13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 3.2.1. Uncoated Grain-boundary weakening is generally responsible for the creep failure of polycrystalline materials [16]. In the hot combustion and turbine sections of gas turbine the coarse-grained or even single-crystalline superalloys are commonly used to minimize the effect of grain boundary on creep behavior [1]. For polycrystalline superalloys (e.g. IN792) the addition of few ppm carbon, by forming carbides along grain boundaries, can improve the resistance of grain-boundary sliding [17]. Carbides detached from grain boundary of the superalloy by tensile force were observed by [18,19], which were also observed in the current study in the serrated crack path (Fig. 5(a)). According to the composition measurement by EDS two types of carbides were detected: (Cr,W)-rich M23C6 and (Ti,Ta)-rich MC. M23C6 was only observed along grain boundaries while MC was common both along grain boundaries and inside of the grains. The same microstructures have been reported in IN792 by other researchers [6,20]. The direction of the alignment of cracks is unsurprisingly perpendicular or near-perpendicular to the creep loading direction. Figure 5. Fractured uncoated sample for 950 °C: (a) serrated grain-boundary crack with carbides, (b) morphology near fracture surface and (c) recrystallization near fracture surface. The creep tests were conducted in air; consequently oxidation occurred at the sample’ surface whether it was covered by coating or not. It has been reported that the surface oxide layer could lower the steady creep strain rate comparing with vacuum creep test (without surface oxidation) [21]. In this study oxidation was also present inside of material when the crack initiated from the surface. However such oxidation was reported to have little effect on the steady creep rate of the sample [6,20]. Fig. 5(b) shows the length-sectional morphology near the fracture surface of the uncoated sample. Since the crack propagates along the grain boundary, the roughness of the fracture surface is grain-size dependent. The appearance of recrystallization along the fracture surface indicates dynamic recrystallization caused by local plastic deformation during crack propagation at the creep testing temperature (Fig. 5(c)). Actually such recrystallization was also commonly observed along the fracture surface in other sample types. 3.2.2. NiAl-NiAl Ductile-brittle transition temperature (DBTT) reflects the ductility of materials at high temperatures. The definition of DBTT is mainly based upon the amount of plastic deformation during failure. For instance Lowrie et al. [6,20] suggested the temperature above which the fracture strain of the
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