The displacement control on notched specimens shall describe the important thermal strain control in gas turbine blades [9]. The strain rate was 10 3 s 1 for all the tests. Usuallywe xedthelifetimeNf at the number of cycles to 5% stress drop in tension with respect to a decreasing regression straight-line including points behind saturation. Except for the test 2, see table 1, which was performed until rupture and an interrupted one, we stopped the tests at 10% stress drop in tension. Some photomicrographs of the surface of these specimens were taken. Then the specimens were broken at room temperature. Some micrographs of their fracture surfaces were taken in order to characterize the crack growth mechanisms in the singles crystal for such geometries. Experimental Results Metallographic observations: The fractographic observations of near h001i notched specimens (Fig. 1, 2) with a scanning electron microscope (SEM) show two main cracks which occured perpendicular to the external load, but indeed they arose not directely at the notch root, instead they are shifted a little away from there. For a near h011i oriented specimen the main cracks seem to be concentrated at the notch root. The \square shape" of the crack front (Fig. 3, 4) allows us to assume that the cracks do not grow homogeneously in the depth. We drew the projection of the secondary crystallographic axes of the material on the micrographs of the fracture surfaces. The direction [100] seems to be close to the maximum crack growth in the depth. Metallographic observations on a h001i notched specimen with 1 mm notch radius in an interrupted test showed a relatively late initiation of a crack within the lifetime, although the stresses are much higher at the notch surface. This seems to indicate the importance of the situation close to the surface in the single crystal, because when a crack is observable there, then the lifetime is reached not much later, at least at 1 mm notch radius. 500 mµ 200 mµ Figure 1: Notch of the h001i SC16 specimen 3 with 0.2 mm notch radius after N = 250 cycles at 950 o C. 2mm 500 mµ Figure 2: Notch of the h001i SC16 specimen 4 with 1 mm notch radius after N = 810 cycles at 950 o C. 2
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