Another part of metallurgy is making parts from metals. These parts must be made so they will not break when they are used. Metallurgists look to make metal that meets the needs of the process it will be used in. Sometimes the metal must be lightweight or low in density. Other times it must be tough (not easily broken). The metallurgist must follow directions when making the part to know what metal to use. the process of metal selection is known as the materials selection process. Metallurgists use mathematical equations to determine what are the most important physical features of the metal. Common desired characteristics include ductility, toughness, corrosion resistance, and strength. Different metals and their associated features are often graphed on Ashby plots.