Study On The Microstructures And Properties Of AZ91D-M Magnesium Alloys

In order to broaden the usage of magnesium alloys, the effects of alloying and heat treatment on the microstructures, mechanical properties, damping capacity and corrosion resistance of AZ91D-M magnesium alloys were studied. Meanwhile, the effects of rapid solidification on the microstructures of AZ91D and Mg-Zn-Y alloys were studied. Microstructures and phase compositions of the alloys were observed by using OM, SEM, TEM and XRD, respectively. The fractographs of the alloys at ambient temperature were observed by using SEM. The tensile properties and hardness of the alloys at ambient temperature were tested. Damping capacity of the alloys was tested by cantilever beam technique. Corrosion resistance of the alloys was tested by weight loss and potentiodynamic polarization curve methods, respectively. The main results are as follows:(1) Element Ce can refine as-cast microstructures of AZ91D alloy and improve its tensile strength and hardness at ambient temperature. The mechanical properties of AZ91D-0.7%Ce alloy are the best, whose tensile strength, yield strength, elongation and hardness are 201.0MPa, 112.1MPa, 1.9% and 71.5, respectively. The damping capacity of AZ91D-0.7%Ce alloy is 2.728 10-3 that is 61% increment compared to that of AZ91D alloy. The corrosion rate of AZ91D-0.1%Ce alloy is 0.35mg/(cm2-d) that is 75% reduction compared to that of AZ91D alloy.(2) Element Y can refine as-cast microstructures of AZ91D alloy and improve its damping capacity. The damping capacity of AZ91D-1.0%Y alloy is 2.306 10-3 that is 36%increment compared to that of AZ91D alloy. The corrosion rate of AZ91D-0.3%Y alloy is 0.66mg/(cm2-d) that is 53% reduction compared to that of AZ91D alloy.(3) Element Si can refine as-cast microstructures of AZ91D alloy and improve its damping capacity. The damping capacity of AZ91D-0.7%Si alloy is 2.055 10-3 that is 21% increment compared to that of AZ91D alloy. The corrosion rate of AZ91D-0.7%Si alloy is 0.56mg/(cm2-d) that is 60% reduction compared to that of AZ91D alloy.(4) Element Ca can refine as-cast microstructures of AZ91D alloy and improve its tensile strength and elongation at ambient temperature. The mechanical properties of AZ91D-0.7%Ca alloy are the best, whose tensile strength, yield strength, elongation and hardness are 185.7MPa, 101.5MPa, 2.9% and 69.4, respectively. The damping capacity of AZ91D-0.1%Ca alloy is 2.960 10-3 that is 74% increment compared to that of AZ91D alloy. The corrosion rate of AZ91D-0.3%Ca alloy is 0.56mg/(cm2-d) that is 60% reduction compared to that of AZ91D alloy.(5) T4 heat treatment can improve the tensile strength and elongation of the alloys and has little influence on the yield strength and hardness. T6 heat treatment can improve the tensile strength and hardness of the alloys, reduce the elongation and has little influence on the yield strength. T6 heat treatment can improve the damping capacity of the alloys. The damping capacity of T6 treated AZ91D alloy is 2.106 10-3 that is 24% increment compared to that of as-cast state. Heat treatment improves the corrosion rate of the alloys, except T6 treated AZ91D alloy.(6) The damping behavior of AZ91D-M alloys can be explained by G-L dislocation pinning theory. The improvement of the volume fraction and reticular degree of phase can improve the corrosion resistance of the alloys.(7) Microstructures of the cross section of the ribbon of rapidly solidified AZ91D alloy are composed of three regions, which are coarse equiaxed grain region on the copper roll side, fine equiaxed grain region on the free surface and the sandwich columnar grain region. The microstructure is supersaturated a-Mg solid solution along the whole section. Quasicrystalline phase Mg3oZn6oYio appears in the microstructures of conventionally solidified Mg-Zn-Yalloys, while it is not found in the microstructures of rapidly solidified Mg-Zn-Y alloys. The rapidly solidified Mg-Zn-Y alloys' microstructures are a-Mg and MgZn phases of 60nm to 100nm in grain size.