The effect of ruthenium on the intermediate to high temperature creep response of high refractory content single crystal nickel-base superalloys

The addition of 3 wt.% ruthenium (Ru) was found to improve both the intermediate temperature/intermediate stress and the high temperature/low stress creep behaviours of two high refractory content single crystal Ni-base superalloys. Creep rupture tests, creep activation energy measurements and microstructural analysis indicate that Ru serves a dual role; it enhances microstructural stability by suppressing the precipitation of deleterious topologically close-packed (TCP) phases and it increases the level of solid solution strengthening of the γ phase. Its effectiveness as a TCP suppressant increases with temperature whereas its potency as a strengthening element decreases with temperature. Ru reduces the stacking fault energy of the γ phase and this is thought to have a significant contribution towards its strengthening benefit at intermediate temperatures. The reduction in γ′ volume fraction upon the addition of Ru, which is central to its effectiveness as a stabilising element, appears to be the principal cause of its diminishing strengthening contribution at elevated temperatures.