The fabrication and creep properties of superalloy-zirconia composites

Many high-temperature structural materials applications require mismatch strain compatibility that can be provided by graded composites. Applications such as ceramic thermal-barrier coatings on superalloy substrates require compatibility to minimize interfacial stresses and degradation effects. The interfaces in such cases could be metal-matrix composites containing a graded distribution of the ceramic phase within the superalloy matrix whose creep properties during elevated temperature service are unknown. This article reviews creep properties of a typical superalloy, René95, containing partially stabilized zirconia. These composites were prepared via powder metallurgy, during which zirconia was found to react with γ′ (Ni₃Al) to form Al₂O₃, resulting in the depletion of γ′ from the superalloy matrix. Due to the combined effects of chemical changes and grain refinement, considerable creep strengthening was achieved at low creep rates, but weakening was observed at higher creep rates. A composite load transfer model is used to isolate the effect of particles on strengthening. For more information, contact A.K. Ghosh, University of Michigan, Department of Materials Science and Engineering, 2300 Hayward Street, Ann Arbor, Michigan 48109-2136; (734) 764-3322; fax (734) 763-4788; e-mail akg@engin.umich.edu.