Research On The Microstructure And Mechanical Properties Of Mg-Zn-Al Alloys

Magnesium alloys, characteristic of high specific strength and toughness, high dimensional stability, excellent shock resistance, good cutting ability and environmental friendly, have wide applications in the fields of aerospace, automobile, electronics and ect. However, their poor high temperature performance limits their applications. Therefore, it is pressing to develop some new heat resistant magnesium alloys with low cost. Mg-Zn-Al is a promising alloy system to meet the above requirements. The aim of the present work is to get a better understanding of alloys of this system.The optimal composition of Mg-Zn-Al ternary alloys was identified. Then, in order to improve the mechanical propertities at elevated temperature of this alloy, small amount of Sn or Si was added to form stable Mg₂Sn and Mg₂Si compounds. The effct of Sn,Si and Ca on the microstructure, mechanical properties and thermal stability of Mg-6Zn-2Al alloy were investigated.The results were enumerated as follows:1) The effct of Al on microstructure and mechanical properties of Mg-6Zn alloys was examined. The alloy containing 2mass%Al possessed a good combination of the tensile strength and plasticity. With the addition of Sn to Mg-6Zn-2Al alloy, the small disperse intermetallic particles of Mg₂Sn were appeared, which increased the yield strenth. With the addition of lmass%Si to Mg-6Zn-2Al alloy, the main strengthening phase Mg₂Si presented as coarse Chinese script distributed arround the grain boundary, which resulted in the decrease of Ultimate Tensile Strength and Elongation. The Mg₂Si phase in Mg-6Zn-2Al-lSi alloy was refined with the addition of 0.2mass%Ca,and thus the tensile strength and plasticity were increased.2)Samples cut from these alloys were heated at 200℃ for very long periods. The variations of the microstructure and hardness with time were investigated. The results showed that the thermal stability of Mg-6Zn-2Al alloy was improved with the addition of Sn and Si.3)The mechanical properties at elevated temperature of the alloys with and without Sn or Si addtition were compared. The addition of Sn and Si to Mg-6Zn-2Al alloy was beneficial to raise the ratio of tensile strength at elevated temperature vs that at RT, and this effect was improved with the increase of the content of Sn or Si.