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refractory metals
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.
==Definition==
Most definitions of the term 'refractory metals' list the extraordinarily high melting point as a key requirement for inclusion. By one definition, a melting point above is necessary to qualify. The five elements niobium, molybdenum, tantalum, tungsten and rhenium are included in all definitions, while the wider definition, including all elements with a melting point above , includes a varying number of nine additional elements, titanium, vanadium, chromium, zirconium, hafnium, ruthenium, rhodium, osmium and iridium. Transuranium elements (those above uranium, which are all unstable and not found naturally on earth) and technetium (melting point 2430 K or 2157 °C) are never considered to be part of the refractory metals.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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