New evidence on the nature of the metastable S″-phase on Al–Cu–Mg alloys

Results obtained from high resolution electron microscopy (HRTEM), the resultant fast Fourier transform (FFT) and the simulated electron diffraction patterns (SEDP) presented in this paper provide a better understanding that suggests that there is no S″-phase and that the effects attributed to S″ are explained by consideration of the various variants of the equilibrium S phase on Al–Cu–Mg based alloys (based only on HRTEM analysis). From the 12 orientation variants, only 3 orientations between the matrix and S-phase precipitates were taking into consideration in this study. It was found that the metastable S″-phase could be explained as an orientation variant of the equilibrium S-phase (Al2CuMg). It is worth mentioning that the third variant has not been reported in the literature using HTREM, therefore, the incorporation of this variant in the analysis provides an alternative explanation for the experimental evidence previously used as support for the S″-phase.

► New evidence on the non-existence of the metastable S″-phase on Al–Cu–Mg based alloys.
► The introduction of Variant III is for the first time experimentally observed.
► The combination of variant III and the “two overlapping networks method” proved to be very useful, for clarifying the non-existence of this S″-phase.
► The S″-phase in Al–Cu–Mg alloys might be seen just as a contrast effect of the interference of Al and S-phase lattices.