Study On Fabrication And Properties Of Magnesium Matrix Composites Reinforced By Mg₂Si Particles

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,and can be widely applied to aerial and aerospace industries,electronic industry,automotive industry,etc.As a typical cast magnesium alloy in Mg-A1 system,AZ91 offers a combination of castability and corrosion resistance,as well as low cost.However,compared with aluminum alloys, this alloy has the disadvantages of low elastic modulus,poor tensile strength,which hinder its applications in industries.Due to the above mentioned disadvantages of AZ91 magnesium alloy,the present paper investigates the microstructures and mechanical properties of AZ91 alloy with alkaline earth additions of Ca,Ba and Sr. Mg₂Si/AZ91 composites were fabricated by in-situ synthesis process,and the effects of Si content in the composites were systemically studied.Finally,the modifying effect of alkaline earth elements and high intensity ultrasonic on Mg₂Si particles was also studied.The results show that,the addition of alkaline earth has an evident modification effect on AZ91 alloy.After adding 0.5%Ca+0.2%Ba+0.1%Sr,the microstructure of AZ91 alloy was greatly refined.The tensile strength and elongation were increased by 18.8%and 2.4%respectively.The the addition of alkaline earth can decrease the volume and size ofβ-phase.The distribution of compounds might pin the grain boundaries and prevent grain-boundary slipping,thereby strengthen the alloy.Adding Si into the magnesium melt can obtain magnesium matrix composite reinforced by in-situ synthesized Mg₂Si reinforcements.The morphology of the Mg₂Si in Mg₂Si/AZ91 composite was affected by the content of Si.It shows mainly Chinese-script shape when Si content is less than 1.0%;and becomes the mixture of eutectic Chinese-script and primary bulk when the Si content is around 1.5%-2.0%; when the Si content reaches 2.0%or higher,the Mg₂Si particles will begins to cluster. When the Si content is around 2.0%,the tensile strength and the yield strength are the highest.The strength of composites will decrease with increasing Si content due to the aggregation of Mg₂Si particles.The modification of Mg₂Si/AZ91 composite was studied.It is found that the addition of alkaline earth elements and the application of high intensity ultrasonic have an evident modification effect on the Mg₂Si phase.After adding suitable alkaline earth elements,the size of the Mg₂Si particles was considerably reduced and its shape was changed from a rough dendritic,bulk or Chinese-script shape to a fine, multi-angle granular shape.As a activator,Ca or Sr can easily absorb on the surface of Mg₂Si phase.Sr probably can enter into Mg₂Si phase and improve its shape.When adding 1%Ca+0.2%Ba+0.1%Sr,the refinement effect is the best.The properties of composites are improved under high intensity ultrasonic.The acoustic cavitation effects of ultrasonic can improve wettability of Si particles with liquid magnesium, and the acoustic streaming make the Mg₂Si particles dispersing homogeneously in the melt.In addition,the effects of the degassing and deslagging of ultrasonic wave in the melt are contributed to the properties of the composites.The results showed that the modification function is the best when the ultrasonic treatment time and power are 6min and 1.2kW respectively.The tensile strength reaches 224.3MPa,which is 17.9% higher than untreated composites.