The Influence Of Mg₁₇Al₁₂ Phases On Microstructure And Mechanical Properties OfAZ91 And SiCp/AZ91 Composites

In this paper,5μmSiCp/AZ91 composites with different SiCp volume fractions were prepared by semi-solid stirring casting technology. As-cast (containing much Mg17Al12 phase) and solution treatment cast (containing a little Mg17Al12 phase) AZ91 alloy and composites are used for hot extrusion. The effect of Mg17Al12 phase on microstructure and mechanical properties of alloy and composites were investigated. Besides, the microstructure evolution and mechanical properties of SiCp/AZ91 composites during aging process at 200℃ were investigated. By comparing with the AZ91 alloy, the influence mechanism of SiCp on the ageing precipitation of Mg17Al12 phase was revealed.The research results show that the larger amounts of Mg17Al]2 phase in direct extrusion AZ91 alloy results in finer grain size and weaker basal plane texture. Thus the combined effect of grain refinement strengthening and texture strengthening may be responsible for the almost same yield strength (YS) of direct extrusion alloy and solution treatment extrusion alloy. The reduced amount of Mg17Al12 phase in solution treatment extrusion alloy is beneficial to reduce the crack source, making the ultimate tensile strength (UTS) and elongation higher than that of direct extrusion AZ91 alloy.After direct extrusion of as-cast composite, Mg17Al12 phases are cracked to fine particles and they distribute uniformly at the newly formed DRXed grain boundaries, while there are very little Mg17Al12 phases in solution treatment extrusion composite. Due to finer grain and larger amounts of Mg17Al12 phase, the UTS and elongation of direct extrusion composite are higher than that of solution treatment extrusion composite.Compared with the direct extrusion matrix alloy, adding a small amount of SiCp (volume fraction of 2%) can make the grain size of matrix refiner. When the SiCp volume fraction is 10%, the grain size of composite is minimum. However, when the SiCp volume fraction increased to 15%, the grain size of composite increases instead. With the increasing of SiCp volume fraction, YS and elastic modulus of SiCp/AZ91 magnesium matrix composites increase continuously, while the elongation decreases.When SiCp volume fraction increased to 10%, UTS of SiCp/AZ91 magnesium matrix composites reached maximum. When the SiCp volume fraction further increased to 15%, UTS decreases instead.Compared with the ageing AZ91 alloy, adding a small amount of SiCp (volume fraction of 2%) can make discontinuous precipitation Mg17Al12 phase decrease while continuous precipitation phase in grain increase. With the increasing of SiCp volume fraction, discontinuous precipitation Mg17Al12 phase reduce further and continuous precipitation phase in the interior of the grains become more dense. Compared with solid solution state SiCp/AZ91 magnesium matrix composites, the YS and UTS of ageing state improved because of the discontinuous precipitation and continuous precipitation Mg17Al12 phase in the process of ageing; When SiCp volume fraction increased from 0% to 10%, the discontinuous precipitated Mg17Al12 phase in composite is easy to generate crack origins, reducing the elongation of ageing composite; However, when SiCp volume fraction is 15%, the second phase is almost layer sheets continuous precipitation of Mg17Al12 phase, which is not easy to produce crack initiation and can hinder the expansion of the crack. As a result, the elongation of the ageing composite is higher than that of solid solution state composite. In addition, compared with the ageing composites, when SiCp volume fraction is 2%-10%, the elongation and UTS of direct extrusion SiCp/AZ91 composites are higher.