Directional solidification of multiphase metallic high temperature materials beyond Ni-base superalloys

该项目的主要目标是确定定向凝固（DS）对熔点远高于镍基高温合金（例如Nb-Si-Cr和Mo-Si-Ti）的合金系统的机械性能的影响。一旦使用参考二元共晶合金Nb-18at%Si的区域熔化装置投入使用，所获得的知识就转移到三相系统Nb-Si-Cr和两相系统Mo-Si-Ti。获得了凝固参数和所得显微组织之间的直接相关性。关于高温蠕变性能，与粉末冶金生产的合金相比，Nb-18原子% Si的DS导致显著增加的抗蠕变性（好约三个数量级）。这种差异可以主要归因于不同尺寸的微观结构特征，在两种材料中的主要机制是扩散蠕变。与现有技术的单晶镍基高温合金（CMSX 4）的比较证实了DS NbSi共晶的抗蠕变性的显著改善。定向凝固Nb-Si-Cr三元合金的首次蠕变实验结果表明位错蠕变是速率控制机制。所观察到的应力指数随着应力的降低而增加，可以通过形成纳米级NbCr 2沉淀物来合理化。出乎意料的是，电弧熔化的参考合金表现出上级抗蠕变性。在该项目的第二阶段，相场模拟将继续在3D中的二元Nb-Si，涉及来自原子模型的各向异性界面能。通过模拟研究，获得了三维层状结构的空间组成的理解和潜在的成核事件的影响进行了分析。与此同时，将开发一个更深入的了解在三元系统Nb-Si-Cr中观察到的蠕变行为。此外，韧脆转变温度（DBTT）和氧化性能的测定是本项目的主要任务。除了三元共晶点的实验识别，相场模拟将扩展到三元Nb-Si-Cr系统。对于基于Pandat数据和实验结果的两个热力学模型，将进行3D微观结构模拟。相排列和微观结构参数将进行比较评估，并对形态进行分类。根据自己的文献数据，合金元素钒将被添加到Nb-Si-Cr系统中，因为它可能提高断裂韧性和抗氧化性。在Mo-Si-Ti系统中，必须确定关于定向凝固可能性的最佳组成。最后，将进行选定的蠕变试验，以比较所产生的微观结构的影响。

Major goal of this project is to determine the influence of directional solidification (DS) on the mechanical properties of alloy systems exhibiting melting points far beyond Nickel based superalloys (e.g. Nb-Si-Cr and Mo-Si-Ti). As soon as the zone melting device had been put into service by using the reference binary eutectic alloy Nb-18 at% Si, the obtained knowledge was transferred to the three-phase system Nb-Si-Cr and to the two-phase system Mo-Si-Ti. A direct correlation between solidification parameters and the resulting microstructure was obtained. Regarding the high temperature creep properties, DS of Nb-18 at% Si led to a significantly increased creep resistance (about three orders of magnitude better), as compared to a powder metallurgically produced alloy. This difference can mainly be attributed to the different size of microstructural features, the predominant mechanism being diffusional creep in both materials. A comparison with a state of the art single-crystalline Nickel based superalloys (CMSX 4) underpins the outstanding improvement of the creep resistance of DS NbSi eutectics. First creep results for DS ternary Nb-Si-Cr alloys indicate dislocation creep to be the rate controlling mechanism. The observed increase of the stress exponent with decreasing stress can be rationalized by the formation of nanoscaled NbCr2 precipitates. Unexpectedly, the arc melted reference alloy, exhibits superior creep resistance. In the second phase of the project, phase field simulations will be continued at the binary Nb-Si in 3D, involving anisotropic interfacial energies derived from atomistic models. Through simulation studies, an understanding of the spatial composition of the lamellar structure is obtained in 3D and the influence of potential nucleation events is analyzed. In parallel, a deeper understanding of the creep behavior observed in the ternary system Nb-Si-Cr will be developed. Also, the determination of the ductile-brittle transition temperature (DBTT) and the oxidation properties is a major task of this project. Besides experimental identification of the ternary-eutectic point, the phase field modelling will be extended to the ternary Nb-Si-Cr system. For two thermodynamic models based on Pandat data and on experimental results, 3D microstructure simulations will be performed. Phase arrangements and microstructural parameters will be comparatively assessed, and the morphologies will be categorized. Based on own literature data, the alloying element Vanadium will be added to the Nb-Si-Cr system, as it potentially improves fracture toughness and oxidation resistance. In the system Mo-Si-Ti an optimum composition regarding the possibility of directional solidification, has to be identified. Finally, selected creep tests will be performed to compare the influence of the generated microstructures.