Creep behavior of thin-cast single crystalline structures

为了进一步提高涡轮机的效率，人们迫切希望提高燃气进口温度，并将涡轮机部件设计得尽可能轻。通过插入冷却通道和丝状结构的设计，可以实现这些目标。然而，在这两种情况下，零件中产生的区域都是薄壁的。一些研究和其他文献发现，镍基高温合金的蠕变抗力随着截面尺寸的减小而降低。虽然在实际应用中，表面很少被加工或不能加工(中空结构)，但通常采用已加工表面的蠕变试件进行研究。为了预测镍基薄壁结构在蠕变变形下的耐久性，有必要知道从薄壁铸造未加工结构得到的结果是否与有已加工表面试件的结果相当。所提交的提案广泛地涵盖了这一专题，引出了三个中心问题：a)铸态试件的薄片尺寸对蠕变抗力的影响与机械加工试件相比有多大？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所不同？近区域表面的枝晶组织与铸件内部区域的枝晶组织是否存在差异？c)在凝固过程中，近区域表面的单晶薄壁组织中是否存在某些元素的聚集，从而对氧化行为和蠕变抗力产生积极或消极的影响？为了回答这些问题，在真空感应炉中用MAR M247合金浇注了单晶零件，最小壁厚为0.3 mm。蠕变试件由铸件制成，并在空气和Ar H2(减少大气影响)下进行测试。将所得结果与已加工表面试件的结果进行了比较。微观结构调查，特别是从靠近区域的表面进行调查，应揭示观察到的差异的原因。在这一点上，枝晶间距特别重要，因为0.2-0.5 mm的典型臂间距与所需的试件厚度相同。不同凝固速度的铸件将导致不同的枝晶间距，应揭示铸件的枝晶结构是否在靠近区域的表面发生变化，或者是否发生偏析对母体/伽马原始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.