AbstractOxidation damage, in conjunction with fatigue, is a concern for nickel-based superalloys utilised as disc rotors in high pressure compressor and turbine of aero-engines. A combined experimental and numerical study has been carried out for alloy RR1000, developed at Rolls-Royce plc through a powder metallurgy route to meet the demands for higher overall pressure ratios, compressor discharge temperatures and rotational speeds for the latest aero-engines.
Cyclic experiments have been carried out for waisted specimens at selected temperatures (700°C-800°C), followed by microscopy examination using Focused Ion Beam (FIB). The results suggest that the major mechanism of oxidation damage consists of the formation of surface oxide scales and internal micro-voids and oxide particles beneath the oxide scales, which becomes more severe with the increase of temperature. Applying a cyclic stress does not change the nature of oxidation damage but tends to enhance the extent of oxidation damage for temperatures at 750°C and 800°C. Further energy dispersive X-ray (EDX) analyses show the enrichment of Cr and Ti, together with lower Ni and Co levels, in the surface oxide scales, suggesting the formation of brittle Cr2O3, TiO2, NiO and Co3O4 oxides on the specimen surface. Penetration of oxygen into the material and associated internal oxidation, which leads to further material embrittlement and associated failure, are evidenced from both secondary ion imaging and EDX analyses.
Scanning electron microscopy (SEM) studies of fracture surfaces have been performed for dwell crack growth, and the results confirmed the transition from transgranular to predominant intergranular cracking in alloy RR1000 for increased dwell time. This change in fracture mode supports the oxidation-assisted crack growth mechanism via grain boundary embrittlement. Oxidation embrittlement has also been supported by the FIB analyses of fracture surfaces which confirmed the oxidation reaction for alloy RR1000 at high temperatures.
A stress-assisted diffusion approach has been used to model oxygen penetration in the waisted specimen based on Fick’s first and second laws. Grain microstructures were considered explicitly in the model using a finite element submodelling technique, and the grain boundary was taken as the primary path for oxygen diffusion. The material constitutive behaviour was described by a crystal plasticity model to consider the effects of heterogeneous deformation at grain level on oxygen diffusion. Two essential diffusion parameters, i.e., oxygen diffusivity and pressure factor, have been obtained from the simulated oxygen penetration as well as the FIB measurements of internal oxidation depth.
Using the obtained diffusion parameters, finite element analyses of a compact tension specimen have been carried out to model oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. A failure curve for crack growth has been constructed based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure curve was then utilized to predict crack growth rates under fatigue-oxidation conditions for selected loading frequencies and dwell periods, with comparison against the experimental results and those predicted from the viscoplastic model alone.
|Date of Award||30 Sep 2011|
|Sponsors||Engineering and Physical Sciences Research Council|
|Supervisor||Liguo G. Zhao (Supervisor) & Jie Tong (Supervisor)|