Effects of Sr on the Aging Behavior of Cast AZ91 Alloys

The effects of Strontium(Sr) additions on the aging behavior of cast AZ91 alloys were investigated with optical microscope,scanning electron microscope and brinell hardness measurements.The results have shown that the addition of Sr slows down the aging kinetics of AZ91 alloy and suppresses the discontinuous precipitation of the β phase.And,with the increase of Sr content,the suppression of the aging processes of the AZ91-Sr alloys becomes more apparent.The peak hardness of the base alloy reaches after 55h while that of the alloy with Sr addition is delayed to 90h or 110h later.The Sr solution content in the α-Mg and the existence of the high-melting point Al4Sr phase at grain boundaries are the main reasons for the suppression of the discontinuous precipitation of the β phrase.The AZ91-0.2Sr alloy obtains better tensile properties at the peak hardness time than the former AZ91 alloy at room temperature and at elevated temperature.