Oxidation and oxygen transport in a commercial polycrystalline Ni-based superalloy under static and loading conditions
Creators: name: Foss, Benjamin
Publisher: name: Imperial College London
Year: 2013

description: The current demands of the aviation industry for increased gas-turbine efficiency pushes polycrystalline nickel-based superalloy turbine discs to their operational limits. This drive for improved efficiency necessitates higher turbine entry temperatures, requiring that alloys exhibit superior oxidation resistance. The synergistic effects of oxidation and high mechanical stresses pose a complex issue and can be responsible for a reduction in the fatigue life of these components. The principal aim of this thesis is to examine the oxidation behaviour and the associated transport mechanisms in the fine grain alloy RR1000 in the polished and shot-peened conditions, for classical and mechanically loaded conditions. To achieve this, experiments were conducted that integrated two-stage isotopic tracing, with 18O serving as a tracer element during the second stage oxidation, combined with focused-ion- beam secondary ion mass spectrometry (FIB-SIMS) and other complimentary techniques. Classical oxidation studies of FGRR1000 between 700-800°C characterised the oxidation behaviour in terms of the kinetics, microstructural changes and transport mechanisms. The alloy formed a mostly chromia external oxide, with discontinuous layers of rutile above and below the main oxide. Preferential grain boundary alumina internal oxidation, γ’-dissolution and recrystallisation occurred in the sub-surface below. The shot-peened alloy demonstrated expedited oxidation kinetics, attributable to short-circuit diffusion paths generation during peening and exposure. Combined isotopic oxidation/FIB-SIMS mapping revealed anionic/cationic growth in the external oxidation. Inward oxygen transport in the alloy, following diffusion through the semi-protective external oxide, took place preferentially along alloy/internal oxide interfaces. The FIB-SIMS maps were used to determine the oxygen tracer-diffusion coefficients in the external scale, which were also higher in the SP alloy. Thermodynamic modelling provided an insight into the γ’-dissolution and recrystallisation, which is not related to the cold work alone, and was subsequently described by oxidation- induced recrystallisation (OIR). A study of compressive and tensile loads on the oxidation behaviour used a purpose-built bending rig for use in an isotopic oxidation apparatus. Loading did not influence the oxidation products formed, but did bring-about expedited oxidation kinetics and changes to the oxide morphology. Arguably, the most significant effects took place in the sub-surface regions. Increased oxidation kinetics were attributed to the development of fast anion/cation diffusion paths as the alloy deformed by creep. Likewise, the creep-deformed microstructure is largely responsible for the morphological differences observed. A study of the residual stress relaxation in the shot-peened alloy due to thermal exposure and dwell-fatigue loading utilised X-ray residual stress analysis, electron back-scattered diffraction, micro-hardness testing and FIB microscopy. Thermal exposure at 700°C for 48 hours resulted in a large reduction in the residual stresses and work-hardening effects in the alloy, but the sub-surface remained in a beneficial compressive state. Oxidizing environments caused recrystallization in the near surface, but did not affect the residual stress-relaxation behaviour. Dwell-fatigue loading at the same temperature and total exposure times caused the residual stresses to return to approximately zero at nearly all depths. The roles of oxidation and oxygen transport during crack initiation and short crack growth of the shot-peened alloy were investigated. This used post mortem examination of specimens that had undergone dwell-fatigue loading at 700°C using isotopic tracing protocols. Preparation of crack cross-sections utilised a tangential FIB milling technique, which provided a suitable analysis region for different materials characterisation techniques. The relative distributions of the 16O-/18O- suggested that oxygen diffusion and the subsequent oxidation took place along grain boundaries, orientated in the direction of the maximum shear, immediately ahead of the crack. The growth of these oxide intrusions appears stress assisted; however, grain boundary primary-γ’ precipitates may impede their continued growth. Complimentary Time-of-Flight SIMS (ToF- SIMS) and Energy Dispersive X-ray Spectroscopy (EDS) revealed the chemistry of the oxidation products. A possible mechanism for initiation and the initial crack growth under the current loading conditions was put forward.