Metals have giant structures of atoms held together by strong metallic bonding. A lot of energy is needed to overcome these strong bonds, so most metals have high melting points and high boiling points.
In a pure metal the atoms are arranged in regular layers. The layers can slide over each other when a force is applied, which is why a pure metal can be bent and shaped. This also means pure metals are often too soft for many uses.
To make a metal harder it is mixed with other metals to make an alloy. The atoms of the other metals are different sizes, so they distort the layers of atoms in the structure. The distorted layers cannot slide over each other easily, so an alloy is harder than the pure metal.
