Creep behavior of thin-cast single crystalline structures

为了进一步提高涡轮机的效率，人们迫切希望提高气体入口温度并将涡轮机部件设计得尽可能轻。通过插入冷却通道和细丝结构的设计，可以实现这些目标。然而，这两种选择会使零件中的区域产生薄截面。自己的研究和文献中的其他研究表明，镍基高温合金的抗蠕变性随着截面尺寸的减小而降低。尽管在真实的应用中，表面很少被加工或不能被加工（中空结构），但一般的研究都是用具有加工表面的蠕变试样进行的。为了预测薄镍基结构在蠕变变形下的耐久性，必须知道，从薄铸造非加工结构获得的结果是否与从具有加工表面的试样获得的结果相当。所提交的提案广泛涉及这一专题，并引出三个核心问题。 与机加工试样相比，薄截面尺寸对铸态试样抗蠕变性的影响有多大？哪些是最重要的影响因素？B） 缩小截面尺寸的铸件结构是否有差异？与铸件内部区域相比，表面附近区域的枝晶结构是否存在差异？c）、 在单晶薄结构的凝固过程中，在表面附近区域中是否存在某些元素的富集，这可能会对氧化行为产生积极或消极的影响，从而影响抗蠕变性？为了回答这些问题，单晶零件铸造与合金MAR M247在真空感应炉在椅子上的金属和合金的最小壁厚为0.3毫米。蠕变试样制成的铸造零件，并在空气和Ar H2（减少大气的影响）下进行测试。所得到的结果，与加工表面的试样的结果进行了比较。微观结构调查，特别是从表面附近区域进行的调查，应揭示观察到的差异的解释。此时，枝晶间距特别重要，因为0.2 - 0.5 mm的典型臂间距与所需试样厚度相同。具有不同凝固速度的铸件，导致不同的枝晶间距，应显示铸件的枝晶结构是否在表面附近区域发生变化，或者如果发生的偏析对基体/γ ′微结构的形成和氧化行为有影响。

To increase the efficiency of turbines further, one is anxious to increase the gas inlet temperature and to design turbine components as light as possible. By inserting cooling channels and the design of filigree structures it is possible to fulfill these aims. With these two options however, regions in the part are generated with thin sections.Own investigations and others found in literature show that the creep resistance of nickel-based superalloys decreases with reduced section size. Generally investigations were performed with creep specimens with machined surface, although in real applications, surfaces are rarely machined or can not be machined (hollow structures).To forecast the durability of thin nickel-based structures under creep deformation, it is essential to know, if the results attained from thin cast non-machined structures are comparable with results attained from specimens with machined surface. The submitted proposal which covers the topic extensively leads to three central questions.a) How big is the influence of thin section sizes on the creep resistance for as cast specimens compared to machined specimens? Which are the most important influencing factors? b) Is there a difference in the casting structure of reduced section sizes? Is there a difference in the dendrite structure in the surface near regions compared to regions inside the cast part?c) Is there an enrichment of certain elements during the solidification of the single crystal thin structures in the surface near regions, which could influence the oxidation behavior and thereby the creep resistance positively or negatively?To answer these questions, single crystal parts are cast with alloy MAR M247 in the vacuum induction furnace at the chair of Metals and Alloys with a minimal wall thickness of 0.3 mm. Creep specimens are made from the cast parts, and tested under air and Ar H2 (reduction of the influence of the atmosphere). The attained results, are compared with results of specimens with machined surface. Microstructural investigations, specifically from the surface near regions shall reveal explanations for observed differences. At this juncture the dendrite spacing is of particular interest, as the typical arm spacing of 0.2 - 0.5 mm is in the same magnitude as the desired specimen thickness.Castings with varying solidification velocities, leading to varying dendrite spacings, shall reveal whether the casting respectively dendrite structure is changing in the surface near region, or if occurring segregations have consequences for the formation of the matrix/gamma prime microstructure and the oxidation behavior.