激光原位反应制备Nb-Si基超高温结构材料的组织演变及韧化机理

Nb-Si based alloys exhibit the greatest potential to replace nickel-based superalloys and have become a new generation ultra-high temperature structural materials. They are believed to have broad application prospects in aero-engine with high thrust-weight ratio., however, low fracture toughness at room temperature is one of the main barriers for their engineering applications. It is an important scientific issue to find a effective way to obtain Nb-Si based alloys with fine, uniform and low segregation and also improve their fracture toughness by fracture mode transition and reveal the toughening mechanism of the alloys..In this project, combining the laser melting deposition with powder in-situ reaction could refine the microstructure and relieve composition segregation of Nb-Si based alloys. Toughening effect of alloying like Ti, Hf will also be used. By the above way, the fracture mode of Nb-Si based alloys at room temperature would transform into ductile fracture from brittle cleavage fracture. According to the idea of this project, in order to solve the problem of low fracture toughness of Nb-Si based alloys, it is necessary to carry out the following research: the microstructure characteristics and formation mechanism of Nb-Si based alloy prepared by laser in situ reaction, the effects of alloying and heat treatment on microstructure and properties evolution, the mechanism of brittle-ductile transition of the alloys at room temperature, On the basis of above, the composition and microstructure of Nb-Si based alloy will be optimized, and the comprehensive mechanical properties at room and elevated temperature will be evaluated. Therefore, this project will be of great significance for the development and application of Nb-Si based ultra-high temperature alloys.

Nb-Si基合金是最具潜力替代镍基高温合金的新一代超高温结构材料，在高推比航空发动机等领域有着广泛的应用前景。室温韧性不足是制约其工程应用的主要障碍之一。如何采取有效的技术途径，获得组织细小均匀、成分近无偏析的Nb-Si基合金，使其室温发生韧性断裂，从而改善合金韧性，并揭示韧化机理，是值得研究的科学问题。.本项目提出，通过激光熔化沉积+粉末原位反应的技术途径，显著细化合金组织、减小成分偏析，同时结合Ti、Hf等元素的增韧作用，促使Nb-Si基合金室温下由脆性解理断裂向韧窝断裂转变。按此思路，为解决室温断裂韧性低的技术难点，拟开展如下研究：分析激光原位反应制备Nb-Si基合金的组织特征及形成机理；研究合金化及热处理等对显微组织及性能演变的影响；揭示Nb-Si基合金断裂机制转变机理；在此基础之上优化合金成分及显微组织，并评价其室高温力学性能。项目具有重要的工程意义和科学价值。

Nb-Si基合金具有高熔点、高刚度、低密度以及优异的高温强度，是最具潜力替代现有镍基高温合金，承温能力达到1200~1400 ºC的超高温结构材料，在新一代高推比航空发动机和高比冲火箭发动机动力装备等超高温领域有广泛的应用，室温韧性不足是制约其工程应用的主要障碍之一。激光熔化沉积（LMD）技术，有望获得组织细小均匀、成分近无偏析的合金，使其室温发生韧性断裂，从而改善合金韧性。.本项目通过激光熔化沉积+粉末原位反应的技术制备Nb-Si合金，系统研究了工艺参数，Si元素含量对Nb-Si二元合金显微组织特征及性能的影响。研究表明，LMD工艺对Nb-16Si二元合金的相组成无影响，合金由NbSS和Nb3Si两相组成。随激光功率由500 W增至1500 W，先共晶NbSS相由枝晶状逐渐转变为细小等轴状，NbSS/Nb3Si共晶组织形貌由细小的层片状共晶转变为不规则共晶，合金维氏硬度由605 HV提高至898 HV。随Nb-Si二元合金中Si元素含量由7 at.%增加至25 at.%，合金显微组织演变：NbSS+细小等轴状的Nb3Si→NbSS (P，初生相)+NbSS/Nb3Si (EU，共晶)基体→NbSS/Nb3Si (EU)→β-Nb5Si3 (P)+ NbSS/Nb3Si (EU)基体。.LMD制备的Nb-Si-Ti合金由NbSS和Nb3Si相组成。激光功率由1000 W增至2000 W，NbSS相由枝晶形貌转变为细小等轴状，Nb-Si-Ti合金的显微硬度值提高了约10%~15%，Nb-18Si-23Ti合金的室温断裂韧性值由12.3 MPa·m1/2提高至14.1 MPa·m1/2。随热处理温度和时间的增加，Nb-Si-Ti合金经历了元素扩散→组织粗化→Nb3Si共析反应的过程。.利用LMD技术制备了具有快速定向凝固特征的Nb-18Si-23Ti。Nb-18Si-23Ti合金中圆球状或短棒状NbSS和板条状Nb3Si平行于LMD生长方向定向排列。随激光功率由1500 W提高至2000 W，合金中板条状的Nb3Si和枝晶状的NbSS相较严格平行于生长方向，枝晶间距仅约1 μm。合金硬度由964 HV增至1072 HV，2000 W制备的Nb-18Si-23Ti合金的断裂韧性达20.3 MPa·m1/2。