铀铌合金低温时效机制的像差校正电子显微学研究

U-Nb alloy obtains various applications due to its high density, superior corrosion resistance and overall mechanical properties, however, the property of which would change significantly after aging. Because regular structural analysis approaches are unable to clearly evidence the subtle structural changes of U-Nb alloys after low temperature aging, it is up to now that the understandings of low temperature aging mechanism of U-Nb alloys are widely divided. In this project we will explore the low temperature aging mechanism of U-Nb alloys at the atomic scale by using a combination of aberration-corrected transmission electron microscopy and first-principles calculations. The evolutions of phases, typical features as well as distributions of niobium will be precisely demonstrated for alloys of as-quenched and aged under different conditions. Combined with first-principles energy calculations, the structural variation behavior of U-Nb alloy during aging will be elucidated. The relationship between the structure and property changes will eventually be clarified. The studies in this project would not only push the understanding of the low temperature aging mechanism of U-Nb alloys to the atomic scale, but also provide fundamental structural information for accurately evaluating the performances of U-Nb alloys after aging.

铀铌合金因具有高的密度，优良的抗腐蚀和综合力学性能而应用广泛，但是该合金会因时效而发生显著的性能改变。由于常规手段难以准确解析该合金低温时效过程中的细微结构变化，导致目前对于该合金低温时效机制的认识仍众说纷纭。本项目拟以先进的像差校正透射电子显微术为主要手段，结合第一原理计算，在原子尺度解析铀铌合金的低温时效机制。通过对淬火态和不同条件时效态铀铌合金的物相、特征结构以及Nb元素分布予以系统详细的解析，建立相应的结构演变图景；结合第一原理计算，理解其演变规律；最终，结合宏观性能阐明时效过程中结构与性能变化的对应关系。本项目的完成可将人们对铀铌合金低温时效机制的认识提升到原子尺度，也将为准确评估时效态铀铌合金的性能提供重要的科学依据。

铀铌合金拥有高的密度，优良的抗腐蚀和综合力学性能，因此应用广泛。但是该合金会因时效而发生显著的性能改变。由于常规手段难以准确解析该合金低温时效过程中的细微结构变化，目前对于其低温时效机制仍然认识不足。本项目利用先进的透射电子显微术在解析了淬火态和低温时效态铀铌合金的微观结构特点，及其随时效时间的演变规律，发现了铀铌合金低温时效结构变化的直接证据。发现低温时效导致U-5.7%Nb合金发生相变，从α″单相结构转变为α″、γ0和γlike三相结构。相变的直接实验证据由STEM高分辨原子像给出， α″、类γ相（γ0、γlike）原子排列规律差异巨大。产生TEM形貌像“明暗区”、“明暗条纹”以及XRD谱图37°新峰的原因是类γ相的出现。证明调幅分解与有序-无序转变的观点不正确，完善马氏体逆相变观点认识不足，提出发生纳米尺度类γ相变的观点。上述研究成果将铀铌合金性能评价与寿命预测提供微观结构基础。