A review of degradation in single crystal nickel based superalloys

Single crystal nickel based superalloys are in use for the front rows of blading in advanced land based gas turbines. This is for their higher creep capabilities as well as outstanding low cycle fatigue properties as compared with conventionally cast nickel based superalloys. The continuous efforts to improve the high temperature creep properties of single crystal (SX) superalloys resulted in the development of the fourth generation of these alloys. Nevertheless, this improvement in the high temperature capability of these materials required the use of higher contents of Re and Ru which increased their densities and costs. The application of SX superalloys at higher temperatures results in microstructural changes called rafting and a topological inversion of their γ/γ′ phases. These two microstructural features reduce the tensile and low cycle fatigue properties of the SX superalloys. The extent of degradation in the microstructure and mechanical properties, in a certain SX superalloy, depends on the exposure temperature, duration and to a lesser extent on the applied stress. The consequences of these changes in microstructure and properties should be taken into consideration in the evaluation of the mechanical properties of SX superalloys.