Research On The Strengthening And Toughening Mechanism Of Al-Mg₂Si Composites By La And Ce Addition

Particle refinfoced aluminium matrix composites possesses excellent mechanical properties,such as high specific strength and stiffness,low thermal expansion coefficient and favorable stability,and it can be extensively used in aerospace,electronic,automobile,weapon equipment and many other fields.Al-Mg₂ Si composites is a typical particle refinfoced aluminum matrix composites.However,in the in-situ casting process,the coarse size and mostly angular shape primary Mg₂ Si phase generated in aluminum matrix,severely fragmented aluminum matrix,reduce the mechanical properties,and the wide application of this composites are limited.Therefore,how to enhance the strength and toughness of Al-Mg₂ Si composites by optimizing the size and shape of the Mg₂ Si phrase have become the most urgent need and concerning problem.Hypereutectic Al-15%Mg₂Si and Al-20%Mg₂Si composites were prepared by in-situ process.In order to optimize the microstructure and improve mechanical properties of Al-Mg₂ Si composites,it is modified by adding different mass fraction of rare earth（Ce or La）and heat treatment,and then observe the size,shape and distribution of primary Mg₂ Si phases in the composites.The optimal adding contents of Ce or La and the best heat treatment process are obtained,as well as revel its strengthening and toughening mechanism.All the research results are as follows:The proper amount of La and Ce improve microstructure and mechanical properties of Al-15%Mg₂Si and Al-20%Mg₂Si composites.It is found that the primary Mg₂ Si phase is refined by adding rare earth element La or Ce,the morphology become smooth and round,and the distribution is relatively uniform.When the addition amount of La is from 0.6% to 0.8% and with 0.6% addition amount of Ce,refining effect is most obvious and the mechanical properties（tensile strength,elongation and hardness）increase observably.The crystal structure,density of state,and elastic constants of Al₄ Ce,Al₄La and Mg₂ Si phases are calculated by first principle calculation and according to the results: Al₄ Ce and Al₄ La phase are more easily form and stably exist,and which stiffness are pretty poor but the plasticity are better.The presence of Al₄ Ce and Al₄ La can improve the strength and plasticity of materials.By changing the solution time and aging time,the morphology of primary Mg₂ Si phase is improved and the optimum heat treatment process is determined.The result shows that the optimum heat treatment process is 545°C×5h solution and 175°C×3h aging for Al-Mg₂Si-Ce composites,as well as 545°C×5h solution and 175°C×4h aging for Al-Mg₂Si-La composites.At this time,the microstructure of Al-Mg₂ Si composites is the best,and the primary Mg₂ Si phase is relatively smooth and mechanical properties（tensile strength,elongation and hardness）are also the best.