Performance of nickel base superalloy components in gas turbines

The topic of this thesis is the microstructural behaviour of hot section components in the industrial gas turbine.

The major part of the work treats the interdiffusion between a Co-36.5Ni-17.5Cr-8Al-0.5Y, MCrAlY coating and the underlying IN738 superalloy. The main purpose of the investigation was to develop reliable methods for the estimation of metal temperature of service exposed components from the observed interface behaviour. The main work on this subject is included in the form of papers in appendices I), II) and III).

Furthermore a number of case studies of the behaviour of hot section components in the Danish gas turbines were performed and are included as part of the main thesis. Two of the case studies Negative Creep of Ni Superalloys and Identification of precipitates in IN792 after long time service exposure merited a separate treatment and are included as separate papers in appendices IV) and V).

I) A new approach for the measurement of average composition profiles in the scanning electron microscope across the interface between superalloys and coatings was developed. The approach allows for the easy measurement of average profiles even though both coating and superalloy contain a large amount of precipitate phases dispersed throughout the microstructure.

II) The developed measurement approach was then used to quantify the interdiffusion taking place at the interface between a Co-36.5Ni-17.5Cr-8Al-0.5Y, MCrAlY coating and the underlying IN738 superalloy in a large matrix of specimens isothermally heat treated for up to 12,000 hours at temperatures 875°C, 925°C or 950°C. Microstructural investigations and calculated phase fraction diagrams using Thermo-Calc showed that a precipitate free zone formed between the coating and superalloy and grew with time. The width of the growing zone was estimated on the basis of average intensity profiles obtained from experimental xray maps measured by energy dispersive spectroscopy in a scanning electron microscope. A simple parabolic growth model was then set up for estimating the metal temperature near the coating/substrate interface based on the growth kinetics of the precipitate free zone. Parameters for the model were extracted from measurements of the width of the growing precipitate free zone with time. The developed model was then used to estimate metal temperatures for a service exposed, first stage gas turbine blade from a Danish gas turbine.

III) Finally the multi-phase, multi-element interdiffusion was modelled using the finite difference software DICTRA and compared with the average composition profiles measured across the interface and the general experimental findings from I) and II).

IV) Negative Creep in Nickel base Superalloys
The case concerns the phenomenon negative creep that was found to occur in Nimonic 80A bolts used for fixation of heat shields in the hot gas ducts of Danish gas turbines. The term negative creep is used to describe a time-dependent contraction of a material rather than the extension normally seen during creep tests. The volume contraction is typically caused by a solid-state transformation, in the case of Nimonic 80A, formation of the ordered Ni2Cr phase after several thousands of service hours.

V) Identification of precipitates in IN792 after long time service exposure
The case paper treats the finding of plate-like precipitates in an uncoated IN792 blade. The precipitates had a morphology that initially pointed in the direction of σ-phase, but advanced microstructural investigations, including the use of focused ion beam milling (FIB) and electron energy loss spectroscopy (EELS) in the transmission electron microscope, established that the precipitates were carbides.