The progress in the further development of monocrystalline nickel-base superalloys as the most advanced turbine-blade materials has been the result of combined efforts in alloy development and microstructural refinement. Some aspects of these developments are reviewed. The questions of microstructural stability with respect to the formation of brittle topologically closed-packed (TCP) phases and, in particular, directional coarsening, i.e., γ/γ′ rafting under service-near high-temperature creep conditions are addressed. Since γ/γ′ rafting is usually accompanied by creep acceleration, attempts have been made to avoid rafting by modifying the microstructure by appropriate thermal/mechanical treatments. These attempts were not successful. On the other hand, it could be shown that pre-rafting in compression, leading to the formation of rafts parallel to the stress axis in the case of alloys with negative γ/γ′ lattice mismatch, enhances both the isothermal high-temperature creep and fatigue strengths. According to current understanding, there exists no optimum γ/γ′ lattice mismatch in the case of negative mismatch but, at best, an optimum compromise between low- and high-temperature behavior. It is speculated that a more suitable situation could be found in alloys with positive lattice mismatch.
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Mughrabi et al. (2000) studied this question.