Preparation And Properties Of Mo-12Si-8.5B-La₂O₃-Al₂O₃ Composite

In order to further improve the efficiency of aerospace engines and industrial steam turbines,the design and development of new structural materials that can withstand higher operating temperatures is a hot research topic at present.Mo-Si-B alloy is one of the hot candidate materials for its high melting point,excellent creep resistance and oxidation resistance at high temperature.Mo-12Si-8.5B alloy is composed ofα-Mo-Mo₃Si-Mo₅Si B₂ternary phase.Due to the continuousα-Mo phase distribution,the alloy has excellent fracture toughness,which has attracted extensive attention.However,the presence ofα-Mo reduces the oxidation resistance of the alloy at high temperature（>800℃）.In this paper,Al₂O₃particles were added to promote the formation of self-healing protective layer at high temperature to improve the oxidation resistance of the alloy,and La₂O₃particles were added as the second phase to enhance the mechanical properties of the alloy.In this paper,the sol gel method and step of hydrogen reduction of preparation of nanometer Mo powder mixes with micron powder,was prepared by SPS sintering alloy bipolar doping,composite materials for room temperature mechanical properties and high temperature oxidation resistance experiment,studies the different Al₂O₃ content on the mechanical property of composite material and the influence of oxidation resistance.The results show that:（1）Through the study of sol gel/hydrogen reduction process for the preparation of powder found:in the p H=1,citric acid than the quality of ammonium molybdate was 1.7,water bath temperature 85℃can get crosslinhing networh of macromolecules,through again after 5 h under 550℃c alcination step is carried out under the 750℃by hydrogen reduction,compound nanometer Mo powder can be mixed evenly.（2）By exploring the effect of one-step hydrogen reduction temperature on the composition and morphology of the composite powder,it is found that the reduction process of Mo O₃ to Mo O₂ follows the chemical vapor transport mechanism in the range of450～850℃.The reduction process of Mo O₂ to Mo follows the pseudo-crystal transition mechanism in the range of 450～～50℃,and follows the chemical vapor transport mechanism when the temperature is higher than 750℃.（3）Composites with different Al₂O₃ content were prepared by SPS sintering method.With the increase of Al₂O₃ content,the density of composites decreased gradually,the hardness decreased from 8.8GPa to 7.34 GPa,the bending strength decreased from 817.MPa to 545.9 MPa,and the fracture toughness decreased by 14%.The decrease from 9.98MPa·m^1/2 to 8.5～MPa·m^1/2 is attributed to the fact that Al₂O₃ grows up and agglomerates into micron particles in the sintering process and distributes atα-Mo phase grain boundary.However,the compressive properties of composites decrease first and then increase,because the increase of the volume fraction of micron particles is beneficial to improve the compressive strength of composites.（4）After cyclic oxidation of the composites at 800～1200℃,it is found that the total mass loss of the composites with 10 wt.%Al₂O₃ content is the lowest（-835.38mg/mm²）at800℃,and the mass loss is reduced by 38%.At 1000～1200℃,the addition of Al₂O₃increases the oxidation loss of the material,because Al₂O₃ reacts with Si O₂ at high temperature to generate polycrystalline stone,which hinders the formation of protective borosilicate layer on the surface of the alloy,resulting in catastrophic oxidation of the alloy.