13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Experimental procedures Creep samples in the form of sheet were produced from cast of nickel based polycrystalline superalloy Inconel 792 (IN792) with the nominal chemical composition of Ni-12.5Cr-9Co-4.175W-4.175Ta-3.975Ti-3.375Al-1.9Mo-0.08C-0.0175Zr-0.015B (wt.%). A drawing of the specimens is shown in Fig. 1. Two kinds of diffusion coatings - NiAl and PtAl - were deposited onto the specimen surface by the CVD process at 1050-1100 °C for 12-20 hrs. The specimens were solution treated at 1120 °C for 2 hrs and then aged at 845 °C for 24 hrs, followed by air cooling to room temperature. The main phases in the alloy are a solid solution of nickel based γ (disordered FCC) and an intermetallic γꞌ (FCC_L12). Four types of specimens were used in this study (Table 1). Specimens coated with only NiAl or PtAl, and specimens coated with NiAl on one side but PtAl on the other including an uncoated reference. The creep samples were tested under different static tensile loads at 850 °C or 950 °C in air until failure. The tested samples were cut as cross sections in their longitudinal direction (loading direction) and then carefully ground and mechanically polished pre, to study the microstructure below the fracture surface. The microscopic characterization was carried out by using a FEG-SEM Hitachi SU-70 scanning electron microscope (SEM) with an INCA Oxford Instruments energy dispersive system (EDS). Figure 1. Dimensions of the sheet specimens with thickness of 1 mm (in millimeters). Table 1. Sample type definition. Sample type Coating covering One side Another side Uncoated uncoated uncoated NiAl-NiAl NiAl NiAl PtAl-PtAl PtAl PtAl NiAl-PtAl NiAl PtAl 3. Results and discussion 3.1. Creep test and effective coating thickness (ECT) Results from the creep tests are presented as applied stress versus creep life (Fig. 2). The creep lifetime varied largely between 205 hrs and 21000 hrs, depending on the testing temperature and applied stress. As expected, the resistances to creep rupture decreases with increasing test temperature, showing either a shorter creep life under the same applied stress or a lower applied stress for the same creep life. While the PtAl-PtAl at 850 °C shows scattered results and the
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