High-temperature behavior of oxide dispersion strengthening CoNiCrAlY

Abstract

To fabricate oxide dispersion strengthened bond coatings, commercial Co–30wt-%Ni–20Cr–8Al–0•4Y powder was milled with 2% additions of Al₂O₃, Y₂O₃ or Y₂O₃ + HfO₂. Low-pressure plasma sprayed, free-standing specimens were oxidised in air + 10%H2O at 1100 °C both isothermally (100 h) and in 500, 1−h cycles. Dry air cyclic testing conducted at both ORNL and FZJ showed remarkably similar results. In general, the water vapour addition caused more scale spallation. Two LPPS specimens without oxide additions were tested for comparison. The specimens with 2%Al₂O₃ addition exhibited the best behaviour as the powder already contained 0•4%Y. Additions of 2%Y₂O₃ and especially 1%Y₂O₃ + 1%HfO₂ resulted in over-doping as evidenced by high mass gains and the formation of Y- and Hf-rich pegs. Scanning transmission electron microscopy of the isothermal specimens showed no Hf and/or Y segregation to the alumina scale grain boundaries in the over-doped specimens.

Keywords:

• CoNiCrAlY coating
• LPPS
• oxidation
• alumina scale
• hafnium
• yttrium
• oxide dispersion strengthening

Acknowledgments

This work is part of a FZJ-ORNL collaboration where work in Germany was funded by the Federal Ministry of Education and Research (BMBF) through the project ‘High-Temperature and Energy Materials’ under grant 03EK3032, and the stay of T. Huang was sponsored by the National Natural Science Foundation of China (grant number 51401097) and the China Scholarship Council. The U.S. work was funded by the U.S. Department of Energy, Office of Fossil Energy, Turbine Program. The microscopy at ORNL was supported (1) through a user proposal at ORNL’s Centre for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy, Office of Science User Facility, and (2) by the U.S. Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D Program and the Nuclear Science User Facilities. At ORNL, D.W. Coffey, T.M. Lowe, G.W. Garner, T. Jordan, T.S. Geer, assisted with the experimental work. S. Dryepondt and J. A. Haynes provided comments on the manuscript. At FZJ, the authors are grateful to their colleagues for spraying the coatings (K.-H. Rauwald, R. Laufs), carrying out the oxidation experiments (A. Kick, H. Cosler) and conducting SEM and EBSD analyses (Dr E. Wessel and Dr. D. Grüner).