13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 9. Summary for the crack initiation and propagation mechanism (a) in substrate and by coating’s effect of (b) NiAl and (c) PtAl. (GB: grain boundary; Loading direction is vertical; small arrows show the cracking direction) By using the Larson-Miller approach the creep results in Fig. 2 can be transferred as new ones presented in Fig. 10(a); the plot shows a somewhat negative effect of the coatings lowering the capacity to carry creep stress. Non-loading carrying by coating part above its DBTT was suggested according to some investigations [27]. Being encouraged by the analyses above the applied mechanical load in the creep sample is considered to be mainly carried by the substrate. After recalculating the creep stress by assuming the coating as load-free and excluding the final effective coating thickness (ECT in Fig. 4) from the total sample thickness, similar performance of coated and uncoated samples is achieved in Larson-Miller diagram (Fig. 10(b)). Figure 10. Creep stress (in log-scale and normalized) vs. Larson-Miller parameter (a) before and (b) after stress being modified. 4. Conclusions In this study the effect of NiAl and PtAl diffusion coatings on the creep fracture mechanism of Ni-based polycrystalline superalloy IN792 is investigated at two temperatures, 850 °C and 950 °C. The following conclusions can be made from this study. 1) The microstructural development of the coating depends on the elemental diffusion during the creep process. The inward diffusion of Al controls the coating thickening rate which seems to be independent of the diffusion coating type. 2) Grain-boundary detachment, which is strengthened by carbides, is the basic cracking mode of the substrate in all samples whether the crack initiated from the coating or the substrate.
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