13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- NiAl (Fig. 6(a) and (b)). Due to the difference of mechanical properties between two characteristic zones, e.g. OZ and IDZ or coating and substrate, some voids/cracks formed at their interface during the creep process at both temperatures (Fig. 7(b) and (c)) but more significant at 950 °C. Toshio [24] also observed such voids in SRZ in PtAl coatings under creep stress. Since the through-coating crack was not formed in PtAl, the “L”-shape crack near the coating part was not observed in this sample type. Either being easily plasticized or producing voids/cracks the coating part lost the ability to carry load during the creep process. 3.2.4. NiAl-PtAl Comparison of the coating behavior of NiAl in NiAl-PtAl (Fig. 8(a)) with that in NiAl-NiAl (Fig. 6) shows no significant difference at 850 °C. The NiAl can be considered to be ductile at 950 °C since NiAl always shows plastic deformation along the whole cross section of the sample with a few cracks only observed near fracture surface (Fig. 8(b)). The behavior of PtAl looks similar as in PtAl-PtAl type samples at two temperatures: i.e. to be ductile but also to induce cracks beneath the coating or inside of coating. Figure 8. Fractured NiAl-PtAl sample of morphology near fracture surface for (a) 850 °C and (b) 950 °C. 3.3. Summary of creep fracture mechanism Based on the experimental observations, the crack initiation and propagation mechanisms by which the coating affects the creep fracture behavior are proposed in Fig. 9. For the polycrystalline superalloy grain-boundary separation is always the failure mode in the substrate at 850 °C and 950 °C no matter that the crack initiates from the coating or the substrate. The grain-boundary carbides are beneficial for creep resistance. Due to formation of γꞌ along grain boundary in outer zone (OZ) at 850 °C NiAl coating can induce active through-coating cracks, which may further penetrate into the substrate along the nearest grain boundary beneath the coating. The γꞌ in OZ is somewhat restricted in NiAl at 950 °C and in PtAl at both 850 °C and 950 °C, which promotes ductile deformation occurring in the coatings. Even though, PtAl promotes the formation of the voids/cracks either beneath the coating or from inside of coating, caused by the difference of mechanical properties between different characteristic zones. The formed voids/cracks are more active to cause the sample failure at higher temperature.
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