THE EFFECT OF COMPOSITION, MISFIT, AND HEAT TREATMENT ON THE PRIMARY CREEP BEHAVIOR OF SINGLE CRYSTAL NICKEL BASE SUPERALLOYS PWA 1480 AND PWA 1484

摘要

Primary creep occurring at intermediate temperatures (650°C to 850°C) and loads greater than 500 MPa has been shown to result in significant primary creep strain (often exceeding 5-10%) during the first few hours of a creep test. This investigation examines how rhenium additions and aging heat treatment affect the primary creep behavior of PWA 1480 and PWA 1484. To aid in the understanding of rhenium's role in primary creep, 3wt% Re was added to PWA 1480 to create a second generation version of PWA 1480. The aging heat treatments used for creep testing were either 704°C/24hr. or 871°C/32hr. For all test temperatures, the high temperature age resulted in a significant decrease in primary creep behavior of PWA 1484 and a longer lifetime for all but the lowest test temperature. The primary creep behavior of PWA 1480 and PWA 1480+Re did not display any significant dependence on age heat treatment. The creep rupture life of PWA 1480 is greater than PWA 1484 at 704°C, but significantly shorter at 760°C and 815°C. PWA 1480+Re, however, displayed the longest lifetime of all three alloys at both 704°C and 815°C (PWA 1480+Re was not tested at 760°C). X-ray diffraction and TEM analysis offer insight into the possible causes in the difference in behavior. PWA 1480 was found to deform by dislocations constrained to the γ matrix while PWA 1484 is dominated by stacking fault shear of theγ' phase. The modified PWA 1480+Re contained stacking fault shear from more than one slip system and displayed the least amount of primary creep. Only after sufficient dislocation debris builds at the γ/γ' interfaces, does secondary creep start.