Effect Of Precipitation Morphology Of β Phase On Microstructure And Mechanical Properties Of AZ91D Magnesium Alloys

The microstructure of as-cast magnesium alloys was inhomogeneous and there were a lot of continual netβ-Mg₁₇Al₁₂ phase on the grain boundary because of the dendritic segregation, which restriced the application of magnesium alloy. In this paper, AZ91D magnesium alloys was studied by OM, SEM, EDS, EPMA, which was treated by solution aging and two-step homogenization treatment. As well as Brinell hardness , tensile test were used to study the mechanical properties of AZ91D, the effect ofβ-Mg₁₇Al₁₂ phase on mechanical properties of AZ91D was discussed.The coarse and brittleβ-Mg₁₇Al₁₂ phase distributed around grain boundaries of as-cast AZ91D were almost dissolved into matrixα-Mg thoroughly after the solution treatment. Prolonging solution time, the distribution of alloying elements in the matrix approached to a uniform state due to the diffusion mechanism, and grains also grew up gradually. Prolonging solution time, the quantity ofβ-Mg₁₇Al₁₂ phase distributed around grain boundaries increased gradually after the aging treatment at 200℃for 16h. Continuously began precipitate at the inner of grains at 415℃for 10h, which inhibit the discontinuous precipitating on grain boundary and the quantity ofβ-Mg₁₇Al₁₂ phase distributed around grain boundaries reduced. Continue to extend the time, continuous precipitation at the inner of grains increased meanwhile coarsened and grew up.β-Mg₁₇Al₁₂ phase mainly continuous precipitated at the inner of grains because of alloying elements in the matrix distributed uniform state at 415℃for 24h, which was extremely beneficial for improving mechanical properties of AZ91D .Tensile strength appeared two peaks after solution and aging treatments. The tensile strength of AZ91D was increased 22.2% after solution treated at 415℃for 10h followed by aging treatment at 200℃for 16h compared with that of the as-cast, while increased 34 % at the same temperature for 24h. Also the corresponding hardness was improved 35.8% and 34.7%, respectively. Because the grain size of AZ91D magnesium alloy ingot is larger, which needed higher temperature and longer time when elements distributed uniform. The hardness and tensile strength was improved 29.49%,24.04% and 31.07%,18.91%after solution treated at 430℃for 28h and 445℃for 22h followed by aging treatment at 220℃for 8h.There has major influence on forming supersaturated organization and precipitation feature ofβ-Mg₁₇Al₁₂ phase after solution treatment by quenching mode. Temperature severely diminished and the cooling speed was fast at cold water cooling, which made alloy supersaturated and also freezed down the space of the high temperature. The bigger supersaturate was and the more the space were, the more the quantity ofβ-Mg₁₇Al₁₂ phase and the shorter shape and layer piece spacing were, which was extremely beneficial for improving mechanical properties of AZ91D. Prolonging solution time, the quantity ofβ-Mg₁₇Al₁₂ phase distributed around grain boundaries increased gradually after the aging treatment at 200℃before 8h. Discontinuous precipitation were reduced at 16h andβ-Mg₁₇Al₁₂ phase began continuously precipitation at the inner of grains, which made alloy hardness reaching peak. Continue to extend the time, the grain began growing up. Improving aging temperature to 220℃, continuous precipitation and the peak of hardness appeared ahead, the hardness of alloy reached the highest at 8h.With the two mechanism of energy and concentration fluctuation working together, theβ-Mg₁₇Al₁₂ phase discontinuously precipitated in grain boundaries in the form of fine strip shape, while continuously precipitated at the inner of grains with granular or short rod shape during the aging process after solution treatments. The size of precipitatedβ-Mg₁₇Al₁₂ phase greatly reduced after solution and aging treatments compared with that of the as-cast. The width and space of fine stripβ-Mg₁₇Al₁₂ phase discontinuously precipitated in grain boundaries were less than 400nm, and the mechanical properties of AZ91D were improved dramatically especially when the width and space of granular or short rodβ-Mg₁₇Al₁₂ phase continuously precipitated at the inner of grains were less than 400nm.After homogenizing annealing treatment, the net segregationβ-Mg₁₇Al₁₂ phase dissoluted intoα-Mg and the lamellar shapeβ-Mg₁₇Al₁₂ phase precipitated again during the homogenizing annealing cooling process. Prolonging solution time, the quantity ofβ-Mg₁₇Al₁₂ phase increased gradually, the size and lamellar spacing became shorter, which was extremely beneficial for improving mechanical properties of AZ91D.β-Mg₁₇Al₁₂ phase began growing up and gathered after 28h, the size and lamellar spacing became wider. After aging treatment, the quantity ofβ-Mg₁₇Al₁₂ phase became larger, the size became shorter and distributed more intensive, which was extremely beneficial for improving mechanical properties of AZ91D. The hardness and tensile strength was improved 25.58%,29.54% and 25.64%,27.56% after two-step homogenization and two-step homogenization followed by aging treatment at 430℃. And the hardness and tensile strength was improved 26.96%%,30.98% and 26.28%,26.92% at 445℃。 The tensile fracture of alloy was scanned analysis. The results show that inter-granular fracture of as-cast became trans-granular fracture after solution treatment. After aging treatment, the fracture morphology became brittle fracture. But there were more the tear slice and the cleavage steps on the tensile fractures after homogenizing annealing treatment, and Performancing trans-granular fracture which needed higher energy.