Multiple twins of a decagonal approximant embedded in S-Al2CuMg phase resulting in pitting initiation of a 2024Al alloy

The pitting of Al–Cu–Mg alloy is believed to originate from the local dissolution of S-Al2CuMg particles, and the dissolution activity differs from one particle to another. Nevertheless, the initial site where the dissolution of the S phase preferentially occurs and the cause of the heterogeneity in the electrochemical dissolution activity remain unknown, hindering our understanding of pitting initiation of Al–Cu–Mg alloys. In this work, we have applied in situ ex-environmental transmission electron microscopy and identified a large number of nanosized Al20Cu2Mn3 approximants of the decagonal quasicrystal embedded in the S phase. We find that the S phase with Al20Cu2Mn3 inclusions is more active than those free of the approximants. Such a preference is clarified to result from the decomposition of Al20Cu2Mn3 approximant prior to the dissolution of the S phase. In addition, we also find that the electrochemical behavior of Al20Cu2Mn3 approximants is different. The approximants with multiple twins are more active than those with few planar defects.