Grain boundary transformations during isothermal exposure of powder metallurgy nickel base superalloys for turbine disc applications

Nickel base superalloys are used for high pressure turbine discs, because they are capable of sustained operation under high mechanical loading at elevated temperatures. In service, turbine discs operate at high temperatures (approaching 700°C at the disc rim), which can lead to various microstructural changes and influence the resulting structural integrity of the component. In the alloys studied in the present work, the development of the topologically close packed phase (TCP) sigma (σ) has been characterised for various time and temperature combinations. The formation of s is particularly important for these alloys, as it is known to have a dramatic effect on fatigue crack growth owing to grain boundary embrittlement. In the present study, various techniques have been used to quantify the amount of σ phase present for given times and temperatures, similar to those seen during operation. Both qualitative microscopy and quantitative X-ray diffraction have been conducted to establish the time–temperature–transformation relationship for σ formation in two experimental turbine disc alloys. Both of the alloys studied here exhibit a low susceptibility to s formation compared with conventional alloys such as Udimet 720Li and RR1000.