【作者】 方欣；

【导师】 郭喜平；

【作者基本信息】 西北工业大学 ， 材料学， 2021， 博士

【摘要】 Nb-Si基超高温合金具有密度低、熔点高以及高温力学性能优良等特点,有望成为下一代应用于航空发动机涡轮叶片的高温结构材料。但是其较低的室温断裂韧性以及较差的高温抗氧化性能阻碍了其在航空工业上的应用。使用定向凝固技术可以有效改善Nb-Si基超高温合金的微观组织,从而提高合金的室温以及高温力学性能。系统研究Nb-Si基超高温合金定向凝固组织形貌演化规律有助于优化Nb-Si基超高温合金成分设计以及优选定向凝固工艺参数,从而进一步改善合金的综合性能。目前有关Nb-Si基超高温合金定向凝固组织的研究中,通常一次实验只能针对合金的一个成分或者一个工艺参数进行研究,实验周期长,成本高。发展Nb-Si基超高温合金高通量定向凝固制备技术可以有效提高实验效率并且节省成本,加速对Nb-Si基超高温合金定向凝固工艺参数的优化。研究了在抽拉速率100-1000μm/s范围内Nb-Si基超高温合金定向凝固组织的演化规律。当抽拉速率为100-300μm/s时,合金定向凝固的固/液界面为胞状形貌,定向凝固组织的排列方向平行于抽拉方向,Nb_SS/γ-（Nb,X）₅Si₃共晶形貌为生长耦合度高的层片状;当抽拉速率为500-1000μm/s时,合金定向凝固的固/液界面为树枝状形貌,凝固组织发散生长,Nb_SS/γ-（Nb,X）₅Si₃共晶形貌为生长耦合度低的不规则棒状。此外,当抽拉速率在100-300μm/s范围内时,随着抽拉速率的提高,Ti和Cr在共晶边缘的偏析程度增大;当抽拉速率为500-1000μm/s时,随着抽拉速率的提高,Ti和Cr在共晶边缘的偏析程度降低。当抽拉速率为100μm/s时,定向凝固层片状共晶中Nb_SS与γ-（Nb,X）₅Si₃沿着试样竖直轴向的生长取向分别为<111>_Nb和<0001>_γ晶向,两相的晶体学取向关系为<111>_Nb//<0001>_γ和{110}_Nb//{10^<sub>10}_γ;当抽拉速率超过500μm/s时,在不规则棒状共晶中,Nb_SS沿着试样竖直轴向的生长取向向<100>_Nb晶向偏移,γ-（Nb,X）₅Si₃沿着试样竖直轴向的生长取向同样偏离<0001>_γ晶向,两相之间不存在晶体学取向关系。在定向凝固过程中进行跃迁变速抽拉,可在一根试样中获得不同抽拉速率下的定向凝固组织,并可揭示跃迁变速抽拉过程中定向凝固组织的演化过程。在Nb-Si基超高温合金跃迁加速抽拉定向凝固实验中,抽拉速率从10μm/s分别跃迁提高到50、100和200μm/s;在跃迁减速抽拉定向凝固实验中,抽拉速率分别从50、100和200μm/s跃迁降低到10μm/s。对比了跃迁变速抽拉后Nb-Si基超高温合金定向凝固组织与相对应的恒定抽拉速率下定向凝固组织的区别。跃迁加速抽拉后,Nb-Si基超高温合金定向凝固组织逐渐细化;而跃迁减速抽拉后,Nb-Si基超高温合金定向凝固组织逐渐粗化。随着跃迁率的提高,跃迁变速抽拉后Nb-Si基超高温合金定向凝固组织达到稳定组织所需的生长过渡区变长。在跃迁变速抽拉过程中,组织的转变是从共晶胞边缘开始的。在跃迁加速抽拉定向凝固过程中,γ-（Nb,X）₅Si₃片在共晶胞边缘重新形核或分枝,使共晶胞边缘组织细化,并且沿着试样竖直轴向规则排列;在跃迁减速抽拉定向凝固中,Nb_SS片在共晶胞边缘合并粗化。随着跃迁率的提高,初生γ-（Nb,X）₅Si₃的体积分数及其平均尺寸以及共晶胞平均尺寸达到其稳定值所需的凝固距离增加,而共晶胞中γ-（Nb,X）₅Si₃的体积分数和共晶胞中平均相间距达到其稳定值所需的凝固距离则比较接近。跃迁加速抽拉后,定向凝固组织形貌特征与对应恒定抽拉速率下定向凝固的非常接近,但是Nb-Si基超高温合金在跃迁减速抽拉定向凝固后却改善了低抽拉速率下定向凝固组织的生长耦合度以及排列的规整性。通过多试样同时定向凝固技术,制备了四根不同Ti含量的Nb-Si基超高温合金棒,并揭示了Ti含量对Nb-Si基超高温合金定向凝固组织的影响规律。当Ti含量为0和10at.%时,Nb-Si基超高温合金定向凝固组织的主要组成相为Nb_SS和α-（Nb,X）₅Si₃;当Ti含量为20 at.%时,合金的主要组成相为Nb_SS、α-（Nb,X）₅Si₃和γ-（Nb,X）₅Si₃;当Ti含量为25 at.%时,合金的主要组成相为Nb_SS和γ-（Nb,X）₅Si₃。随着Ti含量的提高,Nb-Si基超高温合金定向凝固共晶胞发生细化。当Ti含量为0和10 at.%时,Nb_SS/α-（Nb,X）₅Si₃共晶中Nb_SS和α-（Nb,X）₅Si₃沿着试棒轴向的生长取向分别为[001]_Nb和[001]_α晶向,两相的晶体学取向关系为[001]_Nb∥[001]_α和（110）_Nb∥（310）_α;当Ti含量为20 at.%时,Nb _SS/α-（Nb,X）₅Si₃共晶中Nb_SS和α-（Nb,X）₅Si₃的生长取向以及晶体学取向关系与0Ti以及10Ti合金定向凝固组织的相同,但是在Nb_SS/γ-（Nb,X）₅Si₃共晶中,两相之间未发现晶体学取向关系;当Ti含量为25 at.%时,Nb_SS/γ-（Nb,X）₅Si₃共晶中Nb_SS和γ-（Nb,X）₅Si₃沿着试棒竖直轴向的生长取向分_<sub>别为<111>_Nb和<0001>_γ晶向,两相的晶体学取向关系为<111>_Nb//<0001>_γ和{110}_Nb//{1010}_γ。更多还原

【Abstract】 Nb-Si based ultrahigh temperature alloys possess low densities,high melting point temperature and good high temperature mechanical properties,which are expected to be employed as the next generation high temperature structural materials in aviation turbine engine blades.However,their poor room temperature fracture toughness and poor high temperature oxidation resistance restrict their application in aviation industries.Directional solidification process can modify the microstructures and improve their room and high temperature mechanical properties of Nb-Si based ultrahigh temperature alloys.The systematic researches on microstructural evolution during directional