Negative creep of nickel-based superalloys

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The phenomenon of "negative creep" is observed for nickel-based alloys at medium temperatures around 600°C. In this case a sample contracts by -0.04% to -0.07%. This effect occurs in both cases without any external load and at low stresses. In literature various explanations are found which are often highly specific and only valid for one special alloy or loading condition. In the proposed project, a new approach is being pursued. Hypotheses are formulated, which are estimated to account for the negative creep. Possible hypotheses are the carbon content (very slow carbide-formation or -conversion), grain coarsening, redistribution of elements in various phases or close arranged domains. For each of these hypotheses special alloys are melted, so that only that parameter is changed, which is investigated in the hypothesis. The effect of negative creep itself and microstructural changes in the model alloys are compared with the respective starting alloy. This results in four experimental tasks: melting of the model alloys; exposure tests with determination of the length-changes; creep of selected alloys; and intensive studies of the microstructure with the determination of composition with a high spatial resolution. The aim is to quantify the hypotheses and their respective impact on the negative creep or disprove if necessary.