Microstructural aspects of high temperature deformation of monocrystalline nickel base superalloys: some open problems

Some open questions concerning the microstructural mechanisms which govern the high temperature strength of monocrystalline nickel base superalloys will be reviewed in the context of previous work. Emphasis will be laid on microstructural effects observed in microyielding, creep, isothermal and thermomechanical fatigue and on the role of the gamma/gamma' lattice misfit with respect to gamma/gamma' rafting. The conditions for cutting of the gamma' rafts will be stated in a simplified two-dimensional dislocation model. Effects of the connectivity of the coarsened gamma/gamma' microstructure and of the orientation of gamma/gamma' rafts on the strength properties will be discussed, and potentially beneficial effects of a positive lattice misfit will be addressed. Attention will be drawn to counteracting alloying effects which modify both the lattice mismatch and the diffusion kinetics. The main conclusion is that the optimisation of superalloy properties can only be sought in improving the most important properties without deterioration of other desirable properties.