%0 Journal Article
%A Garcia-Fresnillo, L.
%A Chyrkin, Anton
%A Boehme, C.
%A Barnikel, J.
%A Schmitz, F.
%A Quadakkers, Willem J.
%T Oxidation Behaviour and Microstructural Stability of Alloy 625 During Long-Term Exposure in Steam
%J Journal of materials science
%V 49
%N 17
%@ 0022-2461
%C Dordrecht [u.a.]
%I Springer Science + Business Media B.V
%M FZJ-2014-06678
%P 6127-6142
%D 2014
%X Nickel-based alloys are being considered as construction materials for various components in high-efficiency steam turbines with envisaged operating temperatures around 700 °C. In the present study, the steam oxidation behaviour of the nickel-based alloy 625 in the temperature range of 700–800 °C was investigated whereby exposures up to 10000 h were carried out. Gravimetric data in combination with results from a variety of post exposure analysis techniques showed in all cases the formation of protective oxide scales mainly consisting of chromia with minor amounts of outer Cr/Mn spinel and internal silica. The phases found in the bulk alloy after long-term exposure were mainly needle-shaped δ-Ni3(Nb,Mo) phase, μ-phase and Si-rich η-M6C carbide. Microstructural features and phase formation were found to be related to minor variations in the alloy composition, especially iron and silicon content. The oxidation-induced chromium depletion caused a number of microstructural changes in the subsurface depletion layer. Most important was an enrichment of the intermetallic δ-phase at the scale–alloy interface. DICTRA modelling revealed this effect to be caused by uphill diffusion as a result of a negative niobium activity gradient in the subscale chromium depletion zone. Although the available kinetic and thermodynamic data allowed qualitative explanation of the δ-phase enrichment, the databases do not correctly describe the high molybdenum solubility in the δ-phase.
%F PUB:(DE-HGF)16
%9 Journal Article
%U <Go to ISI:>//WOS:000337507100035
%R 10.1007/s10853-014-8344-7
%U https://juser.fz-juelich.de/record/173267