Microstructure and high-temperature strength of monocrystalline nickel-base superalloys

The progress in the further development of monocrystalline nickel-base supe ralloys as the most advanced turbine-blade materials has been the result of combined efforts in alloy development and microstructural refinements. Som e aspects of these developments are reviewed. The questions of microstructu ral stability with respect to the formation of brittle topologically closed -pack (TCP) phases and, in particular, directional coarsening, i.e., gamma/ gamma' rafting under service-near high-temperature creep conditions are add ressed. Since gamma/gamma' rafting is usually accompanied by creep accelera tion, attempts have been made to avoid rafting by modifying the microstruct ure by appropriate thermal/mechanical treatments. These attempts were not s uccessful. On the other hand, it could be shown that pre-rafting in compres sion, leading to the formation of rafts parallel to the stress axis in the case of alloys with negative gamma/gamma' lattice mismatch, enhances both t he isothermal high-temperature and fatigue strengths. According to current understanding, there exists no optimum gamma/gamma' 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 situa tion could be found in alloys with positive lattice mismatch.