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Refractory metals
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Refractory metalsIntroduction
Refractory metalsThe discovery of these elements dates back to earlier times: 1778: discovery of molybdenum, 1781: discovery of tungsten, 1801: discovery of niobium, 1802: discovery of tantalum, 1925: discovery of rhenium. However, it was not until 1925 that refractory metals entered the stage of application development. During this stage, some important technological inventions and advances emerged: 1920: invention of doped tungsten, 1930: development of high-density tungsten alloys. After World War II, arc melting and electron beam melting became the main technologies for the preparation of tantalum and niobium ingots, and powder metallurgy also became the main technology for the preparation of tungsten and molybdenum ingots. Since the 1980s, the development of refractory metals has entered a new stage: new preparation technologies continue to emerge, alloy systems are rapidly increasing, the preparation and analysis level of high-purity refractory metal materials is continuously improving, the size of ingots and deep-processed products is rapidly increasing, and research on rare earth oxide and carbide second-phase reinforced alloys has made progress.
ChinaRefractory metalsResearch and production began in 1950. Between 1960 and 1970, variousRefractory metalsproducts were developed, such as tungsten rocket nozzles, tungsten-rhenium thermocouples, and high-density tungsten alloys. By 1990, it was already possible to trial-produce and produce more than 100 grades and specifications of processed materials, including series of tantalum alloys, various niobium alloys and molybdenum alloys, and tantalum capacitors.
Properties
Refractory metalsIt has the following properties: good high-temperature strength, good corrosion resistance to molten alkali metals and steam, high density, good electrical and superconducting properties, and small thermal neutron absorption cross-section. However,Refractory metalsthere are also some drawbacks, such as poor high-temperature oxidation resistance.Refractory metalsThe main physical properties are shown in Table 1. Among them, the ductile-brittle transition temperature of tungsten and molybdenum is high, and it is difficult to plastically process at room temperature, and the welding performance is poor. However, after adding rhenium, its thermal shock resistance, plasticity and welding performance are significantly improved. Tantalum and niobium have a very low ductile-brittle transition temperature. Table 2 shows the corrosion resistance of refractory metals in boiling solutions. Tantalum has the best corrosion resistance, and molybdenum has good resistance to molten glass corrosion.
Table 1Refractory metalsPhysical Properties
Performance W Re Ta Mo Nb Melting point/℃ 3422 3180 2960 2620 2480 Boiling point/℃ 5555 5690 5400 4800 4840 Density/g/cm³ 19.3 21 16.6 10.2 8.6 Resistivity/μΩ·cm (20℃) 5.4 18.9 13.5 5.7 16 Thermal conductivity/W·(m·K) (0~100℃) 174 47.6 57.55 137 54.1 Table 2Refractory metalsCorrosion resistance in boiling solutions
Solution Ta Nb W Mo 10%HCI Excellent Excellent Excellent Excellent 10%HCI+1%FeCl3 Excellent Excellent Poor Poor 5%HF Poor Poor Excellent Excellent 65%HNO3 Excellent Excellent Poor Poor 10% Oxalic acid Excellent Poor Excellent Excellent 15%H2SO4+Air Excellent Excellent Excellent Poor 40%H2SO4 Excellent Good Excellent Excellent 70%H2SO4 Excellent Poor Good Good 10%NaOH Poor Poor Poor Poor
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Refractory metals
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