Lattice misfit during ageing of a polycrystalline nickel-base superalloy

The temporal evolution of the lattice parameters and lattice misfit of an advanced polycrystalline nickel-base superalloy have been studied in situ during an ageing heat treatment using synchrotron X-ray diffraction. During ageing, the gamma and gamma' lattice parameters were both observed to decrease, a trend that cannot be attributed to a loss of coherency alone. Phase-extracted gamma' replicated this behaviour. Atom probe tomography was used to measure the compositional changes between the start and end of the ageing heat treatment. Using these data, a thermodynamic assessment was made using the software ThermoCalc of the structural change across the interface between gamma and gamma'. Subsequently, the unconstrained lattice parameters were estimated and were shown to be in good agreement with the X-ray diffraction measurements. Thus, the observed anomalous lattice misfit behaviour was concluded to be dominated by elemental exchange between the gamma and gamma' phases during ageing. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.