more than the size of the solvent atom then compressive stresses will be developed in the material near the defect and if the size of the solute atom is less than the size of solvent atoms then the stress developed will be tensile in the material near the defect.

example- copper and nickel

radius of copper atom = 0.128 nm

radius of nickel atom = 0.125 nm

So a nickel atom can make substitutional solid solution in copper.

Interstitial solute atom - For interstitial solid solution to form the size of the solute atom must be small enough to fit in the interstitial sites present between the solvent atoms. The atomic diameter of interstitial impurity is much smaller than that of host atoms. When the atomic size of the alloying element is smaller or larger than the atomic size of the base metal by about 15% then the alloying element may occupy an interstitial lattice site.

Stresses - When the alloying element is smaller the stress produced is tensile and when it is larger the stresses produced is compressive.

example- carbon added to iron

radius of carbon atom = 0.071 nm

radius of iron atom = 0.124 nm

So a carbon atom can easily occupy the space created between the iron atoms.

Vacancy - Vacancy is the imperfection created in the lattice when the atom is missing from its position in the lattice, which occur naturally as a result of thermal vibration. The size of the vacancy created is same as the size of atoms constituting the lattice.

Stresses - In vacancy defect the stresses produced is tensile since the atoms around the vacancy are pulled toward the space created as a result of vacancy defect.