Abstract: The microhardness, microstructure, and composition of precipitated particles of Al-7.7Li-(0.03Er) (at.%) alloys during single-aging at 200 °C and double-aging at 350 °C/1 h+ 200 °C have been studied using microhardness measurements, transmission electron microscopy (TEM), and atom probe tomography (APT). The microhardness results show that the trace addition of Er to the Al-7.7Li alloy increases the peak hardness from 902 ± 27 to 954 ± 45 MPa in the double-aging treatment and 1066 ± 17 MPa in the single-aging treatment. The TEM results show that spherical L1₂ phase particles are formed and distributed uniformly in the three peak-aged samples with average diameters of 31 ± 6, 28 ± 4, and 22 ± 4 nm, respectively. The APT results show that, for the doubly-aged sample, Al₃(Li_0.5Er_0.5) particles are formed during the first-stage aging, and then covered by Al₃Li phase to form Al₃(Li_0.6Er_0.4)/Al₃(Li_0.998Er_0.002) core/shell structure during the second-stage aging. When aged for 24 h, the core/shell structure compositions become Al₃(Li_0.7Er_0.3)/Al₃(Li_0.996Er_0.004), and the particle volume fraction is 4.9 %. While for the singly-aged sample, the core/shell Al₃(Li_0.7Er_0.3)/Al₃(Li_0.997Er_0.003) structured precipitates are formed in the early stage of aging (10 min) and are changed into homogenous Al₃(Li_0.993Er_0.007) phase structured particles at the aging time of 24 h, with a volume fraction of 5.1 %. The classical strengthening mechanism analysis confirms that the antiphase domain boundary energy, γ_APB, of the homogenous Al₃(Li_0.993Er_0.007) phase is 0.200 ± 0.003 J/m², 33 % higher than that of the Al₃Li phase (0.150 ± 0.008 J/m²) in the Al-Li alloy without Er added, which increases the precipitation strengthening effect in the Al-Li(-Er) alloy.

Key words: Al-Li alloy, Er microalloying, Heat treatment, Atom probe tomography, Antiphase domain boundary energy