solidification process can help to optimize alloy compositions and processing parameters of Nb-Si based ultrahigh temperature alloys,which can further improve the comprehensive properties of Nb-Si based ultrahigh temperature alloys.Besides,for the present experimental method of directional solidification,only one composition or one technological parameter of the directionally solidified of Nb-Si based ultrahigh temperature alloys can be investigated in one experiment.It costs much time and recourse.Developing high-throughput directional solidification methods can improve the efficiency and decrease the cost of directional solidification experiments of Nb-Si based ultahigh temperature alloys,which can accelerate the research of directional solidification process for Nb-Si based ultrahigh temperature alloys.The microstructural evolution of Nb-Si based ultrahigh temperature alloys directionally solidified at the withdrawal rates from 100 to 1000μm/s was studied.When the withdrawal rates range from 100 to 300μm/s,the solid/liquid（S/L）interfaces of the directionally solidified alloys present cellular morphology.The microstructures are well parallel to the withdrawal direction,and Nb _SS/γ-（Nb,X）₅Si₃ eutectics present lamellar morphology with high coupling growth degree.When the withdrawal rates range from 500 to 1000μm/s,the S/L interfaces present divergent dendrite morphology.The morphology of Nb _SS/γ-（Nb,X）₅Si₃eutectics transform into irregular rod-like with low coupling growth degree.Besides,when the withdrawal rates range from 100 to 300μm/s,the segregation degree of Ti and Cr in eutectic cellular/dendrite boundary increases with increasing withdrawal rate,while it decreases when the withdrawal rate exceed 500μm/s.Moreover,at the withdrawal rate of 100μm/s,the growth directions are along the longitudinal axis of specimens,and the crystallographic orientations of Nb _SS andγ-（Nb,X）₅Si₃ phases in lamellar eutectic are<111>_Nb and<0001>_γrespectively.Yhe crystallographic orientation relationship is<111>_Nb//<0001>_γand{110}_Nb//{10^<sub>10}_γ.When the withdrawal rate reaches to 500μm/s,the growth direction of Nb _SS deflects to<100>_Nb and the growth direction ofγ-（Nb,X）₅Si₃ also deflects off<0001>_γin rod-like eutectic,and there is no crystallographic orientation relationship between the two phases in rod-like eutectic.The directionally solidified microstructures at different withdrawal rates were carried out by abruptly changing withdrawal rates,and the microstructural evolution after abruptly changing withdrawal rates was studied for Nb-Si based ultrahigh temperature alloys.In the directional solidification experiments of abruptly increasing withdrawal rate,the withdrawal rates were abruptly increased from 10 to 50,100 and 200μm/s respectively.In the directional solidification experiments of abruptly decreasing withdrawal rates,the withdrawal rates were abruptly decreased from 50,100 and 200 to 10μm/s respectively.The comparison of microstructures