13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- c)! b)! a)! Load direction! 2 mm! 1 mm! 2 mm! Figure. 3. Deformation bands on specimen surfaces, a) the 〈011〉 specimen subjected to 750°C in compression (OP), b) the 〈011〉 specimen subjected to 950°C in tension (IP), and c) the 〈001〉 specimen subjected to 950°C in tension (IP). After cutting the specimens parallel to the loading direction all specimens were investigated by SEM. Fig. 4 provides a backscattered electron image where the deformation bands inside the specimen are clearly visible at low magnification. However, at higher magnification the crystallographic deformation bands are often difficult to detect. The SEM investigation shows that the deformation bands are neither twinning nor shearing of the γ/γʹ′-microstructure. Sometimes precipitation of topologically-close-packed (TCP) phases are visible within the deformation bands. Even though most deformation bands are difficult to detect at higher magnification, occasionally more distinct deformation bands are visible, see Fig. 5 for such a distinct deformation band. Most likely, both types of deformation bands are bundles of glide bands. Other than the crystallographic deformation bands, the specimens show very little deformation within the γ/γʹ′-microstructure. However, small amounts of twins are detected in the microstructure for the 〈001〉 specimen subjected to compression at 750°C, see Fig. 6. Here parallel twins propagates through the γʹ′-cuboids. Rafting of the γ/γʹ′-microstructure is visible in specimens subjected to holdtimes at 950°C. The rafting direction is dependent on whether the superalloy has a negative or positive misfit between the γʹ′- and γ-microconstituents, and if the loading is tensile or compressive. The alloy used in this study (MD2) has a negative misfit which implies that tensile loaded specimens show a N-type of rafting, while material loaded in compression show a P-type of rafting. Fig. 7 displays N-type of rafting; the previously cuboidal γʹ′-particles are here coarsened and oriented Normal to the load direction. On the other hand, Fig. 5 shows P-type of rafting where the coarsened γʹ′-particles are oriented Parallel to the load direction.
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