Rejuvenation of Ni-base Superalloys GTD444 and René N5
During service, superalloy turbine components degrade over time by creep and fatigue deformation mechanisms due to a complex combination of stresses at high temperatures. Service degradation is commonly addressed by replacing the component with a new one. The high cost of the original Ni-base superalloy components and, consequently, replacement components has encouraged the development of rejuvenation (restoration) procedures to extend useful service life. Repair procedures used in literature consist of hot isostatic pressing (HIP) and/or solution + aging rejuvenation heat treatments near or above the γʹ solvus temperature. In the limited studies that have performed fatigue testing, rejuvenation has never been successful in recovering fatigue properties in low-cycle, high-cycle, or dwell-fatigue.
In order to elucidate the processes that prevent successful rejuvenation, repeated rejuvenation cycles have been performed. A rejuvenation cycle includes a rejuvenation heat treatment and/or small scale material removal, followed by creep or fatigue testing. Multiple rejuvenation testing is defined in this context as the application of repeated rejuvenation cycles until specimen failure.
As a result of multiple rejuvenation testing, the total creep rupture life of René 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 strain thresholds of both 2% and 3% were used along with solutioning at 28 °C below the γʹ solvus temperature for 2 h and aging at 1079 °C for 4 h. These rejuvenation conditions resulted in the maximum observed increase in total creep rupture life. Rejuvenation of compressive hold-time fatigue damage was also successful with the use of small scale material removal. While rejuvenation of René N5(SX) was considered successful, full recovery of the creep performance was not attainable even with a solution heat treatment at the γʹ solvus. Similar results are expected for GTD444(CG) although rejuvenation heat treatments were only performed below the γʹ solvus temperature.
Transverse grain boundaries limited life during multiple rejuvenation creep testing of both GTD444(CG) and René N5(SX). Due to the tortuosity of the grain boundaries in GTD444(CG), 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 René N5(SX) specimens were more amenable to rejuvenation than GTD444(CG). For both alloys, recrystallization during the rejuvenation heat treatment was responsible for early failure during the subsequent creep test.
Using rejuvenation in a production environment on these alloys represents a significant challenge. There will be a difference in microstructure between lab and industrial-scale components and a difference between service and mechanical testing conditions, requiring detailed characterization and identification of the life-limiting form of damage. Ultimately, the use of rejuvenation on industrial-scale components will depend on a number of factors for each specific case.