Influence of furnace cooling time on precipitation of β-Mg17Al12 phase of AZ91 magnesium alloy

The annealing treatment of AZ91 cast magnesium alloy was carried out at 415 degrees C for 24h and the alloy was then slowly cooled to room temperature in furnace. As a results, the coarse divorced eutectic phase distributed along the grain boundary dissolve into Mg matrix during the isothermal process, and the thin lamellar beta-Mg17Al12 phase precipitates from the magnesium solid solution with a type of pearlite precipitation during furnace cooling. During the cooling process of annealing treatment, the number of beta phases as perlite-type were increased with the furnace cooling time. In the early stage of furnace cooling, the beta phase nucleated preferentially in the grain near the boundary, only a small quantity of beta phase precipitates. In the subsequent cooling process, the rate of precipitate about beta phase increased remarked as the time of furnace cooling prolonging, the number of beta phase increases dramatically, and the lamellar beta phase evenly distributes inside most of the grain.