An alloy is a mixture of a metal with another element, either metal or nonmetal. If we start with a base metal and we add impurity atoms there are two possible outcomes if the two mix. The two different cases are highlighted in the figure below. In the substitutional solid case, the impurity atoms replace the host atoms in the lattice. In the interstitial situation, impurity atoms squeeze between the host atoms.
In addition to mixing, it is possible for regions of a new phase to form. An illustration of the formation of a second phase in a solid solution is shown below. The second phase can have a different composition and often a different structure.
Now watch the following video on alloys and how dislocations harden alloys: Properties of Matter: Alloys and Their Properties.
If the above link does not work, try this YouTube link: Properties of Matter: Alloys and Their Properties.
After watching the video, please proceed to the next section on the development of iron smelting.