The Effect of Cooling Rate from Solution on the Lattice Misfit During Isothermal Aging of a Ni-Base Superalloy
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HE excellent high temperature mechanical properties and environmental resistance of nickel-base superalloys have resulted in their widespread usage in structural applications at elevated temperatures. Their performance is derived from the presence of coherent, L12 c¢ precipitates that maintain a simple cube–cube orientation relationship with the A1 c matrix phase. The size, morphology, lattice parameters, and volume fractions of the precipitates strongly influence the properties of the alloy and must therefore be carefully controlled by judicious selection of alloy chemistry and processing parameters.[1–6] Polycrystalline nickel-base superalloys for gas turbine disc applications typically receive a two-stage heat treatment after the thermo-mechanical processing to obtain an optimal microstructure.[7,8] A solution heat treatment is given to obtain the desired c grain size[9]; this may be performed either above (super-solvus) or below (sub-solvus) the c¢ solvus temperature. For subL.D. CONNOR, Industrial Research Scientist, is with the Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, U.K. Contact e-mail: Leigh.Connor@Diamond. ac.uk H.J. STONE, Assistant Director of Research, and C.M.F. RAE, Reader in Physical Metallurgy, are with the Rolls-Royce UTC, Department of Materials Science & Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, U.K. D.M. COLLINS, Postdoctoral Researcher, is with the Department of Materials, University of Oxford, Parks Road, Oxford OX3 0AZ, U.K. M. PREUSS, Professor of Metallurgy, is with the School of Materials, The University of Manchester, Manchester M13 9PL, U.K. M.C. HARDY, Corporate Specialist—Nickel Alloys Rolls-Royce plc., P.O. Box 31, Derby DE24 8BJ, U.K. Manuscript submitted July 17, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
solvus solution heat treatments, a population of intergranular c¢ precipitates, referred to as primary c¢, are retained. Upon cooling from a solution heat treatment, the supersaturation of the c matrix will be sufficient for homogenous precipitation of intragranular c¢ to occur. Initially, secondary c¢ will nucleate and grow until their diffusion fields overlap, leading to soft impingement.[10] Further cooling, and the associated change in the supersaturation of the c matrix, can lead to another burst of c¢ nucleation, denoted as the tertiary c¢. The kinetics of these nucleation and growth processes are controlled by the cooling rate and hence the size and morphology of the c¢ precipitates can be tailored. The c¢ distributions are coarsened to the desired sizes through a subsequent aging heat treatment, typically between 1023 K and 1073 K (750 C and 800 C).[11–13] After such heat treatments, the coarser secondary c¢ precipitates are typically between 100 and 500 nm and the finer tertiary c¢ precipitates are often between 5 and 100 nm. Historically, the microstructure of a turbine disc has been selected by balancing the varying property requirements across the component. However, this could no
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