【作者】 喻吉良；

【导师】 张凯锋；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 Nb具有高熔点（2468℃）和良好的室温塑性及韧性,Nb-Si系难熔合金有宽的Nb₅Si₃和Nbss（铌固溶体）两相区,可通过共晶和共析反应制备难熔合金。由韧性较好的Nb和Nb-Si金属间化合物组成的难熔合金被认为是最有开发应用前景的下一代超高温结构材料。真空电弧熔炼-铸造法是制备Nb-Si系难熔合金常用的方法,所得材料的铸态组织一般由树枝状的初生Nb和（Nb+Nb3Si）组成。由于Nb3Si处于亚平衡态,因此铸态材料常要进行1800℃/100h的热处理。而且,为了消除材料中的铸造缺陷,热处理后还要进行热挤压处理。由于机械合金化减小了扩散路径和增加了自由能,很容易制备均匀分布的超细晶材料。在本文中机械合金化作为关键的工艺用来制备Nb-Si-Fe系超细晶难熔合金。到目前为止,几乎没有关于机械合金化+热压烧结制备Nb-Si系难熔合金的报道。本文通过机械合金化+热压烧结制备Nb-xSi-2Fe （x=3, 6, 10, 16）多相难熔合金并研究其室温和高温力学性能。令人感兴趣的是发现Nb-16Si-2Fe难熔合金具有优异的超塑性,迄今为止,在Nb-Si系难熔合金中还未见报道。采用机械合金化方法制备了具有良好烧结性能的Nb-Si-Fe复合粉末。分析了球磨时间、球磨速度、无水乙醇含量对粉末形貌和颗粒大小的影响。转速为250 rpm,球磨30h后Nb-16Si-2F混合粉末平均颗粒尺寸为0.3μm,部分颗粒尺寸小于100nm,主要由Nb、Fe、Nb₅Si₃相和亚稳态的Nb3Si相组成。采用真空热压烧结工艺制备了Nb-Si-Fe难熔合金。结果表明,在1500℃保温1h,压力为25MPa条件下烧结的的Nb-Si-2Fe难熔合金相对密度为99.6%。Nb-xSi-2Fe难熔合金均由Nbss、Nb₄Fe₃Si₅、Nb3Si和Nb₅Si₃相组成。晶界轮廓清晰,晶粒几乎呈等轴状,平均晶粒尺寸大约为3μm;金属间化合物颗粒主要位于Nb晶粒交界处,少量细小的金属间化合物Nb₅Si₃分布于Nb晶粒内。1500℃烧结的Nb-Si-Fe系难熔合金表现出良好的室温力学性能。Nb-16Si-2Fe硬度为11.2GPa,弹性模量为330GPa,Nb-3Si-2Fe断裂韧性可达到14.6MPa·m1/2左右,Nb₄Fe₃Si₅相有利于改善其断裂韧性。Si含量不同的Nb-Si-Fe难熔合金在1300℃的抗拉强度为112MPa ²37MP,延伸率分别为54%⁹5%。以初始应变速率2.31×10-4 s-1进行拉伸,Nb-16Si-2Fe难熔合金试样在1350℃、1400℃、1450℃和1500℃的延伸率分别为185%、338%、512%和320%。Nb-16Si-2Fe难熔合金获得最大延伸率的温度高于Nb₄Fe₃Si₅固相线温度91℃,从而生成大量的（体积分数约为24%）Nb₄Fe₃Si₅液相。超塑变形的机理是Nbss, Nb3Si和Nb₅Si₃晶界的滑移或转动,并伴随Nb₄Fe₃Si₅液相的延伸。在进行高温拉伸时,Nb-16Si-2Fe难熔合金的多相结构抑制了晶粒的长大。在1600℃进行了挤压比为30的挤压变形,沿挤压方向形成明显的变形织构组织特征。这种组织有利于提高材料的断裂韧性,由变形前的11.2 MPa.m1/2提高到成形后的20.1 MPa.m1/2。在1600℃实现了发动机推力室模拟件超塑挤压,一次成形,工艺简单,质量良好,材料利用率高,成本低。更多还原

【Abstract】 Niobium has superior characteristic, i.e., high melting temperature （2468℃）, good ductility and fracture toughness at room temperature. Moreover, Nb-Si system refractory alloys have a wide Nb₅Si₃ and Nb solid solution two-phase region; therefore, eutectic and eutectoid reactions offer a variation of microstructures to prepare Nb-Si refractory alloys. So the refractory alloy with Nbss and niobium silicides has the most potential as the next generation ultra-high temperature structural materials.Arc melting is a general method to fabricate the Nb-Si systems refractory alloys. As cast structure consists of original niobium and nonequilibrium Nb+Nb3Si. Because of unstable microsture of Nb3Si phase at high temperature, as cast materials should be heat treated at 1800℃for 100 h. Moreover, in order to remove defect of as cast materials, materials must be hot extruded after heat-treatment.Due to the reduced diffusion path in the refined microstructures and the increased free energy by mechanical alloyed （MA）, it is easy to produced ultra-fine refractory alloys. Therefore, in this research the MA process was introduced as a key process to develop Nb-Si refractory alloys with ultra-fine and homogeneously distributed phase. There is hardly any report about Nb-Si refractory alloys prepared by MA + hot pressing sintering up to now. So in this paper the Nb-xSi-2Fe （x=3, 6, 10, 16） refractory alloys were prepared by MA and hot pressing sintering; mechanical properties at room temperature and