Effect of step-quenching on microstructure of aluminum alloy 7055

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 °C 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 °C results in heterogeneous precipitation of η phase both on dispersoids and at grain boundaries. Isothermal holding at 235 °C 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.