Fig. 1 Microstructure of the solution-treated HEA. a EBSD image, b BSE image, c APT atom maps with Fe, Co, Ni, Ti, and Al elements, d frequency distribution analysis

Fig. 2 Bright-field TEM images of the HEAs aged for a 0.5 h, b 2 h, c 12 h, d 24 h, respectively. The inset in a is the SAED pattern along the [011] zone axis

Fig. 3 Size distribution of L12 precipitates in the HEAs aged for a 0.5 h, b 2 h, c 12 h, d 24 h, respectively. The red curves are generated by Gauss fitting

Fig. 4 TEM analysis of L12 precipitates in the HEA aged for 24 h. a High-resolution TEM image along the [001] zone axis, b, c FFT patterns of the L12 particle and FCC matrix, d HAADF-STEM image and corresponding EDS maps for Fe, Ni, Co, Al and Ti, respectively

Fig. 5 APT atom maps with 35% at.% Ni iso-concentration surface showing the existence of L12 particles a, c, e, and the proximity histograms b, d, f of different elements after aging for 0.5 h, 2 h, and 24 h, respectively

Fig. 6 Vickers hardness of HEAs after different aging time

Fig. 7 Coarsening kinetic of L12 precipitates in the HEA. The blue line represents the volume fraction, while the red line illustrates the cubic average particle size d(t)3, fitted as a linear function of aging time based on the PV model