Study On β-Mg₁₇Al₁₂ Phase Morphology And Mechanical Properties Of As-Cast AZ91D Magnesium Alloy By Three Treatments

The microstructure of as-cast magnesium alloy was inhomogeneous and there were a lot of continual netβ-Mg₁₇Al₁₂ phase on the grain-boundary because of the dendritic segregation, which restricted the application of magnesium alloy. In this paper, AZ91D as-cast magnesium alloy was studied by OM, SEM, TEM ,XRD, EDS, which was treated by slow solidification, solution ageing and homogenizing annealing treatments. As well as Brinell hardness, micro-hardness, tensile and compression tests were used to study the mechanical properties of AZ91D, and the processes of solidification and precipitation were analyzed, the effect of these three methods onβphase was discussed.Because of dendritic segregation, there were a lot of non-equilibrium eutectic structures in as-cast AZ91D magnesium alloy by observing and analyzing, the eutectic morphology included fully divorced-shaped, partially divorced-shaped, lamellar-shaped and fibrous-shaped. By slow solidification, the form ofβ-Mg₁₇Al₁₂ phase gradually changed from the continuous network distribution in cast to intermittent bit or strip distribution along the grain boundary. When the cooling rate was 0.0038 K/s, the volume fraction ofβphase reduced to 2% from 21.3% of as-cast and the grain size increased to 400μm from 100μm, Brinell hardness reduced to 40.7HB from 67.5HB of as-cast.Theβphases were not soluted thoroughly by solution treatment in 380℃50h, when solution in 400℃,420℃and 450℃, the time of solution thoroughly were 24h,16h and 4h. Cooling in cold water, the alloy was fractured.There were continuous and discontinuous precipitates while ageing in 200℃, but only discontinuous precipitates in 250℃ageing. There was non-precipitation region in 200℃ageing 4 hours. The extent of solution homogenization affected the morphology after aging,βphase precipitated around the non-solutionβas lamellar-shaped while ageing in 200℃after 420℃8h solution,βphase precipitated as diamond-shaped while ageing in 200℃after 420℃16h solution. There were lamellar-shaped organizations in ageing, and the lamellar-shaped growed in ((2|-)20(1|-) )_α.The eutecticβphases disappeared, and theβphases precipitated again during cooling with furnace.βphases wouldn't precipitate evenly when 380℃50h, while the time ofβphases precipitating evenly were 34h, 24h and 8h. With the homogenization increasing, the morphologies ofβphases transformed to evenly lamellar from the needle-point at the grain boundary or primaryβphases. There wasn't special orientation relationship betweenβphases and matrix and there was no rule about the direction ofβphase's growth.The brinell hardness reduced to 50HB from 68.5HB and the tensile strength and compression strength increased to 220MPa and 274MPa from 173MPa and 221MPa, while elongation improved to 5.9% form 2% by solution treatment. The fractography demonstrated that brittle fracture has transferred to quasi-cleavage fracture with plastic deformation. After ageing treatment, the brinell hardness increased to 81.6HB, the tensile strength increased to 256MPa, the elongation decreased to 3%. The fractography demonstrated the typical brittle fracture feature. After homogenizing annealing treatment, the brinell hardness increased to 78.9HB, the tensile strength and compression strength increased to 239MPa and 306MPa, elongation was 10%. There were lamellar morphologies in the fractography.