This paper was developed under a SBIR contract. The effect of Sc additions on precipitation strengthening in AI-Zn-Mg-Cu alloys was studied after natural and artificial aging. Phase analysis was performed using transmission electron microscopy and computer simulations. Microhardness of the alloys was studied after different steps of aging and the strengthening mechanisms were discussed. It was found that Sc does not affect the kinetics of aging at room temperature, which ...