high temperature were also researched. It is interesting to note that Nb-16Si-2Fe refractory alloy has excellent superplasticity. Superplasticity of Nb-Si system refractory alloys has not been reported up to now.Nb-Si-Fe powders with excellent sintering property were synthesized by MA. Effects of milling time, milling speed and addition of alcohol on powder sizes and morphlogy were studied. Sub-micro Nb-16Si-2Fe powders with particle size of 0.3μm and some particle size less than 100nm can be obtained by milling for 30h at 250rpm, which mainly consist of Nb, Fe, Nb₅Si₃ phase and metastable Nb3Si phase.Nb-Si-Fe system refractory alloys were prepared by hot pressing sintering. The refractory alloys sintered at 1500℃for 1h has high relative theoretic density （99.6%）. The microstructures of all Nb-Si-Fe refractory alloys consist of the Nbss and uniformly dispersed three kinds of intermetallic Nb3Si and Nb₅Si₃, and Nb₄Fe₃Si₅ phase. The average size of grains of these refractory alloys is about 3μm and the grain shape is essentially equiaxed. The grain boundaries are clear and sharply facetted. Intermetallic particles mainly located at multi-grain junction of Nb grains. But there are sub-micro intermetallic particles within the Nb grians.The as sintered at 1500℃Nb-Si-Fe system refractory alloys have excellent mechanical properties at room temperature. The hardness and elastic modulus of Nb-16Si-2Fe are 11.2GPa and 330GPa repectively. The fracture toughness of Nb-3Si-2Fe reaches 14.6 MPa·m1/2. The Nb₄Fe₃Si₅ phase has improved fracture toughness of Nb-Si-Fe refractory alloys. The tensile elongation of Nb-Si-Fe refractory alloys at 1300℃is between 54% and 95% depening on Si content. The tensile strength of Nb-Si-Fe system refractory alloys at 1300℃is between 112MPa and 237MPa.The tensile elongation of the Nb-16Si-2Fe refractory alloy reaches 185%, 338%, 320% respectively, at 1350℃,1400℃and 1500℃with initial strain rate of 2.31×10-4 s-1. The maximum elongation of the Nb-16Si-2Fe refractory alloy was obtained at a temperature which is 91℃higher than the solidus temperature of 1359℃. A significant amount of liquid phase Nb₄Fe₃Si₅ occurs during superplastic deforming; the volume of Nb₄Fe₃Si₅ liquid phase was about 24%. The deformation strain mainly results from the sliding/rotation of solid Nbss, Nb3Si and Nb₅Si₃ grians accompanied by strectching of the liquid phase Nb₄Fe₃Si₅. Grain growth is restrained by four-phase structure of Nb-16Si-2Fe refractory alloy.The extrusion deformation of a big extrusion ratio of 30 was undertaken at 1600℃. There is texture structure along extrusion directon, which is attributed to fracture toughness. The fracture toughness increases from 11.2 MPa.m1/2 to 20.1 MPa.m1/2 after extrusion. Model of engine combustion chamber was successfully extruded one-off at 1600℃. That material can be processed to parts with good quality by means of simple extrusion technique. Moreover, the utilization rate of the material was high and cost has been greatly cut down.更多还原