G. E. Fuchs, B. C. Wilson, University of Florida, Gainesville, FL
Some second, third and fourth generation single crystal Ni-base superalloys (i.e., Re-containing alloys) have demonstrated a propensity for excessive primary creep at intermediate temperatures. During testing between 650°C and 850°C, the Re-containing alloys can exhibit up to 28-30% creep in the first few hours of a creep test. Secondary creep with a very low creep rate immediately follows and rupture lifetimes for these alloys can be quite long: up to an order of magnitude longer than first generation alloys in similar test conditions. Although rupture times are long, the time to 1% creep can be much shorter than that observed in first generation alloys. This behavior has been attributed to the presence of secondary g’ precipitates in the g matrix channels, as well as on the Re content of the alloys. This investigation examined creep behavior for a common first generation alloy, PWA 1480, a common second generation alloy, PWA 1484, as well as an modified first generation alloy, PWA 1480 with 3 weight percent rhenium added. Results show that while PWA 1484 exhibits high primary creep, the PWA 1480 with Re did not, suggesting that Re is not the sole cause of high primary creep. Additionally, two different aging heat treatments were given to each alloy to either precipitate or prevent the formation of, fine (nm scale) secondary g’ in the g channels. Results of these tests revealed that the presence of the fine secondary γ’ had a significant impact on the extent of primary creep; however, the presence of the fine secondary γ’ was not the only factor controlling the primary creep. The role of the fine scale microstructure and the alloy composition on primary creep deformation will be discussed.
Summary: Results of creep tests at intermediate temperatures revealed that the presence of the fine secondary ã’ had a significant impact on the extent of primary creep; however, the presence of the fine secondary ã’ was not the only factor controlling the primary creep. The role of the fine scale microstructure and the alloy composition on primary creep deformation will be discussed.
