The Preparation And Corrosion Properties Research Of Al₂O 3/Mo₅Si₃ Composites

Mo₅Si₃ has become a focus of material in recent years for the highest melting point, the most wide composition range,good oxidation resistance and creep resistance in Mo-Si system. However, frangibility at room temperature has hindered its widespread application seriously. At present, alloying and composite are commonly used methods to improve Mo₅Si₃ room temperature frangibility. Meanwhile,due to its unique chemical composition and strong bonds with large covalent component, Mo₅Si₃ is anticipated to be a novel corrosion resistant candidate material.The study based on the idea of in situ composite, The Al₂O₃/Mo₅Si₃ composite powder with the structure of nanocrystalline were synthesized by mechanochemistry reduction with MoO3,Mo,Si and Alpowders as raw materials. We have analyzed and characterized the structural evolution, microstructure, micro area composition of preparing composites process by x-ray diffractometer, laser particle size analyzer, scanning electron microscope, optical microscope etc. Moreover we have researched the mechanical properties and corrosion properties of the composites, the resaults show that:After the ball-milling running for ten hours, The raw materials powder has happened the mechanochemistry reaction and generated 20%Al₂O₃/Mo₅Si₃ composite powder, the velocity of reaction is much faster, and process in the way which is similar as self-propagating explosion mode, with the time of ball-milling prolonging, the crystal grain size reduced and the microstain increased, the relation between them is contravariant, after the ball-milling running for 100h, the crystal grain sizes of Al₂O₃ and Mo₅Si₃ respectively are 38.3nm and 39.8nm, they all possessed nano-crystalline structure. The refinement mainly occurs in the early ball-milling stage, after the ball-milling running for 100h, the distribution plot for particle size presents bimetal character of sub-micro zone and micro zone, the particle size is much smaller and between 1-3μm.Main phases of 20%Al₂O₃/Mo₅Si₃ composite materials which prepared by hot pressing sintered were Cu, Al₂O₃, Mo₅Si₃ and a very small amount of Mo3Si. No chemical reaction occurred between Al₂O₃ and the matrix material Mo₅Si₃, composite -component has better thermal stability. The addition of Al₂O₃ refined and toughened the matrix as well as anchored the dislocation and grain boundary. 20%Al₂O₃/Mo₅Si₃ composite powder has the best the over all mechanical properties. The microhardness, pressive strength, bending strength as well as the fracture toughness were 1409HV,1419MPa,346MPa and 5.48 MPa·m^1/2 respectively, which have been increased greatly when compared to the pure Mo₅Si₃ sample. The addition of Al₂O₃ also converted the fracture mode from intracrystalline to the combination of fracture and intergranular fractures. The addition of Al₂O₃ also converted the fracture mode from intracrystalline to the combination of fracture and intergranular fracturesMo₅Si₃ and Al₂O₃/Mo₅Si₃ composites have excellent corrosion properties in H2SO4 and HCl solution, their corrosion properties are much higher than 304 stainless steel. After 100 hours immersing in H2SO4 and HCl solution, corrosion weight loss of the reference sample 304 stainless steel is up to 131.1mg/cm² which is higher 100 times than Mo₅Si₃ and Al₂O₃/Mo₅Si₃ composites. In NaOH solution, because the SiO₂ passive film of Mo₅Si₃ surface and NaOH have reacted to produce Na₂SiO₃, that destroyed the integrity of passive film, the corrosion resistance ability of composites is bad. When adding tough Al₂O₃, the corrosion resistance ability of Al₂O₃/Mo₅Si₃ become excellent, and corrosion resistance ability is similar with 304 stainless steeil.The test materials have excellent corrosion resistance ability, and the corrotion mechanism is that: the composition elements Mo,Si etc of composites have strong passive ability, which can form passive film MoO₂, SiO₂ and Al₂O₃ in the surface of materials to hinder the matrix corroded. In addition, test materials have strong chemical stability that can resistant corrosion by solution ions effectively. Thus the composite have excellent corrosion resistance ability.