alloy (ălˈoi, əloiˈ) [key] [O. Fr., = combine], substance with metallic properties that consists of a metal fused with one or more metals or nonmetals. Alloys may be a homogeneous solid solution, a heterogeneous mixture of tiny crystals, a true chemical compound, or a mixture of these. Alloys are used more extensively than pure metals because they can be engineered to have specific properties. For example, they may be poorer conductors of heat and electricity, harder, or more resistant to corrosion. Alloys of iron and carbon include cast iron and steels; brass and bronze are important alloys of copper; amalgams are alloys that contain mercury; and chromium is an important additive in stainless steel. Because pure gold and silver are soft, they are often alloyed with one another or with other metals. New alloys are being engineered for use in new technology, including materials for the space program. Metallic glasses and crystalline alloys have also been developed, and metal alloys are sometimes bonded with ceramics, graphites, and organic materials as composites.