The quest for higher efficiency and fuel economy has pushed aeroengines to challenging levels. In order to become more efficient, engines must run at higher bypass ratios and temperatures, resulting in extreme operating conditions for their hottest section. Nickel based superalloys have been used for this application for the past 50 years due to high fatigue strength at elevated temperatures. This paper investigates the deformation behaviour and fatigue lives of a powder metallurgy Nickel-based superalloy developed for discs of high-pressure turbines, i.e. the most demanding section of aeroengines. For that six different non-proportional load paths were carefully selected, where five of them present the same degree of load non-proportionality, to explore load path dependency and the effects of non-proportional multi-axial loading on fatigue lives. Results confirm an additional cyclic hardening caused by load non-proportionality and its detrimental effect on fatigue life. Lives for non-proportional tests were around three times shorter than fatigue lives for proportional tests at comparable stress levels.