Sub-Scale Depletion and Enrichment Processes During High Temperature Oxidation of the Nickel Base Alloy 625 in the Temperature Range 900–1000 °C

Abstract

Numerous chromia-forming austenitic steels and nickel-base alloys contain chromium-rich strengthening precipitates, e.g. chromium-base carbides. During high temperature exposure the formation of the chromia base oxide scale results in chromium depletion in the alloy matrix and consequently in dissolution of the strengthening phase in the sub-surface zone. The present study describes the oxidation induced phase changes in the chromium depletion layer in case of alloy 625, a nickel base alloy in which the strengthening precipitates contain hardly any or only minor amounts of chromium. Specimens of alloy 625 were subjected to oxidation up to 1000 h at 900 and 1000°C and analyzed in respect to oxide formation and microstructural changes using light optical microscopy, scanning electron microscopy, energy and wavelength dispersive analysis, glow discharge optical emission spectroscopy, and X-ray diffraction. In spite of the fact that the alloy precipitates d-Ni 3 Nb and/or (Ni, Mo) 6 C contain only minor amounts of chromium, the oxidation induced chromium depletion results in formation of a wide sub-surface zone in which the precipitate phases are depleted. However, in parallel, substantial niobium diffusion occurs towards the alloy surface resulting in formation of a thin layer of d-Ni 3 Nb phase adjacent to the alloy/oxide interface. By modeling phase equilibria and diffusion processes using Thermo-Calc and DICTRA it could be shown that the phase changes in the sub-scale zone are governed by the influence of alloy matrix chromium concentration on the thermodynamic activities of the other alloying elements, mainly niobium and carbon. The d-phase depletion/ enrichment process is caused by a decreasing niobium activity with decreasing chromium concentration whereas the (Ni,Mo) 6 C dissolution finds its cause in the increasing carbon activity with decreasing chromium content.A. Chyrkin (&) Á P. Huczkowski Á V. Shemet Á L. Singheiser Á W. J. Quadakkers

In-Service Oxidation and Microstructural Evolution of a Nickel Superalloy in a Formula 1 Car Exhaust

“…A final feature of note in this region is the strong Nb enrichment directly beneath the oxide scale. Similar enrichment has also been documented in previous experimental and thermodynamic modelling studies on oxidised Inconel 625 and 718 alloys [8,[25][26][27]. Chyrkin et al [8] reported the formation of a localised d phase within the denuded zone beneath the oxide scale, not seen in the bulk.…”

“…The enrichment of Si near/inside the oxide scale should also be noted, consistent with the EPMA data of Fig. 2 and prior spectroscopic measurements [8].…”

“…Thus, both model studies are consistent with the view that the exhaust section was operating at 1000°C for a proportion of the time in use. In terms of the total depth of thermally grown oxide damage, the inner and outer exhaust surfaces display similar trends, with a thin protective chromia scale above a precipitate-depleted zone some 5-6 lm deep, a closely comparable microstructure to that seen following air exposure in a previous study [8]. This previous study demonstrated parabolic oxidation rates at 900°C over 100-1000 h, while weight gain measurements have also shown parabolic oxidation in this alloy up to 1150°C [6].…”

“…7) identifies both Nb and Ti in this region, indicating that 1000°C was attained. Further evidence for this comes from the work by Chyrkin et al [8]. Following a 1000-h furnace treatment at 900°C, their EDX analyses showed two bulk precipitates, one rich in Ni and Nb, and the other (smaller \ 1 lm dia.)…”

“…To understand such effects, previous studies on this and similar alloys have examined oxidation and surface degradation behaviour using a variety of model exposures [6][7][8][9][10][11]. The benefits of protective coatings have also been studied [12,13], along with the effects of corrosion following exposure to salts simulating gas turbine [14] and fuel cell [15] operation.…”