13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3. Results Figure 2 shows some typical stress-strain curves of the specimens without and with liquid gallium on the surface. The test temperature is shown in the figure. Below 400 K, liquid gallium induces a significant reduction of the maximum stress max and the fracture strain f, while at 465 K liquid gallium has no effect on max and no reduction of f is observed. Around the transition temperature (413 K and 423 K in the figure), in the specimens with gallium the stress drops abruptly after the stress-strain curve reaches the maximum, while the decrease of the stress is gradual in the specimens without gallium at all temperatures measured and in the specimen with gallium at 465 K. In Fig. 3 the maximum stress max and the fracture strain f are shown as a function of test temperature T. When the specimen is pulled without gallium, the maximum stress decreases with temperature, while the fracture strain is almost constant. Since the specimen was immersed in liquid nitrogen before the test to prevent supercooling of gallium, embrittlement is not observed below the melting temperature Tm of gallium (303 K). Above Tm the maximum stress and the fracture strain are reduced significantly by liquid gallium. The brittleness disappears above 420 K. This means that ductility trough starts at the melting temperature of gallium and ends around 420 K. Figure 4 shows the SEM images of the fracture surfaces and the optical micrographs of the tensile specimens. When the test temperature is below 413 K, the fracture surface is apparently intergranular, although the surface is partially covered with liquid gallium, and the optical micrograph shows that the fracture surface is nearly perpendicular to the tensile axis. At 465 K, necking is observed in the specimen. The fracture surface is covered with liquid gallium. This Figure 3. Temperature dependence of maximum stress max and frcture strain f of silver specimens tested with liquid gallium (solid circles) and without gallium (open circles). Tm indicates the melting point of gallium.
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