Richard John Chater oral presentation (OA3-Tue1-3-4)
Visualizing ion transport mechanisms through oxide scales grown on Ni-Co model alloys at 900°C using FIB-SIMS techniques
1 Imperial College London - Department of Materials, Exhibition Road, SW7 2AZ London, United Kingdom
2 University of Erlangen-Nurnberg (FAU), Schlossplatz, 4, 91058 Erlangen, Germany
Since the discovery of a ternary Co3(Al,W) compound in the system Co-Al-W by Sato et al. , γ′-strengthened Cobalt-base alloys have recently come back into focus. Although, this new class of super-alloys show a two-phase microstructure, comparable to conventional Nickel-base alloys, several properties change drastically with the transition of the base element from Nickel to Cobalt. The inferior oxidation resistance at temperatures above 800°C [2,3] is particularly negative for the potential usage as high-temperature turbine materials. To develop new alloys with improved performance, detailed knowledge of the oxidation mechanisms is essential.
Two-step oxidation experiments in 16O2 and 18O2 gas atmospheres are an elegant method to draw reliable conclusions on the oxidation rates and mechanisms of alloys or coatings . In the present paper this well-known technique benefits from a combination of nanoscale resolution gallium ion (FIB) sputtering in conjunction with quadrupole-based secondary ion mass spectrometry (SIMS) to quantitatively detect the two stable oxygen isotopic maps in the sequentially formed oxides of the alloy coupon cross-sections .
To systematically address the influence of increasing Cobalt content in the base alloy, a set of single-crystalline model alloys with a nominal composition of Ni-xCo-9Al-8W-8Cr was cast. Five compositions with a Nickel:Cobalt ratio of 100:0, 75:25, 50:50, 25:75 and 0:100 were considered. All alloys were oxidized for a total of 24h (16h in 16O2 + 8h in 18O2) at 900°C.
In all oxide scales, counter-current mass transport with cations diffusing outwards and anions diffusing inwards can be concluded. Besides the thickness of the formed scales, a huge dependence of the diffusion paths of oxygen through the outer oxide layer, from the sample composition, can be seen. An increasing Cobalt content leads to larger grains and enhanced lattice diffusion in regions close to grain-boundaries. These new insights into mass transport assist in modelling the important oxidation mechanisms in Cobalt- and Nickel-base alloys with unique detail.
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