【作者】 王晓丽；

【导师】 张凯锋； 王国峰；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2011， 博士

【摘要】 Nb-Si难熔合金的显微组织通常包含铌基固溶体（Nbss）和铌硅化物（Nb₅Si₃/Nb₃Si）,由于其高熔点、低密度和良好的高温强度等优点,成为极具潜力的满足新一代航空发动机需求的高温结构材料。但目前广泛应用真空电弧熔炼等铸造方法制备的Nb-Si难熔合金因组织粗大和成分偏析等缺陷影响其性能,并且金属间化合物的本征脆性也阻碍了Nb-Si难熔合金作为高温结构材料的实际应用。本文采用机械合金化+真空热压烧结相结合的粉末冶金方法制备了Nb-Si二元难熔合金以及不同Ti、Fe含量的Nb-Si多元难熔合金（Si含量16at.%）,并通过粉末锻造短流程技术成形了发动机推力室模拟件,研究了机械合金化过程中不同混合粉末系统的组织形貌变化以及在不同状态球磨粉末和热压烧结参数下Nb-Si难熔合金的显微组织演变和力学性能,分析了Nb-Si二元合金以及含合金化元素Ti、Fe合金的强韧化机制,考察了材料的高温压缩变形行为以及变形后的显微组织。对Nb-16Si二元和Nb-16Si-18Ti三元粉末系统机械合金化的研究结果表明无水乙醇用量、初始Nb粉末粒度以及Ti粉末的加入均影响球磨进程。球磨过程中,Nb颗粒经塑性变形成片状而后在加工硬化作用下逐渐细化成絮状,Si逐渐固溶到Nb的晶格中形成过饱和固溶体。过量的无水乙醇推迟了球磨进程,而过长时间的球磨导致介质污染的引入而形成富Fe的另一种铌基固溶体。Ti的低密度使得同质量的粉末体积较大,在相同的球磨参数下三元粉末系统的球磨进程明显慢于二元粉末。采用真空热压烧结法制备了Nb-16Si二元难熔合金,分析了烧结温度、烧结时间和球磨粉末状态对烧结材料的显微组织演变及力学性能的影响。在1500°C下保温60min得到了致密度超过99.2%的烧结材料,合金由Nbss、Nb3Si和Nb5Si3三相组成,大量孔隙导致低温烧结材料的力学性能较差。随球磨时间的延长热压烧结材料的显微组织逐渐细化为平均颗粒尺寸仅2μm左右的近等轴状,Nbss体积分数减少,大尺寸和高含量的韧性相能够有效改善材料的室温脆性,由此室温断裂韧性降低而弯曲强度增加。在1600°C下采用粉末锻造技术一次成形了Nb-Si难熔合金推力室模拟件,零件具有良好的力学性能和与烧结材料相似的等轴状组织,材料利用率高且成本低。研究了合金化元素Ti、Fe对热压烧结Nb-Si难熔合金的显微组织演变及力学性能的影响,探讨了组织对性能的影响规律和材料的强韧化机制,考察了高温压缩行为及其变形后的组织。Fe的加入因合成Nb₄FeSi新相而使材料强度提高,但没有改变组织形貌。含Ti的片状复合粉末经烧结后材料的显微组织和性能呈各向异性,在垂直于和平行于热压方向的截面上Nbss分别为平行的条状流线和杂乱的片状。裂纹扩展路径垂直于条状Nbss时塑性变形以及裂纹桥联和偏转等有效提高了材料的断裂韧性,其最高值超过16MPa·m^1/2,弯曲断口为撕裂棱和韧窝韧性断裂特征,而Ti固溶到Nb中引起的强化使材料获得较好的综合力学性能。垂直于流线方向的高温压缩变形中,Nbss发生塑性压缩且变形后条状Nbss变细,韧性相尺寸影响压缩塑性,而沿流线方向压缩时,变形通过颗粒转动和界面滑移带动片状Nbss向垂直于加载方向偏转。更多还原

