PREDICTION OF PLASTIC STRAIN FOR RECRYSTALLISATION DURING INVESTMENT CASTING OF SINGLE CRYSTAL SUPERALLOYS C. Panwisawas 1 , H. Mathur 2 , J.-C. Gebelin 1 , D.C. Putman 3 , P. Withey 3 , N. Warnken 1 , C.M.F. Rae 2 and R.C. Reed 1 1 Department of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 2 Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK 3 Rolls-Royce plc., P.O. Box 31, Derby DE24 8BJ, UK Keywords: Modelling, investment casting, recrystallisation, plastic strain accumulation Abstract Castings for single crystal aerofoils can be prone to re- crystallisation during solution heat treatment; however quantitative information concerning the factors causing this phenomenon is lacking. In this paper, mathemati- cal modelling and targeted experimentation are used to deduce the levels of localised plastic strain needed for recrystallisation to occur. The influences of differential thermal contraction against the shell, specimen geometry and stress concentration factor are quantified. The model predicts that the induced strain in the metal increased with the ceramic shell thickness, and in some geometries, with the solidification height. Negligible plastic strains were predicted in a solid casting with no stress concen- tration features. However, as the geometry became more complex by reducing the casting cross-section, by the in- sertion of a core and introduction of stress concentration features, the induced plastic strains increased significantly. The predicted plastic strain for recrystallisation in a cored casting was in good agreement with experimental critical strain data. The model provides the foundation for a systems-based approach which enables recrystallisation to be predicted and thus avoided, prior to its occurrence in the foundry. Introduction It is well known that turbine blades for gas turbine ap- plications are investment cast, often into single crystal form. But much less appreciated is that during processing, deformation is induced in the nickel-based superalloy dur- ing cooling; this is due to differential thermal contraction of the ceramic shell, core and the metal arising primar- ily from their differing thermal expansion coefficients [1]. In the foundry, this effect has some practical ramifica- tions. First, account needs to be taken of the shrinkages [2] which occur – for example, the final casting will not exhibit the same dimensions as the wax model. Second, plastic strains can be produced which are large enough to induce recrystallisation during subsequent solutioning heat treatment. Particularly for components cast in single crystal form, the occurrence of recrystallisation cannot be tolerated – the associated high angle grain boundaries degrade the creep [3, 4] and fatigue [5, 6] properties signif- icantly. Work has been done to study the recrystallisation behaviour of single crystal superalloys under the influ- ence of different annealing conditions and microstructural features [1, 5, 6, 7, 8, 9]. However, from the processing per- spective, very little attention has been given to developing a systematic approach to controlling this problem. This paper is concerned with the mathematical mod- elling of investment casting, with particular emphasis on thermal-mechanical effects so that processing-induced plas- ticity can be predicted and rationalised. The overarching goal is to build a physics-based tool for the prediction of recrystallisation during the processing of single crystal parts. Traditionally, the avoidance of recrystallisation has been dealt with in a rather empirical way, with reliance placed on existing casting practice, experience and rules of thumb. Mathematical modelling represents a method by which the physical effects causing recrystallisation can be anticipated, which is obviously advantageous. Such modelling might be used for the optimisation of process- ing conditions, so that the likelihood of recrystallisation can be reduced. If sufficiently robust, it might also be used during the early stages of design process to influence the geometry chosen for the turbine blade – as part of a systems-based approach to component design. 547 Superalloys 2012: 12 th International Symposium on Superalloys Edited by: Eric S. Huron, Roger C. Reed, Mark C. Hardy, Michael J. Mills, Rick E. Montero, Pedro D. Portella, Jack Telesman TMS (The Minerals, Metals & Materials Society), 2012