RESULTS AND DISCUSSIONS The results to be discussed in this section are for CT specimens with a fixed content and uniform distribution of precipitates. Two cases are examined: an alloy with a precipitate volume fraction of 68%, which corresponds to that of CMSX4 at the start of life, and an alloy with a volume fraction of 58%, which is a typical volume fraction found near crack faces after long-term exposure to an oxidising environment. In on-going work, the effect of time and spatial evolution of the γ’ volume fraction and size on the deformation and stress state is being studied. 1 2 3 0.00 3.00E-3 5.83E-3 8.67E-3 1.15E-2 1.43E-2 1.72E-2 2.00E-2 4.89E-1 20 m [010] [100] [001] (a) (b) f = 0.68 p V V 1 2 3 0.00 3.00E-3 5.83E-3 8.67E-3 1.15E-2 1.43E-2 1.72E-2 2.00E-2 7.62E-1 [010] [100] [001] f = 0.68 20 m p V Figure 5: Contributions to the overall accumulated inelastic strain around the notch tip from the (a) octahedral and (b) cubic slip systems for the vf =0.68 case shown in Fig. 3 Figures 4 to 6 show results for the CT specimen loaded with a stress intensity factor of 10 MPa √m at 850 ◦C. In Fig. 4, contours of accumulated inelastic strain are presented for (a) vf = 58% and (b) vf =68%after 320 hours exposure. The contours are shown at the central notch root region of the specimen where fracture is expected to initiate. It is worth noting the greater magnitude and extent of the accumulated overall slip for the vf = 58% case, which is a considerably softer material (see Fig. 1). The contributions to the overall accumulated inelastic strain around the tip of the sharp notch for the vf = 68 % case (see Fig. 4(b)) from the octahedral and cubic slip system families are shown in Figs. 5 (a) and (b), respectively. Since the specimen is loaded along the [010] direction, one would expect the octahedral slip systems to be dominant, as it is the case under uniaxial <001 >(homogeneous) deformation. However, Fig. 5 reveals that the contribution from the cubic slip systems is equally important. This is a result of the multiaxial nature of the stress distribution near the notch region. In Figs. 6(a) and (b), the distribution of the stress component normal to the crack faces, σ010, and the mean hydrostatic stress, σh = (σ100 +σ010 +σ001)/3, respectively, are given in terms of the distance from the tip of the notch and the γ’ volume fraction. The peak normal stress in Fig. 6(a), found at approximately 23 µm from the sharp notch root, decreases by 34% for a 10 % decrease in the precipitate volume fraction. Thus, the crack driving force for cleavage fracture is considerably reduced. It is known that the rate of growth of voids within the single crystal at high temperatures depends on the magnitude of σh and the local slip rates. Therefore, from the results of Figs. 4 and 6(b), the evolution of ductile damage could be inferred. 5
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