Characterization of the Creep Behavior of a Nickelbase Superalloy under Non-Isothermal Conditions and Modification of the Creep Life by Means of High Current Density Impulses

本课题将研究一种镍基高温合金（CMSX-4）在非等温蠕变条件下的蠕变行为。此外，还分析了通过大电流脉冲处理来增加蠕变寿命的预处理或中间处理。初步研究表明，在循环变化的温度下对单晶镍基高温合金进行蠕变试验，其蠕变速率比在相同机械载荷下等温进行的试验要高。非等温蠕变试验中的蠕变损伤不能用随时间累积蠕变应变的线性损伤累加法准确预测。本研究项目的目的是对单晶CMSX-4进行非等温蠕变试验。通过对样品进行导电加热和主动冷却，产生高循环温度变化，分析温度变化频率和所选最高温度的影响。通过扫描电子显微镜、透射电子显微镜和x射线断层扫描，研究了导致循环热载荷下蠕变速率增加的微观结构机制。这些发现将用于描述基于非等温蠕变物理模型的蠕变寿命，考虑确定的微观结构损伤机制。此外，还将分析电流密度高达10 kA/mm2的大电流脉冲处理对材料抗蠕变性能的影响。随后，将研究这种处理可以在多大程度上提高已经蠕变损坏的样品的强度。初步研究表明，脉冲处理可以影响局部化学成分。这种效应将用于预处理，通过在相界面富集稳定元素来抑制增强γ′沉淀的降解。此外，对于已经蠕变损伤的条件，脉冲处理应该通过影响材料γ矩阵中的位错网络来进行强化。为此，将研究与电流密度和电流引入方向相关的微观结构机制。该研究项目将在能量转换动力学研究网络（德语：Dynamik der Energiewandlung， DEW）的框架内进行，为此，2019年9月在加布森校区开设了一座新的研究大楼“能量转换动力学”。在这座研究大楼里，来自不同学科的科学家们共同努力，开发能量转换过程、燃烧、传热、应用流体力学和电磁能转换的新概念，同时考虑到周期性的热机械负荷。

In this research project, the creep behavior of a nickel-based superalloy (CMSX-4) under non-isothermal creep conditions will be investigated. Additionally, a pre- or intermediate treatment by means of a high current impulse treatment to increase the creep life will be analyzed. Preliminary work showed that creep tests on a single-crystal nickel-based superalloy with cyclically changing temperature show an increased creep rate compared to tests carried out isothermally under the same mechanical load. The creep damage in the non-isothermal test cannot be predicted correctly from isothermal creep tests by linear damage accumulation by summing up the creep strains over time. The objective of this research project is to carry out non-isothermal creep tests on single crystalline CMSX-4. By conductive heating and active cooling of the samples high cycle temperature changes will be produced to analyze the influence of the temperature change frequency and the selected maximum temperature. The microstructural mechanisms that lead to an increase of the creep rate under cyclical thermal load will be investigated by means of scanning electron microscopy, transmission electron microscopy, and X-ray tomography. These findings will be employed to describe the creep life based on a physical model of non-isothermal creep, considering the determined microstructural damage mechanisms.In addition, the influence of a high current impulse treatment with a current density of up to 10 kA/mm2 on the creep resistance of the material will be analyzed. Subsequently, it will be invesitgated to what extent the strength of already creep-damaged samples can be increased by this treatment. In preliminary work, it was shown that the local chemical composition can be influenced by impulse treatment. This effect will be used for pretreatment in order to inhibit the degradation of the strength-increasing gamma′-precipitates by concentrating the gamma′-stabilizing elements at the phase’s interfaces. In addition, impulse treatment of already creep-damaged conditions is supposed to undergo strengthening by influencing the dislocation networks in the gamma matrix of the material. For this purpose, the microstructural mechanisms in dependence of the current density and the direction of the current introduction will be examined.The research project will be carried out within the framework of the research network Dynamics of Energy Conversion (German: Dynamik der Energiewandlung, DEW), for which a new research building "Dynamics of Energy Conversion" was opened at the Campus Garbsen in September 2019. In this research building, scientists from various disciplines work together to develop new concepts for energy conversion processes, combustion, heat transfer, applied fluid mechanics and electromagnetic energy conversion, taking into account cyclical thermomechanical loads.