Refractory metals are a class of
metals that are extraordinarily resistant to
heat and
wear. The expression is mostly used in the context of
materials science,
metallurgy and
engineering. The definition of which elements belong to this group differs. The most common definition includes five elements: two of the
fifth period (
niobium and
molybdenum) and three of the
sixth period (
tantalum,
tungsten, and
rhenium). They all share some properties, including a melting point above 2000 °C and high
hardness at room temperature. They are chemically inert and have a relatively high density. Their high melting points make
powder metallurgy the method of choice for
fabricating components from these metals. Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, and chemical reaction vessels in corrosive environments. Partly due to the high melting point, refractory metals are stable against
creep deformation to very high temperatures.