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

点击翻译按钮获取中文摘要

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.