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.
Castings for single crystal aerofoils can be prone to recrystallisation during solution heat treatment; however quantitative information concerning the factors causing this phenomenon is lacking. In this paper, mathematical 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 concentration features. However, as the geometry became more complex by reducing the casting cross-section, by the insertion 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.
Abstract. The effect of composition on recrystallisation of single crystal Ni-based superalloys was studied in a series of alloys that vary systematically in composition. Following room temperature macro-indentation and subsequent annealing, alloys containing higher Co concentration showed greater susceptibility to recrystallisation, and this has been attributed to the effect on the γ solvus temperature and the stacking fault energy of the γ phase. Mo, W and Ru did not appear to influence the recrystallisation behaviour systematically, although high concentration of all these elements led to the highest recrystallisation. Moreover, with grain orientation mapping, it was found that the nucleating recrystallisation grains form in orientations similar to the deformed regions, and subsequently twin to form higher angle boundaries and proliferate further within the deformed microstructure. The effects of the interaction between grain boundaries and secondary phases, such as the γ and topologically close packed phases, were also studied.
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