【Abstract】 Nb-Si refractory alloys usually consist of Nbss, Nb₅Si₃ and/or Nb₃Si, and they have been developed as the new class high temperature structural material because of their high melting point, low density and excellent high temperature mechanical properties. Arc melting is the general method to fabricate Nb-Si alloys, while the casting flaws of component segregation and coarse microstructure damage the properties. Moreover, the intrinsic brittleness of silicides also hinders their practical applications. In the present research, Nb-16Si binary alloys and multi-element ones with the addition of Ti and/or Fe were prepared by mechanical alloying and uniaxial hot pressing and a model of trust chamber were fabricated by powder forging. Meanwhile, the evolution of mechanical alloyed powders, and the effects of mechanical alloying, sintering parameters and alloying elements of Ti and Fe on the microstructures and mechanical properties have been investigated. Moreover, the reinforcing and toughening mechanisms, high temperature compressive deformation behaviors and the defromed microstructures have been studied.The research results of mechanical alloying of Nb-16Si and Nb-16Si-18Ti powder systems showed that the amount of absolute alcohol, particle size of initial Nb powders and addition of Ti affected the ball milling process. During milling, Nb powders were refined into flocculent morphology due to work-hardening resulting from plastic deformation, and a supersaturated solid solution was formed by Si atoms dissolving into Nb lattices. Excess absolute alcohol reduced the refining rate of powders, and the long milling time induced the contaminate to form another solid solution rich in Fe. In addition, under the same milling parameters, the milling progress of ternary powers was slower than that of binary ones due to the lower density of Ti.Nb-16Si binary alloys were prepared by vacuum hot pressing sintering, and the effects of sintering temperature, sintering time and mechanical alloying on the microstructures and mechanical properties were analyzed. The density was mainly determined by sintering temperature, and over 99% relative density were obtained when hot pressed at 1500°C for 60min. Sintered materials contained three phases of Nbss、Nb₃Si and Nb₅Si₃, and pores resulted in the poor properties at lower temperatures. With milling time increasing, the microstructure refined and changed into equiaxed one with average size of about 2μm, and the volume fraction of Nbss decreased. Furthermore, fracture toughness reduced and flexural strength increased because the large size and high content of ductile phase improved room ductility. A model of trust chamber was successfully fabricated by one-time powder forging, and the part contained nice performance and the similar microstructure to sintered material. Moreover, material utilization was improved and the cost was reduced.Influence of the addition of Ti and/or Fe on the microstructures of hot-pressed Nb-Si alloys, and effect of microstructural control on the mechanical properties were studied. The reinforcing and toughening mechanisms of materials with alloying elements were discussed and high temperature compressive deformations and the defromed microstructures were investigated. With the additon of Fe, the strength was enhanced due to the synthesizationof new phase Nb₄FeSi, and nevertheless the microstructure did not change. It is worth noting that the anisotropic microstructures and properties were observed in the materials hot pressed from flaky powders containing Ti. Nbss phases were respectively in shapes of parallel streamlines and mussy sheets on the cross sections parallel and perpendicular to the pressed direction. When propagation directions of the cracks were perpendicular to long strips of Nbss, the fracture toughness was severely improved and the highest value exceeded 16 MPa·m1/2 as a result of plastic deformation, crack deflection and crack bridging. The fracture morphology showed ductile fracture characters of dimples and tear ridges. In addition, flexural strength was enhanced due to solution strengthening when Ti dissolving into Nb, and Nb-Si alloys with Ti consequently possessed superior comprehensive room temperature mechanical properties. In the elevated compressive deformation, when the loading direction was perpendicular to streamlines, strips of Nbss became narrow after being compressed, and the compression ductility was relative to the size of trips. On the other hand, materials deformed through particle rotation and interfacial slip which drove strips of Nbss deflect toward the direction perpendicular to compressive axis.更多还原