A metal may exist as a plasma, gas, liquid, or crystalline solid. Plasmas and gases exist only at high energy levels. Strength, hardness, wear resistance, shock resistance, and electrical and thermal conductivity are important metallic properties. A metal melts if heated to a high enough temperature. An alloy consists of two or more metals, or at least one metal and a nonmetal, mixed intimately by fusion or diffusion. Equilibrium diagrams are presented to demonstrate the effects of temperature change. Carbon is the basis for the wide range of properties obtainable in iron and steel. It forms different compositions with iron when combined in different ways and amounts. Thus carbon is the primary means for making iron or steel soft and ductile, tough or hard. Most other alloying elements, in effect, modify or enhance the benefits of carbon. The fundamental effects of carbon on iron are shown by the iron and iron carbide equilibrium diagram. Testing is performed on materials, components, and assemblies. It consists of measuring fundamental properties or responses to particular influences such as load, temperature, and corrosive substances. Most nondestructive test (NDT) methods are intended to detect internal flaws likely to cause fatigue or static load failure.