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The effect of step-quenching on the microstructure of aluminum alloy 7055 after artificial aging was studied by hardness testing and transmission electron microscopy (TEM). Step-quenching leads to decomposition of solid solution and heterogeneous precipitation of equilibrium phase mainly on dispersoids and at grain boundaries; thus lower hardness after aging. Prolonging isothermal holding at 415 ℃ results in coarser and more spaced η phase particles at grain boundaries with wider precipitates free zone, and lower density of larger η′ hardening precipitates inside grains after aging. Isothermal holding at 355 ℃ results in heterogeneous precipitation of η phase both on dispersoids and at grain boundaries. Isothermal holding at 235 ℃ results in heterogeneous precipitation of η phase first, and then S phase. Precipitates free zones are created around these coarse η and S phase particles after aging. Prolonging isothermal holding at these two temperatures leads to fewer η′ hardening precipitates inside grains, larger and more spaced η phase particles at grain boundaries and wider grain boundary precipitates free zone after aging.