Study On Microstructure And Properties Of Al₂O₃ Doped Molybdenum Alloys Prepared By Hydrothermal Synthesis Method

Molybdenum products such as aerospace hot nozzle and nozzle, the molybdenum head of perforated stainless steel tube need molybdenum alloy has good mechanical properties and high temperature recrystallization temperature. Someone in my research group had used the sol-gel and powder metallurgy process for the preparation of a rare earth lanthanum oxide doped molybdenum alloy, significantly improved the strength of the molybdenum plate and the recrystallization temperature. In this paper, a new type of dispersion strengthened molybdenum alloy is prepared by low-cost alumina replace valuable rare earth lanthanum oxide, to improve the tensile strength of the doped molybdenum plate and recrystallization temperature of molybdenum alloy, to reduce production costs, and to save rare resources.Alumina-doped molybdenum alloy powders were prepared by hydrothermal synthesis method, with（NH4）2Mo4O13·2H2O and Al（NO3）3·9H2O as raw materials. The mass fractions of Al₂O₃ were 0%, 0.25%, 0.5%, 0.75% and 1.0%. The molybdenum alloys were obtained after low-temperature calcination, first reduction, secondary reduction, cold isostatic pressing and sintering in hydrogen protection processes, which final work a plate with of 1.0 mm. The plates after annealing at different temperatures were tested of tensile properties at room temperature, and the performance of hightemperature tensile were tested by thermal simulator to study alumina doped molybdenum alloy recrystallization temperature. We analyzed morphology of Al₂O₃ doping phase, organization of doped molybdenum billet, fracture morphology and organization of sheets at room temperature and high temperature tensile by XRD, SEM, EDS and other means of detection in molybdenum alloy preparing at different stages, and the strengthening mechanism was discussed. The main conclusions are as following:In hydrothermal synthesis process, there were MoO3 of hexagonal structure and hydrated alumina AlO（OH） bamboo-like sheet were generated. After low temperature calcination, intermediate phase Al2（MoO4）3 was generated in powders doped. And MoO3 of metastable hexagonal structure turned into stable orthorhombic structure, and AlO（OH） dehydration turned into Al₂O₃. After a reduction, the intermediate phase Al2（MoO4）3 was eventually reduced to MoO2 and Al₂O₃ by hydrogen. After the second hydrogen reduction, MoO3 like sheet completely was reduced to spherical Mo particles. The aluminum-containing phase in the form of α-Al₂O₃ existed in Mo powders. In the reduction process, the alumina particles could play a role in refining molybdenum powder particles. In molybdenum blank, α-Al₂O₃ is dispersed as point in the molybdenum matrix, as compared to the molybdenum powders doped Al₂O₃, particles growth, sizing between 500 nm 1 μm. With increasing of alumina content, the material microhardness is increased significantly, and density is closer to theoretical density.The Al₂O₃ added make recrystallization temperature of molybdenum plates increase to 1200 ℃-1300 ℃. Recrystallization nucleus formation and dislocation movement is closely related, but the movement of dislocations is strongly impeded by the Al₂O₃. Recrystallization core formation will be delayed, recrystallization temperature is increased. The Al₂O₃ make the recrystallization grains refiner. In the recrystallized grains growth stage, Al₂O₃ particles play a role to them growing, thus alumina particles delay the recrystallization growth process.Al₂O₃ doped molybdenum plate is significantly higher than the pure molybdenum plate of the tensile strength. Elongation curve of pure molybdenum sheet appeares inflection point at 1000 ℃, but the inflection point of doping molybdenum plate does at 1100 ℃, Al₂O₃ doping amount of 0.5% molybdenum plate shows good overall performance. With the increase of annealing temperature, the hardness is decreased of the molybdenum sheets. With the increase of Al₂O₃ doping amount, microhardness is increased significantly of the doped molybdenum board. Pure molybdenum sheets after annealing at 1100 ℃ show the brittle fracture, and Al₂O₃ doped molybdenum plates fracture form begin to change after 1200 ℃ annealing. Alumina joined postpones the recrystallization of molybdenum, changing the fracture mechanism.With the temperature improvement of the stretching, tensile strength is decreased of the pure molybdenum and aluminum-doped molybdenum plates. But the tensile strength of the aluminum-doped molybdenum alloy is significantly higher than of the pure molybdenum. With the increasing of stretching temperature, the elongation of pure molybdenum and alumina molybdenum plates is increased first and then decreased. The elongation of pure molybdenum plate reaches the maximum value at 1000 ℃, but alumina molybdenum plate at 1100 ℃. The plasticity of alumina molybdenum plates is higher than pure molybdenum sheets. The fracture surface of alumina molybdenum plates is based to dimple in high temperature. It is always plastic fracture with stretching temperature from 900 ℃ to 1300 ℃, while pure molybdenum plate fracture mode becomes from ductile fracture to brittle fracture when the tensile temperature reaches 1100 ℃. The alumina added to molybdenum plates improves the strength and the dynamic recrystallization temperature, thereby increasing the use of molybdenum at high temperature range.