Thermo-Calc and DICTRA modelling of the β-phase depletion behaviour in CoNiCrAlY coating alloys at different Al contents

The β-phase depletion kinetics in MCrAlY coating alloys at different Al contents has been investigated by diffusion simulations and comparing with available experimental data. The thermodynamic software, Thermo-Calc, and the finite difference software, DICTRA, were employed to calculate phase equilibriums and to perform diffusion calculations using the thermodynamic database, TTNi7, and the mobility database, MOB2. The phase fractions and compositions in MCrAlY alloys with different Al contents were calculated by Thermo-Calc. At high temperature exposures, the β-NiAl phase in MCrAlY coating alloys depletes progressively due to the consumption of Al during oxidation and interdiffusion. The Al flux due to oxidation was approached from the oxide growth kinetics at the oxide/coating interface in the DICTRA diffusion model. Initial oxides and the associated initial β-phase depletion zones were assumed as the initial conditions. It is found that the MCrAlY with highest Al content exhibits the least β-phase depletion due to its large β-phase fraction and high Al content. In addition, it is shown that the loss of β phase to the substrate due to interdiffusion is more significant than the β-phase depletion from oxidation.