PSI - Issue 25

Francesco Leoni et al. / Procedia Structural Integrity 25 (2020) 348–354 Francesco Leoni / Structural Integrity Procedia 00 (2019) 000–000

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Fractographic observations Figure 5 shows selected fracture surfaces of tensile specimens tested at different temperatures. Necking in combination with considerable dimple formation are observed at all four temperatures, indicating that the tensile ductility of the AA6082 filler wire is satisfactory. Moreover, a closer inspection of Figure 5 reveals that the tendency to necking and dimple formation become more pronounced with increasing temperatures. However, this trend is not fully reflected in the corresponding fracture strain data in Figure 3, which are based on elongation measurements and not on observations of the reduction in the cross-sectional area. The reason for this inconsistency is not known. When the temperature is raised to 500 °C the fracture mode becomes completely different, as shown in Figure 6, indicating a change in the failure mechanism. At this temperature necking is no longer noticeable (see Figure 6A). Figure 6B shows a lateral view of the same fracture surface. It appears rough, probably because intergranular slip and grain boundary sliding become predominant at high temperatures, which, in turn, promote separation between grain boundaries when the material is subjected to tensile stress. This type of failure, being associated with negligible necking and intergranular slip, is typical of creep rupture Spigarelli (1999) .

Figure 5: SEM fractographs of tensile specimens tested at 25 °C (A), 100 °C (B), 200 °C (C) and 350 °C (D).