Fig. 1 Typical ADF-STEM images of the precipitate microstructure in the conventional Al-Cu-Mg alloy aged at 180 °C for a 0.5 h, b 10 h of peak-aging time, c 50 h, d 75 h. e-h Local magnification of a, b, respectively, further showing the type and size of the formed precipitates in the matrix. Viewing direction: [001]Al crystallographic zone axis

Fig. 2 Typical ADF-STEM images viewed along the [001]Al direction showing the precipitate microstructure in Al-Cu-Mg-Sc alloy aged at 180 °C for a 0.5 h, b 25 h of peak-aging time, c 50 h, d 75 h. e-h Local magnification of a, b, respectively, further showing the type and size of the formed precipitates in the matrix. (i) and (j) Low-magnification and locally enlarged ADF images acquired from the peak-aging Al-Cu-Mg-Sc alloy subjected to the secondary solution and peak-aging treatment

Fig. 3 Low-magnification SEM-EDS mappings and the associated EDS spectra on the right a, b showing the existing mode of Sc in the peak-aged Al-Cu-Mg-Sc alloy, as well as the schematics revealing the mechanism underlying the distinct difference of precipitate formations in Al-Cu-Mg c, d Al-Cu-Mg-Sc e alloys. The elemental mappings shown in different colors are acquired from the green rectangular region in the SEM micrograph. Insets in the spectra of a, b show the corresponding elemental content of the spectrum. The orange Miller planes of (120)Al and (100)Al shown in c, d indicate the positions of Cu layers in early-stage GP zones

Fig. 4 Size distributions of the precipitates in the peak-aged alloy samples. a Rod-like S phase in the Al-Cu-Mg (10 h) and needle-like GPB zone in the Al-Cu-Mg-Sc (25 h). b Plate-like θ’ phases in the Al-Cu-Mg (10 h). The insets in the figure schematically show the morphology and size of the different precipitates

Fig. 5 Experimental comparison of the corrosion resistance between peak-aged Al-Cu-Mg and Al-Cu-Mg-Sc alloys. a Schematic of the electrochemical testing setup. b Potentiodynamic polarization curves of two different alloys studied. Inset shows the extracted values of Ecorr and Epit. c, d Typical current transient records for the Al-Cu-Mg and Al-Cu-Mg-Sc alloys, respectively. Both samples were held potentiostatically at - 700 mV SCE during the current transient collection. Insets present local magnifications of corresponding single metastable pitting events

Fig. 6 Strain distribution at the interface between precipitates and Al matrix in the peak-aged Al-Cu-Mg a-d and Al-Cu-Mg-Sc (e-h) alloys. a-d Atomic-resolution ADF image of S and θ’ phases, matrix lattice distortion of εxx, εxy and εyy surrounding the precipitates, respectively. e-h Atomic-resolution ADF image of GPB zone, matrix lattice distortion of εxx, εxy and εyy surrounding the precipitates, respectively. Two vertical g{200}Al vectors were used for the lattice-distortion measurement. The lattice strain fields in the ADF images were calculated in relative to the strain-free Al area and shown in the corresponding images. The color scale represents the strain variations, and the positive value corresponded to the tensile strain and vice versa

Fig. 7 Cross-sectional examination of intergranular corrosion by optical microscopy a, c and the corresponding PFZ-widths by ADF observations b, d in peak-aged samples: a, b Al-Cu-Mg alloy; c, d Al-Cu-Mg-Sc alloy

Fig. 8 Hardness and electrical conductivity vs. aging time curves of Al-Cu-Mg alloys without and with Sc upon aging at 180 °C a, b and the engineering stress-strain curves of the peak-aged Al-Cu-Mg and Al-Cu-Mg-Sc, as well as solid solution-treated Al-Cu-Mg-Sc alloys c