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

Abstract

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.

Detection of Rhenium-Rich Particles at Grain Boundaries in Nickel-Base Superalloy Turbine Blades

“…Other strains can be induced in the base metal due to the microscale plasticity which is caused by differential thermal contraction of metal, mould, and core [47][48][49] . It was estimated that when the mould and core do not crush or deform suf ciently or crack the metallic components can impose a strain value of about 2 3% 46) , which is over the critical plastic strain (0.2%) for the recrystallization of turbine blades 47) .…”

“…These processes can lead to extensive plastic deformation being imparted to an as-cast turbine blade at high temperature and this can be responsible for strain relaxation through recrystallization during subsequent heat treatments. In addition, as the recrystallized grains are initiated from the as-cast surface, the surface condition of turbine blades can play an important role in the amount of recrystallization observed 48,49) . Since the recrystallized grains introduce disadvantageous orientations and high-angle grain boundaries, which can dramatically reduce the creep rupture strength and fatigue life of the turbine blades 38,[50][51][52] , there are a number of studies on the recrystallized grain which have been exposed on the surface of the turbine blades.…”

The role of stress relaxation and creep during high temperature deformation in Ni-base single crystal superalloys – Implications to strain build-up during directional solidification

“…To a lesser extent, re-crystallisation is also dependent on the alloy composition [17], as well as the presence of the non-equilibrium γ/γ / eutectic; the latter has been proposed to act as pinning sites for the grain boundary and thereby retards growth of the re-crystallised grains [18]. Some key questions persist on the nucleation mechanism of these re-crystallised grains [19 -23] and the critical "inelastic" strains and their temperature dependence [16,24,25]. Cox et al.…”