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 degrees C results in coarser and more spaced eta phase particles at grain boundaries with wider precipitates free zone, and lower density of larger eta' hardening precipitates inside grains after aging. Isothermal holding at 355 degrees C results in heterogeneous precipitation of eta phase both on dispersoids and at grain boundaries. Isothermal holding at 235 degrees C results in heterogeneous precipitation of eta phase first, and then S phase. Precipitates free zones are created around these coarse eta and S phase particles after aging. Prolonging isothermal holding at these two temperatures leads to fewer eta' hardening precipitates inside grains, larger and more spaced eta phase particles at grain boundaries and wider grain boundary precipitates free zone after aging.