Microstructure and creep properties of dendritically solidified nickelbase alloys over a wide range of cooling rates

壁厚低于1毫米的中空涡轮叶片可提高效率、减轻重量并节省原材料。目前，叶片后缘的壁厚低至0.2毫米。在项目的第一阶段，建立了一条工艺路线，用于生产镍基合金的单晶样品，其壁厚低至0.4 mm。观察到初生枝晶间距依赖于铸件的几何形状，以及明显的枝晶间距方向依赖。该项目的第二阶段的目标是通过实验工作和多尺度模拟链的结合来研究和理解薄壁单晶的枝晶凝固。此外，还研究了枝晶偏析对合金在980℃时蠕变性能的影响。请两名研究助理，每人36个月来执行这个项目。在Bridgman工艺中，单晶薄壁（最大0.4 mm）和圆柱形样品（直径15 mm）将以不同的提取速率生长。树突间距要小于100µm ~大于1000µm。通过对铸造过程的有限元模拟，可以确定部件内部的局部温度分布以及凝固区域的形状。这些参数决定了树突的局部分布。用光学、扫描和偶尔用透射电子显微镜以及能量色散x射线能谱测定了偏析的微观结构。基于模拟温度场的相场研究将用于描述微观组织的形成。比较了实验结果和模拟结果中元素的枝晶间距和偏析系数。在现有枝晶间距形成知识的基础上，建立非均匀温度分布模型。蠕变试样将由铸造试样制成，将在980°C真空下进行测试。研究不同枝晶间距的残余偏析对合金蠕变行为的影响。局部位错密度的选择性透射电镜研究将阐明枝晶和枝晶间蠕变性能的差异。铸造试样的蠕变特性将通过蠕变试验的有限元模拟来解释。为此，枝晶和枝晶间区域的局部蠕变特性通过改变现象学蠕变模型的参数来近似。通过实验确定的局部位错密度，模拟验证了枝晶和枝晶间组织的局部变形。

Hollow turbine blades with wall thicknesses below 1 mm lead to an increase in efficiency, weight and raw material savings. Currently, wall thicknesses down to 0.2 mm are used at the trailing edges of the blades. In the first phase of the project, a process route was established for the production of single-crystalline samples from a nickel-based alloy with wall thicknesses down to 0.4 mm. A dependence of the primary dendrite spacing on the geometry of the castings was observed, as well as a pronounced directional dependence of the dendrite spacing. The goal of the second phase of the project is to investigate and understand the dendritic solidification of thin-walled single crystals through a combination of experimental work and a multiscale simulation chain. In addition, the effect of dendritic segregation on creep properties at 980°C will be investigated. Two research assistants are requested for 36 months each to carry out this project. In the Bridgman process, single-crystalline thin-walled (up to 0.4 mm) and cylindrical samples (diameter 15 mm) will be grown at different withdrawal rates. Dendrite spacings of less than 100 µm to greater than 1000 µm are to be achieved. By accompanying finite element simulations of the casting process, the local temperature distribution in the component, as well as the shape of the solidification zone will be determined. These parameters determine the local distribution of the dendrites. The segregation microstructure is determined experimentally by optical, scanning and occasionally transmission electron microscopy, as well as by energy dispersive X-ray spectroscopy. Phase field studies, based on the simulated temperature fields, will be used to describe the microstructure formation. The dendrite spacing and the segregation coefficients of the elements from experiment and simulation will be compared. Based on existing knowledge about the formation of dendrite spacing, a model for inhomogeneous temperature distributions will be established. Creep specimens will be fabricated from the cast samples, which will be tested at 980°C under vacuum. The effect of residual segregations with different dendrite spacing on the creep behavior of the alloys will be investigated. Selective transmission electron microscopy studies of the local dislocation density will clarify how the creep properties differ between the dendritic and interdendritic regions. The creep properties of the cast specimens will be explained by finite element simulations of the creep tests. For this purpose, the local creep properties of dendritic and interdendritic region are approximated by varying the parameters of a phenomenological creep model. The local deformation of the microstructure of dendrite and interdendritic region is validated in the simulation with the experimentally determined local dislocation densities.