Refractory metal materials mainly refer to tungsten, molybdenum, tantalum, niobium, rhenium, and vanadium, all of which are materials with a melting point above 2000 degrees Celsius. Refractory metal materials and their alloys have the characteristics of high melting point, high strength under a high-temperature environment, strong corrosion resistance to liquid metal and processing plasticity. Refractory metals and their alloys are generally used at temperatures between 1100 and 3320 degrees Celsius, which is much higher than those of high-temperature alloys.
Niobium alloy has good welding characteristics and room temperature plasticity. As one of the least dense refractory metals, it can be processed into products with complex shapes. Niobium alloy can be divided into low strength, medium strength and high strength according to the different strength of alloy, and into low density and high density according to the different density of alloy.
The research and development of niobium alloys in the United States and Russia are different, and there are as many as two dozen kinds of niobium alloys. In the United States, W, Hf, and Mo were used as strengthening elements of niobium, while in Russia, Zr, W, and Mo were used as the main strengthening elements, while in the second phase, C was used as the main strengthening elements.
The melting point of molybdenum alloy is lower than that of tantalum and tungsten. It has the advantages of highest elastic modulus, smaller density and expansion coefficient, and excellent creep performance at high temperature. The welding properties of molybdenum alloy are very good, the strength and plasticity of its welding seam can reach certain conditions, and its technological properties are above tungsten.
The development of molybdenum alloys should be represented by Russia. Molybdenum alloys can be divided into 14 categories according to the different alloying elements. The added elements are mainly Ti, C, Re, and Zr, and some Ni, Nb, and B elements are added to change some properties of the materials. Of all the molybdenum additive elements, rhenium is the only one favorable for its plasticity at low temperature. Rhenium can not only improve the plasticity of molybdenum in the low-temperature environment but also improve its strength and welding performance, significantly reduce the brittle tendency after recrystallization, as well as improve its high-temperature stability, especially against thermal shock.
Tantalum alloy has a high melting point, small expansion coefficient, excellent thermal shock resistance and forming toughness. Its disadvantage is that the antioxidant performance is very poor when working in the environment above 500 degrees Celsius, so the antioxidant coating treatment is needed on the surface. The creep properties and strength of tantalum alloy in the high-temperature environment have been developed in the United States and Russia. In the static air environment, the refractory metal compounds of tantalum alloy exhibit good oxidation resistance.
The most promising tungsten alloy coating is the protective layer of boron and insoluble oxide with high strength and thermal stability. The research direction of tungsten alloy coating is to cover the protective surface with silicides with self-healing ability, and the mixture of refractory oxides and silicides is to cover the silicides with blocking substrates so that the products can work in a specific service environment for a long time.
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