超高温Nb-Si基合金中固溶体NbSS相由解理向韧窝型断裂转变的机理研究

In the ultra-high temperature structural Nb-Si based alloys with a NbSS/Nb5Si3 microstructure, the NbSS phase supplies the room temperature fracture toughness, which is one of the most important properties of the structural materials. We have found that the as-cast NbSS phase usually had a grain size up to dozens of micrometers and failed in a cleavage mode, which obstructs the NbSS phase taking the best of toughness to the NbSS/Nb5Si3 microstructure. When the NbSS grain size is below 10 micrometers, the active dislocations of {1 1 0}<1 1 1> operate in the fine NbSS phase, whereas in the coarsened NbSS phase the immovable dislocations of {0 0 1} <1 0 0> operate. This change in the slip systems leads to a fracture mode transition of the NbSS phase from the cleavage to the dimple, subsequently, improving the fracture toughness of the fine NbSS/Nb5Si3 microstructure with a dimple failure of NbSS by ~200%, as compared to the samples with the coarsened NbSS phase failed in a cleavage mode. The grain size, volume fraction, stress triaxiality (Rc), cleavage surface energy and Peierls-Nabarro barrier energy UP-N of the NbSS phase in the NbSS/Nb5Si3 microstructure are confirmed to be the keys that affect the failure mode transition of the NbSS phase from the cleavage mode to the dimple mode. In this proposal, the fracture toughness of the NbSS/Nb5Si3 microstructure and the deformation and failure modes of the NbSS phase dependent on the grain size and volume fraction of the NbSS phase and the alloying (elements Ti, Cr etc.) will be investigated. The stress triaxiality, the cleavage surface energy and the Peierls-Nabarro barrier energy of the dislocation movement of the NbSS phase in the NbSS/Nb5Si3 microstructure will also be analyzed by Finite Element Method and Analytical Method under the aforementioned conditions, and the mechanism and criteria that the NbSS phase fails from the cleavage mode to the dimple mode will be identified. Finally, the optimization principle based on refining the NbSS grain and alloying for toughening of the NbSS/Nb5Si3 microstructure will be proposed.

具有NbSS/Nb5Si3组织的超高温Nb-Si基合金中NbSS相提供室温韧性，是结构材料关键性能之一。我们发现铸态合金中NbSS晶粒尺寸往往达数十微米，发生解理断裂，妨碍了NbSS相韧性充分发挥。当NbSS晶粒尺寸降至微米级，其变形滑移系由大晶粒解理面上{001}<100>变成小晶粒易滑面上{110}<111>，断裂方式由解理转向韧窝型，Nb-Si合金韧性提高~200%。NbSS相晶粒尺寸、体积分数、应力三轴性(Rc)、解理能与位错运动P-N能(UP-N)是影响NbSS由解理到韧窝型断裂转变的关键因素。项目研究粉末冶金NbSS/Nb5Si3组织中NbSS晶粒尺寸、体积分数、合金化（Ti、Cr等）对NbSS变形断裂方式的影响规律，应用数值法计算NbSS相的Rc、解理能和UP-N值，揭示NbSS从解理到韧窝型断裂转变的临界条件和机理，提出基于细化晶粒和合金化的Nb-Si基合金韧化理论与方法。

新一代高推比航空发动机和高比冲火箭发动机的关键热端部件如叶片、燃烧室、喉衬、尾喷管等需要承温能力更高的超高温结构材料以满足更高动力和工作效率的需求。Nb-Si基合金是具有承温能力达到1200ºC~1400ºC的新型金属间化合物基超高温结构材料，可用于制造新一代航空航天器动力装置关键热端部件，是当前结构材料领域的研究热点之一。新一代高推比动力装置要求超高温结构材料必须在高温强度、蠕变抗力、室温韧性、抗氧化性和密度等方面达到综合性能平衡，因而通过多相组织匹配设计可使材料获得上述性能的综合平衡。. 本项目应用不同Nb和Nb5Si3粉末粒度搭配设计制备了具有不同显微组织形态的Nb-Si基合金，应用有限元方法研究了不同Nb晶粒尺寸和Nb5Si3分数下Nb/Nb5Si3显微组织的应力三轴性分布和应力应变场，揭示了Nb相变形约束行为以及提高Nb/Nb5Si3组织断裂韧性的力学因素；应用第一性能原理计算了Ti元素含量对Nb-Ti固溶体的表面能γs和P-N势能U_(P-N)的影响，揭示了Ti合金化对Nb相的变形断裂行为和断裂韧性的影响规律与机制。研究结果表明，随着Nb/Nb5Si3显微组织中Nb相的细化，其所受到的应力三轴性水平降低，塑性应变增大；其变形形式由{0 0 1}面位错运动变成{1 1 0}）<1 1 1>/2可动位错运动，断裂方式由{0 0 1}解理型断裂向韧窝+撕裂+解理混合方式断裂。有利于提高Nb/Nb5Si3显微组织的断裂韧性。Ti合金化提高Nb的表面能，降低了U_(P-N)能，使Nb向韧性断裂转化。. 根据上述研究结果，提出基于细化NbSS晶粒和合金化的Nb-Si基合金组织优化设计的理论与方法。即设计以细化的NbSS晶粒为基体的显微组织，降低Nb/Nb5Si3组织中NbSS晶粒及晶界的应力三轴度，提高NbSS晶粒塑性变形能力，使其变形以<1 1 1>/2可动位错运动为主，产生韧窝型断裂，有利于提高Nb/Nb5Si3显微组织的断裂韧性。其学术价值在于探明了细化晶粒引起合金断裂方式由解理向韧窝转化的力学因素及机理，为合金组织韧化设计提供了理论依据。该成果也可以用于指导其他强/韧合金体系的韧化设计，为航空航天用新型金属间化合物基高温结构材料的发展提供支持。