Question

why an alloy is hardened when the solubility of solute in solvent is exceeded. discuss this with an example

Answer 1

When the solute atom is the solution, it is obvious that there will be some amount of size difference between solute and solvent atom and due to this difference, there will elastic stress fields around the solute atom could be compressive or tensile.

If the solute atom is larger than solvent atom there will be compressive stress field around it.

If the solute atom is smaller than solvent atom there will be tensile sress field around it.

Either of the stress fields interact with gliding dislocations and lock them or pin them by interacting with the stress field along the dislocation.

Thus more stress is to be supplied to move the dislocation further and this results in the increase in hardness.

Steel if the carbon content or nitrogen content is increased, nitrogen or carbon will interact with the dislocations and lock it giving higher hardness as you increase carbon content when you harden it.