Phase Transformations in Co-Ni-Cr-W Alloys During High Temperature Exposure to Steam Environment
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Three model alloys, Co-10Ni-20Cr-15W, Co-30Ni-20Cr-15W and Ni-20Cr-15W (all in wt.%) were investigated in Ar-50%H2O at 700 and 750 °C for up to 3000 h reaction. The results showed the formation of thin chromia scales on the sample surfaces in all cases. For the Co-base alloys, accompanied by the Cr2O3 formation, a chromium depletion zone was detected underneath the oxide scale along with Co3W with some amount of dissolved Cr and Ni. This kind of Co3W was enriched along the oxide/alloy interface. The formation of this intermetallic phase was considered to follow nucleation and subsequent growth based on morphology and composition analyses. Increasing nickel content reduced the amount of Co3W formation. For the nickel-base alloy Ni-20Cr-15W, no intermetallic phase was detected. The formation of the Co3W intermetallic was discussed based on phase transformation induced by chromium depletion. The effect of nickel on this phenomenon was also discussed according to thermodynamic phase equilibrium analysis.
Keywordschromia scale Co-Ni-Cr-W alloys Co3W intermetallic Cr depletion phase transformation steam oxidation
The authors are grateful to Mr. H. Cosler, Ms. A. Kick, and R. Mahnke for carrying out the oxidation tests, Dr. E. Wessel and Dr. D. Grüner for SEM/EBSD investigations, and Mr. M. Borzikov for SNMS measurements. The Central Institute for Engineering, Electronics and Analytics (ZEA-3) is also acknowledged for ICP-OES analyses.
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