| MCrAlY type bond coat alloys, where M= Co, Ni or their combination, are commonly applied in land based gas turbine systems to prevent high-temperature oxidation of underlying components. Microstructural stability and alloy lifetimes are of critical importance since the coating degrades overtime through oxide formation and interdiffusion with the substrate. Present research work aims to develop better understanding towards the effects coating composition have on alloy phase transformations and oxidation lifetimes. A series of isothermal oxidation tests were performed on cobalt and nickel rich MCrAlY alloys with varying concentrations of aluminum and rhenium. Isothermal tests were conducted in temperature range of1000-1250 oC for up to 1000 hours.The influence of coating composition on alloy phases was investigated both experimentally and by means of thermodynamic modelling. Alloy microstructures were modelled via Thermo-Calc software using two thermodynamic databases,namely SSOL5 and TTNI8. TTNI8 proved to be exceptionally reliable in predicting the phase type and amounts, but the SSOL5 database made inaccurate predictions.Effects of aluminum and rhenium concentrations on phase contents of cobalt and nickel rich MCrAlY alloys were investigated. Results revealed that a 1 wt.% increase of aluminum cause an ~8 vol.% increase in alloy's ? phase. Cr-Re phases(? or ?)precipitate above a critical aluminum concentration in rhenium containing alloys with? particles in nickel rich alloy and ? in cobalt rich alloy.Isothermal oxidation revealed sub-parabolic growth of Al2O3 in all investigated alloys. A detailed investigation of subscale concentration and microstructural changes was carried out by examining the sample cross-sections. The concentration changes in the subscale were modelled using software DICTRA and verified experimentally.Experimental findings proved the predicted concentration profiles produce excellent qualitative and quantitative phase information in oxidized alloys. Lifetimes caused by chemical failure during high-temperature oxidation of cobalt and nickel rich MCrAlY alloys were investigated by modelling the ? depletion as a function of oxidation time.Chemical life of an alloy was found to depend critically on its oxidation rate and aluminum diffusivity. Rhenium and aluminum additions increased the lifetime by providing higher ? fraction and by lowering the oxidation rates and aluminum diffusivity. The effects of rhenium were more pronounced in cobalt rich alloys than in nickel rich alloys. This difference is attributed to different phases in two alloys with the former being ?, ?, and ? and the latter having ?, ? and ?.Moreover, two types of subscale chromium and rhenium enrichments were seen beneath the oxide of two batches of oxidized Co32Ni21Cr10Al3.5Re Y alloys.Despite their similar compositions, one alloy developed a supersaturated Cr-Re rich layer and another formed Cr-Re rich ? precipitates beneath the oxide in the subscale?-phase depleted region. Thermochemcial modelling using DICTRA showed the enrichments resulted from decreased chromium and rhenium chemical potentials with aluminum depletion. The difference in accumulation forms is a result of different phases in two alloys, with a microstructure of ?, ?, ? and ? in first alloy and ?, ? and ?only in the second. This phase difference as well as the different tendency of the enriched chromium and rhenium to precipitate as particles at the interface is believed to be related to different levels of impurities in two alloys. The less pure alloy had a faster ? to ? transformation and the accumulated chromium and rhenium precipitated as ?-phase particles at the interface while the pure alloy formed a Cr-Re enriched layer within the ? phase. |
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