found: About.com website, Mar. 12, 2013(The term 'refractory metal' is used to describe a group of metal elements that have exceptionally high melting points and are resistant to wear, corrosion and deformation. Industrial uses of the term refractory metal most often refer to five commonly used elements: Molybdenum (Mo), Niobium (Nb), Rhenium (Re), Tantalum (Ta), Tungsten (W), However, broader definitions have also included the less commonly used metals: Chromium (Cr), Hafnium (Hf), Iridium (Ir), Osmium (Os), Rhodium (Rh), Ruthenium (Ru), Titanium (Ti), Vanadium (V), Zirconium (Zr))
found: Wikipedia, Mar. 12, 2013:Iridium (Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element (after osmium) and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C) 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. Most definitions of the term 'refractory metals' list the extraordinarly high melting point as a key requirement for inclusion. By one definition, a melting point above 4,000 °F (2,200 °C) 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 2,123 K (1,850 °C), includes a varying number of nine additional elements, titanium, vanadium, chromium, zirconium, hafnium, ruthenium, osmium and iridium.)
