Prediction of recrystallisation in single crystal nickel-based superalloys during investment casting

¹ Department of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, UK
² Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK
³ Institute für Metallkunde und Metallphysik, RWTH Aachen University, 52056 Aachen, Germany
⁴ Rolls-Royce plc., PO Box 31, Derby DE24 8BJ, UK
⁵ Department of Engineering Science, University of Oxford, Parks Road, Oxford OX1 3PJ, UK

^a Corresponding author: c.panwisawas@bham.ac.uk

Abstract

Production of gas turbines for jet propulsion and power generation requires the manufacture of turbine blades from single crystal nickel-based superalloys, most typically using investment casting. During the necessary subsequent solution heat treatment, the formation of recrystallised grains can occur. The introduction of grain boundaries into a single crystal component is potentially detrimental to performance, and therefore manufacturing processes and/or component geometries should be designed to prevent their occurrence. If the boundaries have very low strength, they can degrade the creep and fatigue properties. The root cause for recrystallisation is microscale plasticity caused by differential thermal contraction of metal, mould and core; when the plastic deformation is sufficiently large, recrystallisation takes place. In this work, numerical and thermo-mechanical modelling is carried out, with the aim of establishing computational methods by which recrystallisation during the heat treatment of single crystal nickel-based superalloys can be predicted and prevented prior to their occurrence. Elasto-plastic law is used to predict the plastic strain necessary for recrystallisation. The modelling result shows that recrystallisation is most likely to occur following 1.5–2.5% plastic strain applied at temperatures between 1000 ^∘C and 1300 ^∘C; this is validated with tensile tests at these elevated temperatures. This emphasises that high temperature deformation is more damaging than low temperature deformation.