An investigation into static crack growth and dwell-fatigue in the nickel based superalloy RR1000 at elevated temperatures

This study aims to investigate the crack growth performance of the high strength superalloy RR1000 at elevated temperatures. Within the aero-engine RR1000 is subjected to fatigue and dwell-fatigue where crack growth can occur via creep and oxidation in addition to fatigue. The potentially dominating static crack growth mechanism has resulted in a focus upon static load testing in this study. Post-testing fractography, grain size measurements and metallographic examination have been carried out in order to better understand crack growth behaviour. Parent specimens were tested under fatigue-dwell conditions over the temperature range 600-700°C. The tests revealed that extended periods of dwell have a detrimental effect upon crack growth resistance in air. Such crack growth exhibited a fully intergranular cracking mechanism. It has been shown that dwell-fatigue loading conditions are greatly influenced by increasing test temperature and therefore suggested that the rate of oxygen diffusion is a controlling factor causing crack growth. Inertia welded (IW) specimens were tested in air over the temperature range 500-700°C with two samples also tested in vacuum at 700°C. IW specimens exhibited a significantly lower crack growth resistance compared to the parent alloy with evidence that sustained loading has a dominant effect on crack growth for high values of stress intensity factor, K. A vast improvement in crack growth resistance is apparent for the vacuum samples in comparison to the samples tested in air. Welding processing parameters were investigated by static load testing as-welded and post-weld heat treated (PWHT) linear friction welded (LFW) specimens over a range of welding energy input rates. It was revealed that energy input rate has a notable effect on crack growth in the as-welded condition but a negligible effect after PWHT. The more symmetrical crack profiles produced as a result of PWHT have suggested a controlling influence of residual stress upon such crack growth resistance curve