PSI - Issue 68

R.J. Mostert et al. / Procedia Structural Integrity 68 (2025) 351–357 R.J. Mostert et al. / Structural Integrity Procedia 00 (2025) 000–000

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Fig. 7. The secondary cracking as studied using X-Ray Computed Tomography (XRCT) on a sample removed from the gauge length of the tensile specimens, as shown. Image (b) is a virtual section taken of (a).

The secondary surface cracks were observed on both alloys, and the nature and characteristics were found to be similar. High-resolution X-Ray Computed Tomography (XRCT) was used to investigate the cracking (Fig. 7). A small section was cut from the gauge length edge of a tensile sample, some distance from the fracture surface. In this region, the crack widths ranged from 0.01 to 0.1 mm and the depth was in the order of 0.1 mm. Interrupting a tensile test at 2 % of axial strain was sufficient to demonstrate the formation of this surface cracking. The hydrogen degassing heat treatment referred to before (e.g., Table 1 and Fig. 4) did not have any influence in mitigating the occurrence of surface cracking. Sections taken through the regions showing cracking in Zone (iv) were analysed using scanning electron microscopy (SEM) and energy-dispersive X-Ray Spectrometry (EDS) (Fig. 8). It was found that, immediately below and on the exposed surface, scattered small regions of localized corrosion (LC), typically 5 x 10-micron elongated pit-like damage, were observed. In these unetched samples, a grain boundary network, indicating inter-granular corrosive attack (IGC) was evident, up to a depth of ~ 100 micron. EDS-SEM analysis demonstrated that the grain boundary regions showed high levels of oxygen, while a number of associated constituent particles were also observed. EDS analysis showed these to be mostly S -phase constituents.

Fig. 8. (a) Section through surface in Zone (iv), (b) enlarged image at a depth of ~ 100 µm below the surface, with associated EDS scan, for AA 2024 T3.