燃烧合成(Mo,Nb)Si2材料中含Nb相的微观组织演变与强韧化机制

Transition-metal disilicides have been of great interest for use as ultra-high-temperature structural materials, i.e. above 1200℃. They have good potential for improving gas turbine engine performance. MoSi2 is one of the most promising candidates for such structural applications because of its high melting point and good oxidation resistance. Unfortunately, applications of MoSi2 are limited by its low fracture toughness at room temperature and poor creep resistance at high temperature. Thus the key issue is to improve fracture toughness and creep resistance of MoSi2. An significant route currently being pursued to increase silicide properties is the alloying approach, and Nb is considered as an important toughening and strengthening element. Recently, (Mo,Nb)Si2 powder were prepared from Mo, Si and Nb powders by combustion synthesis (Self-propagating high-temperature synthesis) in our experiment. The objective of this study is to investigate the microstructural evolution during combustion synthesis, strengthening and toughening mechanism of niobium-containing phase of (Mo,Nb)Si2. The combustion mode will be analyzed by the combustion videos and images. Thermocouple wires of WRe3-WRe25 will be used to record the combustion synthesis temperature. The microstructural evolution during combustion synthesis will be discussed by XRD, SEM and EDS. The formation of niobium-containing phase will be researched basing on the discussion of diffusion and move of niobium in alloys. The physical model will be founded. And then the (Mo,Nb)Si2 powder prepared by combustion synthesis will be made into the target materials pellets by hot-pressing method. The morphology, distribution and phase structure of niobium-containing phase will be studied. The strengthening and toughening mechanism of (Mo,Nb)Si2 will be discussed by crack propagation, dislocation and interface characters. The relationship among composition, structure and properties will be established. And the contribution of niobium to properties will be made clear..This research will develop the strengthening and toughening theories and methods of MoSi2 and other transition-metal disilicides. It will also provide important theoretical and experimental evidence for new silicide-based structural materials used in the temperature range of 1200-1600℃.

在前期以元素粉末为原料，采用燃烧合成技术低成本快速制备了(Mo,Nb)Si2合金粉末的基础上，针对过渡族硅化物存在的室温韧性差和高温强度低的问题，本课题拟通过对含Nb相的组织动态演变规律、形成机制和显微力学性能分析，揭示(Mo,Nb)Si2的强韧化机理。通过研究燃烧模式、反应激活能、微观结构演变特征和产物组成，明确(Mo,Nb)Si2的合成机制，阐明Nb的扩散和迁移过程，确立含Nb相在合成过程中相结构和组织演变规律，探明含Nb相形成机理，并建立相关物理模型。明确Nb元素以及含Nb相在热压烧结致密化(Mo,Nb)Si2中的存在方式、微观形态和分布特征，从裂纹扩展、位错结构和界面特征三方面探明(Mo,Nb)Si2的力学行为，建立成分、结构和性能之间的关系，确立Nb对材料性能的贡献，明确(Mo,Nb)Si2的强韧化特征。为丰富和发展过渡族金属硅化物的强韧化理论和方法提供必要的理论依据和技术支持。

MoSi2是一种非常重要的高温结构候选材料，但存在室温韧性差和高温易蠕变的不足。针对上述问题，本项目以Nb单一合金化和Nb、Al协同合金化MoSi2为思路，采用热力学计算理论绝热温度，通过燃烧合成技术获得超过饱和固溶体并考察其转变机理，运用真空热压烧结和放电等离子烧结技术使其致密化，并对其组织和力学性能进行研究。结果表明，Nb的固溶引起C11b相和C40相热容及热焓的变化，从而改变体系的绝热温度。以Mo、Si、Nb、Al元素粉末为原料，通过燃烧合成技术快速制备了(Mo1-xNbx)Si2和(Mo1-xNbx)(Si1-yAly)2材料。Nb、Al合金化促进了MoSi2燃烧合成反应放热，使燃烧温度上升，燃烧模式向稳态转变，其反应机制为：熔融-溶解-析出。燃烧合成产物主要为固溶有Nb(Al)的C11b相和C40相，存在过饱和固溶。C11b相中过饱和固溶的Nb/Al在烧结过程中发生脱溶形成C40相，Nb合金化促进NbSi2型C40相形成，Nb、Al协同合金化促进Mo(Si,Al)2型C40相形成。热压烧结使材料相组成趋近于平衡态，快速低温的放电等离子烧结材料则仍为非平衡态。少量Nb固溶使C11b相中最强及次强键的键能下降，而共价电子数比例 增加，导致材料的硬度降低而强度增加；C40相为硬脆相，大量Nb/Al合金化时形成大量C40相，会导致(Mo1-xNbx)Si2和(Mo1-xNbx)(Si1-yAly)2材料硬度上升，而强度和断裂韧性下降。同成分SPS样弯曲强度高于热压样，且其强度峰值出现在更高Nb含量（x=0.12）材料中，主要是细晶强化效应和C11b相过饱和固溶强化所致。适量Nb协同微量Al（y=0.03）有助于提高MoSi2的力学性能，Nb在高温下起到固溶强化作用，并且有助于产生层错强化效应；少量Al可减少晶界SiO2玻璃相使强度上升，但大量Al合金化会降低材料的强度及断裂韧性。(Mo0.85Nb0.15)(Si0.97Al0.03)2材料具有最佳综合力学性能，其1400℃压缩强度比MoSi2提高了115%，弯曲强度（322 MPa）和断裂韧性（5.91 MPa/m1/2）相对MoSi2（250 MPa，4.68 MPa/m1/2）都有明显改善。

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