Influences of Heat Treatment on Microstructure of AZ91D Magnesium Alloy

The microstructure of as-cast, solution-treated and aged AZ91D magnesium alloy was studied. The influences of heat treatment on microstructure and microhardness of the alloy were analyzed. The results show that the thermal stability of laminar α phase and blocky β phase are not good, but can be eliminated by solution treatment at 380 ℃ and 410 ℃ respectively. Al-Mn phases, on the other hand, are the morphologies of granule and aciculae with the dimension of 10 to 50 μm, and keep almost unchanged during solution treatment (410 ℃× 24 h). A large amount of second phases in the forms of rod with dimension of 10 μm, ф1~ф2 μm are precipitated after aging at 200 ℃ for 12 h, and are perpendicular to or crossed with the matrix. Such precipitates will change into the ones which are mainly parallel to the matrix when aging temperature increases further. As a result, the strengthening effect of precipitates declines.