Creep behavior of thin-cast single crystalline structures

为了进一步提高涡轮的效率，人们迫切希望提高燃气入口温度，并设计尽可能轻的涡轮部件。通过插入冷却通道和细丝结构的设计，可以实现这些目标。但是，使用这两个选项，部件中的区域将生成薄切片。自己的研究和其他文献发现表明，镍基高温合金的抗蠕变性能随着截面尺寸的减小而降低。通常研究是用蠕变试样进行加工表面，尽管在实际应用中，表面很少加工或不能加工（空心结构）。为了预测薄镍基组织在蠕变变形下的耐久性，有必要知道，从薄铸非机加工组织获得的结果是否与从机加工表面的试样获得的结果相比较。提交的提案广泛地涵盖了这一主题，导致了三个核心问题。a)与机加工试样相比，薄截面尺寸对铸造试样抗蠕变性能的影响有多大？哪些是最重要的影响因素？b)缩小截面尺寸的铸件结构有区别吗？表面附近区域的枝晶组织与铸件内部区域的枝晶组织有区别吗？c)在单晶薄结构凝固过程中，在表面邻近区域是否存在某些元素的富集，从而对氧化行为产生积极或消极的影响，从而对抗蠕变性能产生影响？为了回答这些问题，在金属和合金的椅子上用合金MAR M247在真空感应炉中铸造单晶零件，最小壁厚为0.3 mm。蠕变试样由铸件制成，并在空气和氩气（减少大气的影响）下进行测试。所得结果与加工表面试样的结果进行了比较。微观结构调查，特别是来自表面附近区域的调查，将揭示对观察到的差异的解释。在这个结合处，枝晶间距是特别有趣的，因为典型的臂间距为0.2 - 0.5毫米，与期望的试样厚度相同。不同凝固速度的铸件，导致不同的枝晶间距，应该揭示铸件的枝晶结构是否在表面附近区域发生变化，或者发生的偏析是否对基体/ γ初晶组织的形成和氧化行为产生影响。

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.