Microstructural changes and negative creep of Inconel 783

Some materials used in structural components in aircraft turbines show a volume contraction in service. This phenomenon is often referred to as “Negative Creep”, however the mechanisms behind are not yet well understood. Inconel 783 is an oxidation-resistant low expansion nickel-cobalt-iron alloy and therefore is considered for applications with tight clearance such as rings, casings, shrouds and seals in aircraft turbine engines. The objective here is to find correlations between microstructure evolution and negative creep in Inconel 783. Samples of this alloy are isothermally annealed without external load for a long time at various temperatures. To measure dimensional changes high resolution length measurement is used. The lattice parameters of different annealing states are determined by X-ray diffraction. The phase morphology and stability and their influence on the lattice parameters as well as effects of oxidation are considered. Equilibrium phase fractions and compositions of matrix, γ’ and β phase are assessed by the calculation of phase diagram method (CALPHAD) is used.