Fabrication and creep properties of superalloy-zirconia composites
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9/27/03
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Fabrication and Creep Properties of Superalloy-Zirconia Composites R.K. ORUGANTI and A.K. GHOSH 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 thermalbarrier 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 René 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 (Ni3Al) to form Al2O3, which resulted in the depletion of 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.
I. INTRODUCTION
FUNCTIONALLY graded composites have been proposed as a means to reduce stress buildup at abrupt metalceramic interfaces. While the concept of a gradation in microstructure has been used for a long time, such as in casehardening of steels,[1] its use for thermal-stress reduction has been pursued more recently. The principle underlying this concept is illustrated in Figure 1, which shows two kinds of metal-ceramic interfaces—one in which the transition from one material to the other is abrupt (Figure 1(a)), and another where the transition is gradual (Figure 1(b)). The abrupt interface, upon undergoing a temperature change, would develop a stress distribution (component normal to the thickness direction of the coating) that is also abrupt. Such a temperature change can occur when this laminate is cooled from the fabrication temperature at which the laminate is generally stress-free. The large compressive stresses thus developed in the ceramic could lead to buckling failure,[2] or the ceramic could fail in tension during a heating cycle when the metal expands rapidly. The graded interface, on the other hand, shows a less-steep stress gradient due to the gradual change in composition from one material to the other. Here, the absolute magnitude of the stress is also reduced, which may prevent possible failure of the coating. The actual values of the stresses developed in such a system in the elastic regime can be calculated as described in Reference 3. A potential application of the graded-material concept is in thermal barrier coatings (TBCs) on
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