Lattice misfit during ageing of a polycrystalline nickel-base superalloy D.M. Collins a,⇑ , L. Yan b , E.A. Marquis b , L.D. Connor c , J.J. Ciardiello d A.D. Evans e , H.J. Stone f a Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK b Department of Materials Science & Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA c Diamond Light Source Ltd., Harwell Science and Innovation Campus, Didcot OX11 0DE, UK d Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EQ, UK e Institut Laue-Langevin, BP 156, 6 rue Jules Horowitz, 38042 Grenoble Cedex 9, France f Rolls-Royce UTC, Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK Received 25 February 2013; received in revised form 13 July 2013; accepted 1 September 2013 Available online 1 October 2013 Abstract 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 c and c 0 lattice parameters were both observed to decrease, a trend that cannot be attributed to a loss of coherency alone. Phase-extracted c 0 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 c and c 0 . Subsequently, the unconstrained lattice parameters were estimated and were shown to be in good agreement with the X-ray dif- fraction measurements. Thus, the observed anomalous lattice misfit behaviour was concluded to be dominated by elemental exchange between the c and c 0 phases during ageing. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Misfit; Synchrotron radiation; X-ray diffraction (XRD); Nickel alloys; Coarsening 1. Introduction Nickel-base superalloys are capable of operating under high mechanical loads at elevated temperatures whilst simultaneously providing exceptional resistance to environ- mental degradation [1]. These alloys derive their strength from an A1, c matrix with a dispersion of L1 2 , c 0 precipi- tates. The size and distribution of these precipitates are carefully controlled during processing and subsequent heat treatments to optimize mechanical performance. To mini- mize changes in precipitate morphology and size, the com- position of the c and c 0 are tailored to control and limit their lattice misfit and hence maintain interfacial coher- ency. In particular, refractory elements, which are added to improve high-temperature properties have a strong influence on the lattice parameters [2,3] and are carefully balanced to provide an appropriate lattice misfit between the two constituent phases. In addition, the cooling rates following heat treatment affect the partitioning of elements between the c and the different c 0 distributions, where pres- ent, and must therefore also be carefully considered [4,5]. Modern nickel-base superalloys processed by powder metallurgy for gas turbine disc applications typically have a trimodal c 0 distribution. Primary c 0 is formed at the grain boundaries, with precipitate diameters between 1 and 5 lm. These precipitates pin grain boundaries, inhibiting bound- ary migration during subsolvus thermal exposures. In addi- tion, intragranular distributions of secondary and tertiary c 0 are present following precipitation during cooling. The size and distribution of these precipitates varies for differ- ent alloys, and can be controlled through processing to 1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.09.018 ⇑ Corresponding author. E-mail address: david.collins@materials.ox.ac.uk (D.M. Collins). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com ScienceDirect Acta Materialia 61 (2013) 7791–7804