Rejuvenation of Directionally Solidified and Single-Crystal Nickel-Base Superalloys

Abstract

During service, superalloy turbine components degrade over time by creep and fatigue deformation mechanisms due to a complex combination of stresses at high temperatures. The high cost of fabricating Ni-base superalloy components and, consequently, replacement components has encouraged the development of rejuvenation (restoration) procedures to extend useful service life. The processes that limit repeated rejuvenation of directionally solidified GTD444 and single-crystal Rene N5(SX) alloys have been studied in detail. A rejuvenation cycle includes a rejuvenation heat treatment and/or small-scale material removal, followed by creep or fatigue testing. With the application of multiple rejuvenation cycles, the total creep rupture life of Rene N5(SX) tested at 982 °C and 206 MPa was extended by a factor of 2.8 over the baseline rupture life. To produce this increase in rupture life, creep strains were limited to 2 or 3 pct prior to application of a rejuvenation cycle, involving solutioning at 28 °C below the $$\gamma ^{\prime }$$ solvus temperature for 2 hours and aging at 1079 °C for 4 hours. Rejuvenation of compressive hold-time fatigue damage was also successful with the use of small-scale material removal. Due to the tortuosity of the grain boundaries in GTD444(DS), some boundaries are initially oriented transverse to the growth direction. The enhanced plasticity near these grain boundaries may be the primary reason why the initially single-crystal Rene N5(SX) specimens were more amenable to rejuvenation than GTD444(DS). For both alloys, recrystallization after multiple rejuvenation cycles was responsible for early failure during the subsequent creep test. Resonant ultrasound spectroscopy was successfully implemented to detect the presence of recrystallized grains.