Molybdenum and its alloys had a low coefficient of thermal expansion, superior electrical and thermal conductivities, good high-temperature strength and creep resistance, and excellent high-temperature dimensional stability. Therefore, as high-temperature components, molybdenum and its alloys were widely used in aerospace, electronic communications, electrical equipment, space vehicles and other high-temperature parts. However, to some extent, the low recrystallization temperature (about 1000℃), inherent brittleness and insufficient strength hindered their practical application. It was well known that ultrafine grained or nanocrystalline materials have better mechanical properties than that of the conventional coarse-grained materials. Therefore, preparing ultrafine grained or nanocrystalline composite materials was an effective way to enhance the mechanical properties. Moreover, Y2O3 partially stabilized ZrO2 had a great potential in improving the comprehensive mechanical properties of molybdenum and its alloys. However, the study had not yet reported the preparation of the Mo-ZrO2(Y2O3) nanometer composite powders. The nanometer Mo-ZrO2(Y2O3) powders were prepared by sol-gel and high temperature hydrogen reduction, the details were as follows: Firstly, the raw materials of Zr(NO3)4•5H2O, Y(NO3)3•6H2O and C6H8O7•H2O were dissolved in the distilled water according to a certain proportion and continuously heated with thermostat water bath cauldron at 85℃ to form wet gel. Secondly, the wet gel was dried at 110℃ for 10 h to obtain the dry precursor powder, and the powder subsequently calcinated at 400~650℃ for 5 h in a muffle furnace. At last, the effects of different reduction time and temperatures on the phase composition and microstructure of the Mo-ZrO2 (Y2O3) composite powders after high temperature hydrogen reduction were also analyzed. The reduction parameters were discussed as follows: (1) reduction time: the powders were reduced at 550℃ for 1~5 h in H2 atmosphere; (2) reduction temperature: the powders were reduced at 350~900℃ for 3 h in H2 atmosphere. Phase composition and microstructure of the precursor composite powders were investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS). The calcinated powders were composed of single MoO3 when the dry precursor powders were calcined at 400 to 650℃ for 5 h. An increase in the grain size of the calcined powders was observed with the enhancement of the calcination temperature. When the calcination temperature was varying from 400 to 550℃, the calcined MoO3 powders showed the characterization of lamellar and smooth microstructure, and the agglomeration phenomenon appeared in the powders. When the calcination temperature increased up to 600 or 650℃, the grain size of MoO3 powders continued to grow. The powders showed the shape of rod after the dry precursor powders were calcined at 650℃. It indicated that the particle size distribution of the composite powders was relatively uniform after the dry precursor powders were calcined at 550℃ for 5 h. When the calcined powders were reduced at 550℃ for 1 h, the powders were composed of MoO3 and a small amount of MoO2, and the particles of the reduced powders showed the characterization of lamellar (MoO3) and granular (MoO2) morphologies. With extending the reduction time to 3 or 5 h, the diffraction peak of MoO3 disappeared and the MoO2 phase appeared, and a small amount of the Mo phase formed, which indicated that the reduction time of transforming MoO3 to Mo phase at 550℃ needed at least 3 h. According to the relationship between reduction temperature and products, the reduction temperature could be divided into three stages. With an increase of the temperature from 350 to 600℃, MoO3 powders were first reduced to Mo4O11 and subsequently were transformed into MoO2. When the powders were reduced at 650℃, the reduced products were composed of MoO2 with trace amount of Mo. When the reduction temperature varied from 650 to 900℃, the products were composed of Mo. It was obvious that the temperature of reducing all the calcined MoO3 powders to Mo should be over 650℃. When the reduction temperature was 350℃, the morphologies of the powders were lamellar (MoO3) and granular (MoO2). When the reduction temperature increased up to 450℃, the morphology of the powders gradually changed from lamellar to granular structure. After reducing at 550℃, the powder exhibited the irregular granular. When the reduction temperature was 600℃, the irregular granular particles gradually decreased and changed into the spherical Mo particles. When the reduction temperature was further increased up to 650-850℃, the agglomeration phenomenon of the powders was more obvious, and the agglomerated particles were composed of fine near-spherical particles. The Mo powder particles tended to grow up due to the high reduction temperature. The TEM image indicated that the agglomerated composite powders were made of nanometer Mo-ZrO2(Y2O3) particle with the size of 80~100 nm, and the EDS results further proved that the composite powders were composed of Mo and ZrO2(Y2O3) particles. In addition, Mo-ZrO2(Y2O3) powders contained 0.11% C, 1.53% O and 0.07% N. It indicated that the combination of sol-gel and high temperature hydrogen reduction could prepare the nano-grained and high purity Mo-ZrO2 (Y2O3) composite powders. TEM analysis of Mo-ZrO2(Y2O3) composite powders reduced at 750℃ for 3 h further confirmed that the composite powders were composed of nano-Mo and ZrO2(Y2O3), and the elements of Zr and Y were uniformly distributed in the Mo matrix in the form of oxide ZrO2 and Y2O3. The results showed that the compositions of the precursors, which were calcined at 400-650℃, were composed of MoO3. With an increase in the calcination temperature, the morphologies of the powders evolved in the form of flakes-lamellar-long rods. The reduction temperature was 550℃ when MoO3 were initially reduced to Mo. The temperature should be over 650℃ using a one-step reduction method when the MoO3 was completely reduced to Mo powder. Nanometer Mo-ZrO2(Y2O3) composite powder with high purity was prepared by sol-gel and high-temperature hydrogen reduction method. The agglomerated composite powder was made of nanometer Mo-ZrO2(Y2O3) particle with the size of 80~100 nm, and ZrO2(Y2O3) particles were uniformly distributed in the Mo matrix powder. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.
