As-cast Microstructures And Mechanical Properties Of AZ31 Based Alloys With Additions Of Cd, Sb Or RE

It is well known that magnesium alloys have many excellent and unique properties, such as low density, high specific strength and stiffness,perfect damping capacity and shock energy absorption capacity,easy processing, as well as a good recycling capacity and compatibility with environment,and the resource of the magnesium is very abundant in the nature.Therefore, it is believed that magnesium alloy can be the most promising green material of the 21st century.But because of its poor plastic deformation ability, low yield stress and fracture toughness, the use of magnesium alloys is limited for a long time.AZ31 alloy is one of the most useful deformation magnesium alloy.In the present work,AZ31 based alloys with additions of Cd, Sb and RE have been investigated,in order to improve mechanical property of AZ31 alloy.In this dissertation,the effects of Cd, Sb and RE additions on the microstructure and mechanical property of AZ31 magnesium alloy are systematically studied by optical microscope (OM), scanning electron microscope (SEM), X-ray diffractometer (XRD), energy dispersive spectrometer(EDS), microhardness, tensile test,impact toughness etc. The main research work and conclusions are presented as follows:1. The small addition of Cd can decrease the crystal grain size of magnesium and AZ31 magnesium alloy remarkably,the more Cd,the less crystal grain size. The mechanism of grain refinement of Cd on Mg and AZ31 alloy is attributed to Cd on the solid/liquid interface front during soildification strongly restricts theα-Mg grain growth. When 0.7% Cd is added,the microstructure of AZ31 alloy is made up of theα-Mg matrix riched Al and the uniformly distributed ofβ-Mg17Al12 .Cd dissolved into matrix of AZ31 alloy and no new phase is formed.The impact toughness and Brinell hardness of experimental alloy increased 68.6%,10.3% respectively.2. When Sb is added, the microstructure of magnesium and AZ31 magnesium alloy are refined effectively,the phase ofβ-Mg17Al12 become more uniformly distributed,and new phase Mg3Sb2 is formed.Mg3Sb2 has the same close-packed hexagonalst ructuer asα-Mg,which conformed to the fundamental of heterogeneous nucleation,and is the favorable nucleating center.Mg3Sb2 precipitation,which has the higher melting point, is formed before any other phases and restrained the growth of the grain andβphase.The small addition of Sb can increase the mechanical properties of AZ31, the impact toughness and Brinell hardness of experimental alloy increased 58.8% and 4.8% respectively.3.The microstructure of magnesium and AZ31 magnesium alloy are refined by RE addition. RE are combined with Al form Al11RE3(Al11Ce3) needles near the grain boundaries. The role of grain refinement of RE is that, on the one hand, RE enriched on the solid/liquid interface when concreting,this will increase the undercooling,thereby block the growth of grain; on the other hand, the formation of Al11RE3 restrained the growth of the grain,decreased the Al solution and theβ-Mg17Al12 phase in alloys. The addition of RE can increase the Brinell hardness of AZ31, but reduce the impact toughness .4. The effect of cadmium, antimony and rare earth,in different groups with orthogonal design,additions on the microstructure of AZ31 are different.Through the analysis, that the better mechanical properties of experimental conditions are: A(3)B(3)C(1)(Sb content of 0.3%, Cd content of 0.7%, RE content of 0.2%) and A(3)B(3)C(2)(Sb content of 0.3%, Cd content of 0.7%, RE content of 0.3%).