directionally solidified by abruptly changing withdrawal rates with those at corresponding constant withdrawal rates were also investigated.After abruptly increasing withdrawal rates,the directionally solidified microstructures refine gradually.After abruptly decreasing withdrawal rates,the directionally solidified microstructures coarsen gradually.The length of transition zone before the directionally solidified microstructures reach to steady state increases with increasing abruptly changing ratios.The microstructural evolution of eutectic cells after abruptly changing withdrawal rates begin at the cellular boundary regions firstly.After abruptly increasing withdrawal rates,the refined and alignedγ-（Nb,X）₅Si₃ lamellae nucleate and branch at cellular boundaries.After abruptly decreasing withdrawal rates,Nb _SS lamellae are emerged and coarsened at cellular boundaries.With increasing abruptly changing ratios,the solidification distances for volume fraction and average size of primaryγ-（Nb,X）₅Si₃ and average size of eutectic cells reaching their steady values increase,while the distances for volume fraction ofγ-（Nb,X）₅Si₃ and average interphase spacing in eutectic cells are not changed significantly.Moreover,the microstructural characteristics of Nb-Si based ultrahigh temperature alloys directionally solidified upon abruptly increasing withdrawal rates and at corresponding constant withdrawal rates are similar with each other.However,the coupling growth degree and arrangement of the microstructures of Nb-Si based ultrahigh temperature alloys directionally solidified upon abruptly decreasing withdrawal rates are modified compared with that at the corresponding low withdrawal rate.The high throughput directional solidification of Nb-Si based ultrahigh temperature alloys with four Ti contents were carried out by multi-specimen simultaneously withdrawing pocess,and the effect of Ti contents on microstructural evolution of directionally solidified Nb-Si based alloys were studied.When Ti contents are 0 and 10 at.%,the main constituent phases are Nb _SS andα-（Nb,X）₅Si₃;when Ti content is 20 at.%,the main constituent phases are Nb _SS,α-（Nb,X）₅Si₃ andγ-（Nb,X）₅Si₃;when Ti content is 25 at.%,the main constituent phases are Nb _SS andγ-（Nb,X）₅Si₃.With increasing Ti contents,the microstructures of Nb-Si based ultrahigh temperature alloys are refined and the morphologies of eutectics become more regular.When Ti contents are 0 and 10 at.%,the crystallographic orientation relationship between Nb _SS andα-（Nb,X）₅Si₃ is[001]_Nb∥[001]_αand（110）_Nb∥（310）_α.When Ti content is20 at.%,the crystallographic orientation relationship between Nb _SS andα-（Nb,X）₅Si₃ are similar with that in the alloy with 0 and 10 at.%Ti contents.However,no crystallographic orientation relationship between Nb _SS andγ-（Nb,X）₅Si₃ exists.When Ti content is 25 at.%,the crystallographic orientation relationship between Nb _SS andγ-（Nb,X）₅Si₃ is<111>_Nb//<0001>_γand{110}_Nb//{10^<sub>10}_γ.更多还原