【作者】 孙飞；

【导师】 张建新；

【作者基本信息】 山东大学 ， 材料学， 2014， 博士

【摘要】 镍基单晶高温合金具有优良的高温性能,是目前制造先进航空发动机和燃气轮机叶片的关键材料。随着镍基单晶高温合金的开发,通过添加大量的难熔元素来提高高温合金强度。然而,由于合金呈现很高的过饱和度,增大了拓扑密排相(TCP相)的析出倾向。TCP相的析出不仅消耗了大量的固溶强化元素,往往也作为裂纹的发源地和裂纹迅速扩展的通道,导致单晶高温合金的持久寿命降低,塑性和韧性明显恶化,严重地影响了合金的高温力学性能。第四代镍基单晶高温合金中加入了铂族元素Ru,能明显抑制TCP相的析出,进而提高单晶合金的高温蠕变性能,但其作用机制尚不清楚。有关Ru元素对镍基单晶高温合金中基体与TCP相界面结构及其合金元素赋存状态的研究还非常有限。本文采用电子显微学分析方法研究了镍基单晶高温合金蠕变断裂之后γ/γ’界面的精细结构以及合金元素的分布行为,并探讨了Ru元素对基体与TCP相界面结构及合金元素分布行为的影响规律。通过第一性原理方法研究了合金元素在γ/γ’界面、γ’相和基体/TCP界面的分配行为及择优占位倾向。(1)1100℃/137MPa蠕变实验研究表明：合金元素在γ/γ’界面两侧的分布趋势很明显：Co、Re和Cr元素容易富集在Y相中,而Ni、Al、Ta和W元素较容易向丫’相中偏析。在γ/γ’界面形成的扭结结构的尖端存在2-3nm的Re原子团簇。蠕变过程中,这种扭结结构的形成是由于γ/γ’界面附近的重金属原子团簇和界面位错芯部结构的交互作用。γ相和丫’相以共格的台阶形式相结合。在高温和外加应力的作用下,丫’相发生筏排化,γ/γ’相界面处形成了六角形和四边形的位错网。(2)1100℃/800h热暴露测试研究表明：在不含Ru元素合金1中,TCP相呈细长的针状,基体/TCP相界面结构呈不规律的高度不一的台阶状；在含Ru元素合金2中,TCP相呈短棒状,基体/TCP相界面结构呈规律的两个原子层高度的台阶状。Ru元素的添加,改变了合金中其他合金元素在基体和TCP相间的分布行为,减少了TCP相的析出数量,也改变了基体/TCP相界面的精细结构以及TCP相的形貌。(3)采用第一性原理方法研究了Ru元素在γ/γ’界面和γ’相中对其他合金元素分配行为的影响规律,揭示了Ru元素与Re元素之间的强化机制是由于Re原子和Ru原子间p-p轨道的杂化而产生较强的相互作用。(4)采用第一性原理方法研究了Re原子易于向TCP相偏聚的微观机制。由于Re原子在6相中特殊的键合特征以及6相独特的晶体结构,Re原子倾向于占据6相中非密排面上的W原子。更多还原

【Abstract】 The superior high-temperature properties of Ni-based single crystal superalloys make them the best candidates for use in jet engines and for the blades in aerospace turbine engines. To improve the high-temperature strength, solid solution strengthening elements have been added to create advanced-generation superalloys. However, when Ni-based superalloys are composed of excessive amounts of such refractory elements, instabilities are promoted. These instabilities can lead to the formation of deleterious topologically close-packed (TCP) phases as a result of the depletion of the solid solution strengthening elements when the superalloy is exposed to elevated temperatures. Furthermore, these TCP phases can serve as damage accumulation sites that initiate or accelerate crack propagation, which ultimately cause the creep strength of the material to decrease. The platinum group element Ru plays as a symbol element in the fourth-and fifth-generation, Ni-based single crystal superalloys. It is known that the fundamental mechanisms by which Ru additions are able to suppress the formation of TCP phases and improve the high-temperature phase stability of Ni-based superalloys are still controversial. There have been only a few detailed reports describing the influence of Ru on the TCP/matrix atomic interfacial structure in single-crystal Ni-based superalloys. In this study, the γ/γ’ interfacial structure at the atomic scale and related distribution characteristics of alloying elements have been investigated by high-resolution transmission electron microscopy. The influence of Ru on the TCP/matrix interfacial structure and the related redistribution behaviors of alloying elements have been analyzed. In addition, the site preference and partition behaviors of alloying elements have also been investigated by the first principle calculation.(1) The experimental results of the investigated superalloys after creep rupture test at1100℃/137MPa indicate that the distribution behaviors of alloying elements at both sides of the γ/γ’ interface are obvious:Ni, Al, Ta and W mainly partition into the γ’ phase, while Cr, Co and Re mainly partition into the y phase. Local clustering of Re (clusters～2-3nm in size) appears at the apexes of the kink structure. During creep, the formation of the kink structure results from the interactions of heavy atoms clusters and the core structure of the interfacial dislocation. An atomic structural model of the coherent γ/γ’interface includes a step. During high temperature low stress creep of single crystal superalloys, y’rafting occurs and the γ/γ’ interfacial dislocation networks display six-fold and four-fold symmetry.(2) The study of microstructural stability during long-term thermal exposure at1100℃/800h indicates that the TCP phase in Ru-free alloy has the typical long, slender needles shaped morphology and the corresponding TCP/matrix interfacial structure exhibits irregular step-shaped with overlapping minor and major steps. However, with the addition of Ru, the morphology of TCP phases changes to short, board-plate-shaped, and the corresponding TCP/matrix interfacial structure represents regular two-atomic-layer step shape. The addition of Ru changes the distribution behaviors of alloying elements between the matrix and TCP phases, suppresses the precipitation of TCP phases and changes TCP morphology as well as the related TCP/matrix interfacial structure.(3) The influence of Ru on the site preference of alloying elements at y/y’ interface and γ’ phase has been investigated using the first-principles method based on the density functional theory. The strength mechanism between Ru and Re have been clarified that there is a strong interaction between Re atom and Ru atom through p-p hybridization.(4) The micro-mechanisms of Re atoms preferentially partitioning to TCP phase has been investigated using the first-principles method. Re is prone to occupy the W site in a less close-packed plane of the σ phase due to the bonding characteristics and interstitial spaces in the crystal structure of σ phase.更多还原