Fabrication and creep properties of superalloy-zirconia composites

Graded composite interfaces have been proposed as a means to reduce thermally induced stresses between dissimilar materials. This is expected to be useful in applications such as ceramic thermal-barrier coatings (TBCs) on superalloy substrates. The interfaces, in such cases, are metal-matrix composites containing the ceramic phase within the superalloy matrix, whose creep properties during elevated-temperature service become critically important. This study was carried out to assess the creep properties of a typical superalloy-ceramic combination, namely, a Rene 95 alloy containing partially stabilized zirconia. Composites of these materials were prepared via powder metallurgy. Microscopy and X-ray work revealed that the zirconia reacted with gamma' (Ni3Al) to form Al2O3, which resulted in the depletion of gamma' from the matrix. The creep behavior of the composites was markedly different from that of the unreinforced matrix. In addition to showing different stress exponents, the composites were stronger than the unreinforced material at low strain rates and weaker at the higher strain rates. A composite load-transfer model is used to isolate the effect of particles on strengthening. It is found that strengthening by the ceramic particles is smaller than strengthening arising from the change in chemistry of the matrix due to the addition of ZrO2.