铀及铀合金是重要的核原料，目前已广泛地应用在核电站核燃料组件、医学放射治疗和诊断以及核动力航母等多个领域。但铀元素具有放射性，且其相变行为较为复杂，这使得通过实验手段对铀及铀合金进行研究具有较大的局限性，因此计算模拟自然就成为了重要的手段。金属铀在固态时存在α、β、γ三种相，其中γ相是体心立方结构，具有高度对称性并且综合性能最为优异。但γ相在常温下不稳定，通常通过掺入一些合金元素比如铌元素来获得稳定的结构。实践已经证明，铌元素的掺入可以显著提高铀的抗氧化、抗腐蚀、抗辐照性能与力学性能。因而γ相铀铌合金已成为核工程中重要的结构材料，而其作为结构材料使用时，对加工和服役性能都提出了很高的要求，故提高其综合力学性能成为非常重要的研究目标。因此，本文首次采用计算模拟的方法，根据一般体心立方结构可能存在的滑移系和孪生系，分别对γ相铀铌合金的滑移/孪生变形、孪生变形/马氏体相变竞争机制展开研究，以期为铀和铀铌合金的研究和应用提供有意义的参考。在滑移/孪生变形开动行为的研究中，本文采用第一性原理计算和分子静力学方法计算了γ相铀铌合金的广义平面层错能（GSFE）和广义堆垛层错能（GPSE）。随后通过改进的P-N模型结合GSFE确定了位错起动的派纳力，进而对不同滑移系开动的难易程度进行判定，同时结合施密特定律确定其加载各向异性；通过比较GSFE和GPSE得到滑移与孪生变形开动的关系。结果表明在γ相U-15at.%Nb中，单晶体全取向拉压加载下变形均为{110}?111〉滑移系优先开动，并且有一定可能性会在{112}?111⟩系统中形成孪晶，且合金越靠近相变成分点，这种可能性就越大。在孪生变形/马氏体相变竞争机制的研究中，本文通过动力学模拟方法分别沿[110]和[100]方向拉伸γ相U-15at.%Nb合金，发现沿着[100]方向进行拉伸几乎不发生马氏体相变，而沿着[110]方向进行拉伸会发生较为严重的马氏体相变生成综合性能更差的α,,相。随后利用晶体在拉压加载条件下的外形变化，分析了{112}?111⟩系统的孪生变形与马氏体相变之间的竞争及其潜在机制，结果表明可以将孪生变形/马氏体相变进行抗拉/压分区，在对γ相铀合金加工的过程中，应该选择有利于孪生发生而抑制马氏体发生相变的取向，从而在塑性变形后保留更多的γ相合金。
Uranium and uranium alloys are important nuclear raw fuels, which have been widely used in nuclear fuel assemblies, nuclear power plants and so on. However, uranium is radioactive, and its phase transition behavior is relatively complex, which makes the research on uranium and uranium alloys by experimental means have lots of limitations. Therefore, computational simulation has naturally become an important means.Metal uranium has three phases, α, β, and γ in solid state. Among them, the γ phase is a body-centered cubic structure with high symmetry and has the best comprehensive performance. However, the γ phase is unstable at room temperature, and a stable structure is usually obtained by incorporating some alloying elements such as niobium. Practice has proved that the incorporation of niobium can significantly improve the oxidation resistance, corrosion resistance, radiation resistance and mechanical properties of uranium. Therefore, γ-phase uranium-niobium alloy has become an important structural material in nuclear engineering, and when it is used as a structural material, it has put forward high requirements for processing and service performance, so improving its comprehensive mechanical properties has become a very important research goal. Therefore, in this paper, the method of computational simulation is used for the first time, according to the possible existence of slip systems and twin systems in the general body-centered cubic structure, to study the competitive mechanisms of slip/twin deformation and twin deformation/martensitic transformation of γ-phase uranium-niobium alloy. The research is carried out in the hope that this paper can provide a meaningful reference for the research and application of uranium and uranium niobium alloys.In the study of the slip/twin deformation start-up behavior, the generalized stacking fault energy(GSFE) and generalized plane fault energy(GPSE) were calculated by first-principles calculations and molecular statics methods. Then, the Peierls-Nabarro stress of dislocation initiation was determined by the improved P-N model, and then the difficulty of initiation of different slip systems was determined, and the loading anisotropy was determined in combination with Schmid factors. By comparing GSFE and GPSE, the relationship between slip and twinning deformation actuation is obtained. The results show that in the γ phase U-15at.%Nb, all oriented deformations are preferentially activated by the {110}?111〉 slip system, and there is a certain possibility that twins will be formed in the {112}?111⟩ system, and the closer the alloy is to the transformation point, the more likely it is.In the study of the competitive mechanism of twinning deformation/martensitic transformation, the γ phase U-15at.%Nb was stretched along the [110] and [100] directions by dynamic simulation method. It is found that stretching along the [100] direction hardly occurs martensitic transformation, while stretching along the [110] direction will cause more serious martensitic transformation to form α,, phase with worse comprehensive properties. Then, the competition between twinning deformation and martensitic transformation of the {112}?111⟩ system and its potential mechanism were analyzed by using the shape change of the crystal under tension and compression loading. The results show that the twinning deformation/martensitic transformation can be divided into tensile/compression zones. In the process of processing γ phase uranium alloys, the orientation that is conducive to twinning and inhibits martensite transformation should be selected, so as to ensure that the More γ phase alloys remain after plastic deformation.
