What is Alloys?

An alloy is a metallic solid made of several elements. In a substitutional alloy the atoms of the added element replace atoms of the host on its lattice; this requires radii within about 15 % of each other (copper and nickel, copper and zinc in brass). In an interstitial alloy small atoms ($\ce{H}$, $\ce{C}$, $\ce{N}$, $\ce{B}$) occupy interstitial sites of the host (carbon in steel).