In Crystal defects there are either imperfections that change crystal properties by swapping the atom (substitutional) or squeeze in an atom (interstitial). Example of substitutional: Brass. Cu + Zn. Copper atoms are replaced by zinc atoms, they are similar size so they substitute to make a stronger mental.

Example of Interstitial defects: Steel. Smaller carbon atoms squeeze into holes between iron atoms. This forms steel and makes it harder and stronger than iron alone.