13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- The displacement rate has a small effect on the mechanical behavior. The main difference is that at 470 °C the four tests at 2 mm/min gave a brittle behavior while at 0.2 mm/min two tests showed a brittle behavior the other two giving a ductile response. At temperature of 475 °C, there is a slight change in curve shape with different displacement rates. The most significant change is the shape of the curve after the peak force was reached. At lower displacement rate, the decrease is more gradual compared to the slope at 2 mm/min. This behavior is reasonable, because at 0.2 mm/min, the alloy has more time to compensate the deformation. 3.2 Fracture surface analysis To analyze the behavior during fracture, we performed SEM analysis of the fracture surfaces at four different conditions, i.e. at two deformation rates and at solid fractions of 0.99 and 0.94, reflecting the transition from brittle to ductile behavior (see Figures 3–5). From all of the fracture analysis performed, we found that in general, the fracture mode is mixed between inter-granular (with dendritic morphology) and intra-granular. We observed such a mixed fracture mode at both solid fractions and displacement rates (figure 7). Moreover, features that possibly attest for broken dendrite arms (features within the dashed white ellipses in Figure 8a) were found for all conditions. Figure 7. Examples of mixed-mode fracture surface at fs = 0.99 with 0.2 mm/min displacement rate (a) and fs = 0.94 with 2 mm/min displacement rate (b). The higher solid fraction (fs = 0.99) and lower displacement rate (0.2 mm/min) shows deformation of the solid and interdendritic liquid phases as evidenced in Figure 8b. While with a higher displacement rate (2 mm/min) and same solid fraction the interdendritic liquid phases have different shape (Figure 8c). The shape differences could be explained as the interdendritic liquid has more time to deform and rearrange at lower displacement rate compared to the higher one. At lower solid fraction (fs = 0.94), for both displacement rates, the interdendritic liquid raptures [11] (features within the dashed white ellipses in Figure 8d) and the solid bridges fracture in a brittle fashion as illustrated in Figure 8d. Interestingly, such interdendritic liquid features are rarely found at the higher solid fraction.
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