Mechanisms of Dwell Fatigue in Single Crystal Ni-Based Superalloys at Intermediate Temperatures

Single crystalSingle crystal (SX) nickel-based superalloysNickel-based superalloys have been developed specifically to withstand the high temperaturesHigh temperature that would engender severe creepCreep degradation in a lesser alloy. Use in gas turbine bladesTurbine blades has long been a driver for the development of ever more capable alloys that can survive higher temperatures without significant material damage (Hashizume et al. Proc Int Symp Superalloys 53–62 (2004)). The drive for increased financial efficiency has included a desire for increased duration, and longer time between overhauls, as well as reduced specific fuel consumption. The long-term behaviour of SX alloys at lower temperatures, such as that experienced at the blade root fixings, has not historically influenced alloy designAlloy design, but has become increasingly important as the target time between overhauls increases. This study looks at the behaviour of an example of a second, third, and fourth generation SX alloy at a temperature commensurate with that outside the gas path, in keeping with that around the blade root. The influence of dwell on both the predicted life and consequent fractography is examined. In addition, the dependence of crack growth rate with secondary orientation is assessed. Finally, the crack trajectory close to the crack tip is considered and the possible influence of different regions of the loading cycle is discussed.