The role of molybdenum alloy elements
Molybdenum is usually strengthened by microalloying, carbide strengthening, solution strengthening and dispersion strengthening. (1) Micro alloying is mainly to add trace Ti, Zr, B, La and other alloy elements, the amount of which is 0.1% ~ 1% for solution strengthening. Zr, Hf and Ti have significant effect on increasing the recrystallization temperature of molybdenum. Such a small amount of alloying elements can significantly improve the properties. In addition to these elements can refine the grain size, remove or weaken the harmful effects of C, O2, N2, etc., and improve the processing and welding properties, there is also an elastic stress field around the dislocation and impurities. When a small amount of solute element is added, it will be placed in the maximum dislocation accumulation area of the solid solution. As a result, the elastic stress and the internal energy of the metal are reduced. The internal energy of the microalloyed alloy is less than that of the pure metal. If the elastic equilibrium of the alloy is destroyed, more energy will be needed. In addition, heterogeneous atoms pile up around the dislocations, which makes it difficult for dislocations to migrate, so the recrystallization temperature of the alloy increases. The results show that the optimum content of titanium is about 0.5% and that of zirconium is 0.2%. The strengthening effect of zirconium is better than that of titanium. (2) Carbon and active metals Ti, Zr, Hf, etc. are added to carbide strengthening, and Nb or other carbide forming elements are added to form refractory carbide phase to strengthen the carbide phase, and carbon also plays the role of deoxidization. The thermal strength of molybdenum alloy strengthened by Zr, Hf and Ti metal carbides is much higher than that of micro alloying molybdenum alloy, but the strengthening effect of molybdenum alloy strengthened by carbide is below 1700c. (3) The alloying elements such as Ta, W and re are indirectly reasonable to improve Mo alloy atoms and form solid solution. Because Ta can deteriorate the deformation property of Mo, Mo-W and MoRe alloys are the main solution strengthening Mo alloys. Mo-W alloy is developed for its chemical corrosion resistance, which is a low-cost and light-weight material to replace pure tungsten. MoRe alloy is due to the "rhenium effect" results in a very low plastic brittle transition temperature. (4) Dispersion strengthened molybdenum alloys such as MH and KW doped with Al, K and Si can stabilize the forging structure, hinder the recrystallization or stabilize the elongated recrystallization grain structure, avoid transforming into equiaxed grains, and improve the low-temperature plasticity significantly. The results show that the doped MH and KW Mo alloys have excellent creep resistance and high recrystallization temperature, although they have no special improvement in strength at low temperature. In addition, the active metal oxide dispersion phase is also used to improve the high strength and creep resistance of molybdenum. For example, 0.5% ZrO2 is added to Z-6 molybdenum alloy for dispersion strengthening. The improvement of creep rate is similar to that of dopants. Recently, the strengthening of rare earth oxide molybdenum alloy has been studied. For Mo alloy with 0.2% ~ 0.3% La2O3, the intergranular fracture is reduced, the fracture strength is improved and the low temperature brittleness is improved. For example, the hardness, room temperature tensile strength and compressive strength of molybdenum alloy with Y2O3 added from 0% to 1.5% increase. In addition to the above-mentioned strengthened molybdenum alloys, in order to improve the oxidation resistance of molybdenum, Al, Ni, Si and other elements are often added. For example, when 0.17% AI is added into molybdenum, its oxidation resistance is improved. It is higher than that of pure molybdenum. When 30% Ni and 20% Co are added into Mo, the oxidation resistance life can reach more than 100 h at 940 ℃. Molybdenum silicide is a kind of good high temperature oxidation resistant material. It can still be used as heating element when heated to 1600 ~ 1700C in air. However, molybdenum silicide is brittle and difficult to process, so it has not been widely used.
