13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- NiAl-PtAl has only two data points, a linear relationship between the applied stress and creep life, both plotted in logarithm scale, could be found for the other coated samples. Figure 2. Creep lifetimes of specimens under different applied stresses (normalized) at 850 °C and 950 °C. Based on their distinctive metallographic features, three characteristic zones are defined for the coatings (Fig. 3): outer zone (OZ), interdiffusion zone (IDZ) and second reactive zone (SRZ). The matrix phases in OZ, IDZ, SRZ and the substrate are respectively β, γꞌ (or β), γꞌ and γ. After long creep time at 900 °C the OZ shrinked in NiAl (Fig. 3(a)) but was well stabilized in PtAl (Fig. 3(b)), which indicates that Pt can restrict the β-to-γꞌ transformation by blocking the outward diffusion of Ti, Co, W and Ta. The enrichment of Cr, Mo and Co in those zones was responsible for the formation of some specific precipitates (i.e. TCP, Cr- ) which were observed in both NiAl and PtAl coatings. The phase identification in this research is achieved by comparing the EDS-detected composition with those given in literatures [1,9-11]. A more detailed analysis of the microstructural changes in the PtAl-PtAl samples could be found in [12]. Figure 3. Microstructure and composition profiles (averaged from three different areas) on (a) NiAl side and (b) PtAl side in a NiAl-PtAl sample after creep test at 950 °C with lifetime 10244 hrs. The micro-hardness tests of cross sections at room temperature show a variation along the diffusion coatings (Fig. 4(a)), verifying the difference in mechanical properties between coating and substrate. This means that the stress partitioning needs to be recalculated by considering the newly developed coating thickness. To encompass the growth of coating due to the diffusion of alloying elements
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