Study On Strengthening And Toughening Of AZ81 Magnesium Alloy With Sb And Al-Mn-Sb Master Alloy Additions

Magnesium alloy, the lightest engineering structural material, is praised as"new green engineering material and one of the materials with great development and utilization potential"because of its low density, low pollution, high specific strength, specific stiffness, superb damping capacity, sound electrical conductivity, heat conductivity, excellent machinability and good recycle ability. So it has extensive application prospect in the fields of transportation, aero-space and electronic communications. However, its poor plastic deformation capacity, low impact-toughness and low tensile strength seriously hinder its industrial application. AZ81 magnesium alloy is one of the most widely used magnesium alloys. In view of high strength and poor impact-toughness of the Mg-Al series magnesium alloys, the method of micro-alloying was adopted to improve the impact-toughness of Mg-8Al-0.8Zn (AZ81) magnesium alloy by adjusting the addition level of Sb. Effect of Sb and Al-Mn-Sb master alloy additions on the microstructure, mechanical properties and fracture mechanics were systematically investigated using OM, XRD, SEM, EDS, hard-meter, tensile testing machine, impact testing machine and fracture analysis. And its strengthening and toughing mechanism was discussed.Results show that:Small amount of Sb addition can significantly refine grains and second phase of AZ81 alloy as well as results in formation of Mg₃Sb₂ phase. The finest grains were obtained when Sb addition is 0.1 wt. %. And the binary Mg17Al12 network is broken into small particles homogeneously distributed in the matrix. In addition, it is seen that some black Mg₃Sb₂ particles are dispersively distributed at grain boundaries. However, further increase in Sb-addition causes the change in morphology of Mg17Al12 precipitates from particles to continuous network. Moreover, increase in Sb-addition leads to increase in the volume of Mg₃Sb₂ phase including the change of its morphology from finer particles to a thicker needle shape. Particularly, as Sb addition is up to 0.6 wt. %,β-Mg17Al12 phase becomes continuous and coarse again and Mg₃Sb₂ phase appears thick needle shape. The presence of needle-like Mg₃Sb₂ phase is harmful to both tensile strength and impact-toughness, especially the tensile strength. It counteracts the strengthening effect of Sb addition because of rending effect and stress concentration occurring at the ends of needle-like precipitates.Sb addition causes increases of mechanical properties to some extent. When Sb addition is 0.1 wt. %, both the impact-toughness and tensile strength reach the highest points, 17.928J/cm2 and 190MPa,which are 31.3% and 11.5% higher than that of mother alloy respectively. Excessive Sb addition leads to sharp decrease in both impact-toughness and tensile strength. The hardness keep increasing trend with increasing Sb addition.When Sb element is added in form of Al-Mn-Sb master alloy, effect of Al-Mn-Sb master alloy on the microstructure of AZ81 alloy is the same as that of pure Sb particles. The difference is that the modification effect of Al-Mn-Sb master alloy is better than that of pure Sb particles. Compared with Sb added alloys, the microstructures of Al-Mn-Sb added alloys are finer and the second phases are more dispersive when the Sb content is the same. Therefore, the mechanical properties of Al-Mn-Sb added alloys are higher than that of Sb added alloys. When 2 wt. % Al-Mn-Sb master alloy was added, the impact-toughness and tensile strength reach the highest points, which are 34% and 17% higher than that of mother alloy. The hardness still increases with increasing Sb addition. The optimum amount of Sb addition is 0.1wt. % .Sb addition makes tensile fracture of AZ81 alloys change from cleavage fracture to quasi-cleavage fracture and impact fracture change from brittle quasi-cleavage fracture to ductile quasi-cleavage fracture.