【作者】 徐轲；

【导师】 马勤；

【作者基本信息】 兰州理工大学 ， 材料学， 2011， 硕士

【摘要】 Mo₅Si₃在Mo-Si系中具有最高熔点、最宽的成分范围、良好的高温抗氧化性能和抗蠕变性能而成为近年来研究的热点。但是其室温脆性严重阻碍了它的广泛应用。目前,合金化和复合化是改善Mo₅Si₃室温脆性的常用方法。同时,Mo₅Si₃因具有高的共价键组成及良好的化学稳定性而可望具有好的腐蚀性能。本研究基于原位复合化的思想,以MoO3粉、Mo粉、Si粉和Al粉为原料,采用机械化学还原法制备了具有纳米晶结构的Al₂O₃/Mo₅Si₃复合粉体,并结合热压烧结方法制备了Mo₅Si₃及20%Al₂O₃/Mo₅Si₃复合材料。利用X射线衍射仪、激光粒度分析仪、扫描电子显微镜、光学金相显微镜和能谱仪等对复合材料在制备过程中的结构演变、组织形貌及微区成分进行分析和表征,并对复合材料的力学性能和耐腐蚀性能进行研究,结果显示:原料粉体在球磨10h后,发生机械化学反应生成了20%Al₂O₃/Mo₅Si₃复合粉体,反应速度较快,以类似自蔓延的爆炸模式进行。随球磨时间的延长,复合粉体的晶粒尺寸减小,显微应变增大,二者呈逆变关系。球磨100h后,Al₂O₃和Mo₅Si₃的晶粒尺寸分别为38.3nm和39.8nm,具有纳米晶结构。粉体细化主要发生在球磨初期,球磨100h后粉体粒度曲线呈亚微米和微米的双峰分布,颗粒尺寸较小,在1～3μm之间。热压烧结制备的20%Al₂O₃/Mo₅Si₃复合材料主要物相为Al₂O₃、Mo₅Si₃和极少量的Mo3Si。Al₂O₃与基体材料Mo₅Si₃之间未发生化学反应,复合材料组元之间具有较好的热稳定性。Al₂O₃的加入细化基体材料组织,钉扎了晶界和位错,强韧化基体;20%Al₂O₃/Mo₅Si₃复合材料具有较好的综合力学性能。其显微硬度、抗压强度、弯曲强度及断裂韧性分别为1409HV、1419MPa、346MPa和5.48 MPa·m^1/2,较纯的Mo₅Si₃试样有了较大提高。随强韧相Al₂O₃的加入,复合材料断裂模式由穿晶断裂转变为沿晶-穿晶的混合断裂。Mo₅Si₃和20%Al₂O₃/Mo₅Si₃复合材料具有较好的耐H₂SO₄和HCl溶液腐蚀性,其耐腐蚀性远好于传统耐蚀材料304不锈钢。在H₂SO₄和HCl溶液中浸泡100h后,参比试样304不锈钢的腐蚀失重则高达131.1mg/cm²,为Mo₅Si₃和20%Al₂O₃/Mo₅Si₃复合材料腐蚀失重的100多倍。在NaOH溶液中,Mo₅Si₃表面的SiO2钝化膜与溶液中的NaOH发生反应,生成Na₂SiO₃,破坏钝化膜的完整性,耐蚀性较差。加入强韧性Al₂O₃后,20%Al₂O₃/Mo₅Si₃复合材料的耐腐蚀性明显提高,耐腐蚀性与304不锈钢相当。试验材料具有好的耐腐蚀性,其腐蚀机理为:复合材料组成元素Mo、Si等具有强的钝化能力,能在材料表面形成MoO₂、SiO₂及Al₂O₃钝化膜,阻止基体被腐蚀。此外,试验材料具有高的化学稳定性,能有效抵抗腐蚀溶液离子的侵蚀,具有好的耐腐蚀性。更多还原

【Abstract】 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.更多还原