An alloy involves its constituents in its solid structure. This means the combined properties are different than the properties of the constituents individually. Brass, for example, is made of copper and zinc, but has neither the color, nor the melting point, nor the same hardness nor the same lubricity of either copper or zinc.
Melting points can be depressed by alloying, owning to the stresses between the atoms helping the alloy to disassemble with less heat energy, even though those same stresses can make the solidified alloy stronger than its individual constituents. For example:
Pure lead melts at just over 621°F.
Pure tin melts at just over 449°F.
63% tin and 37% lead alloy melts at 369°F; lower than either constituent metal melts at.
If you melt lead and add some tin, then let it cool, you don't see the lead start to freeze until below 621°F. The more tin you add, the lower the temperature the lead starts to solidify at until you get to that 63% tin and 37% lead alloy with its 369°F melting point. Before getting to that much tin, the mix would start to solidify at a higher temperature (called the liqudus) and not finish solidifying until 369°F (called the solidus). There is a slush temperature range as lead solidifies until the remaining liquid has that optimal 63:37 concentration, at which point it finishes solidifying. 63:37 is called a eutectic alloy of tin and lead, characterized by its liquidus and solidus being the same temperature.
From the
Wikipedia entry on Alloy:
"When a molten metal is mixed with another substance, there are two different mechanisms that can cause an alloy to form, called atom exchange and the interstitial mechanism. The relative size of each atom in the mix plays a primary role in determining which mechanism will occur. When the atoms are relatively similar in size, the atom exchange method usually happens, where some of the atoms composing the metallic crystals are replaced with atoms of the other constituent. With the interstitial mechanism, one atom is usually much smaller than the other, and so cannot successfully replace an atom in the crystals of the base metal. The smaller atoms become trapped in the spaces between the atoms in the crystal matrix, called the interstices."