Timur Galiullin

Modelling compositional changes in nickel base alloy 602 CA during high temperature oxidation

Abstract The high chromium nickel base alloy 602 CA is used as construction material for high temperature components in aggressive environments such as gas burners or heat exchangers. The high temperature oxidation behaviour of this alloy is characterised by the formation of an external Cr2O3 scale and internal precipitation of alumina. The main goal of the present investigation was to elucidate the effect of aluminum on the subscale chromium depletion phenomena. The chromium concentration profiles were found to be flat and sometimes even increasing towards the oxide-alloy interface. The experimental observations were interpreted in terms of alloy thermodynamics, i.e. effect of oxidation induced compositional changes on chemical potentials of the alloy constituents. The concentration profiles in alloy 602 CA observed during high temperature exposure were modelled using the CALPHAD approach for thermodynamic and kinetic calculations. Thermodynamic calculations revealed that the Al depletion in the alloy subsurface due to internal alumina precipitation leads to enhanced Cr diffusion towards the alloy surface.

Predicting the microstructural evolution in a multi-layered corrosion resistant coating on a Ni-base superalloy

Abstract Protective metallic MCrAlY or diffusion type (NiAl) coatings enhance the oxidation and corrosion resistance of the underlying high temperature materials employed in aeroengines and industrial gas turbines by ensuring the growth of a slowly growing protective alumina scale. However, a chromia forming coating would provide a better resistance against sulphur induced corrosive attack. A hybrid coating system combining both chromia and alumina forming coating layers would provide optimum protection in oxidising-sulphidising environments. The microstructural stability and applicability of such a coating system (SmartCoat) containing alternate layers rich in chromium and aluminium respectively on the Ni-base superalloy CMSX-4 was evaluated after various exposure times at 800 C. Scanning electron microscopy (SEM) and electron microprobe analyses (EPMA) provided the element concentrations. Phases were identified by electron backscatter diffraction, and correlated with SEM and high-resolution TEM/EDX analyses. A computational approach was employed to describe the mechanisms of the phase transformations occurring in the coating system.