Effect Of Rare Earth Element La Addition On Al-25wt.%Mg₂Si Composites

Al-Mg₂Si composites are the typical particle-reinforced aluminum-based composites,which are widely used in various fields.However,in the Al-Mg₂Si composites,the primary Mg₂Si phase prepared by the traditional melting process has coarse grains,complex morphology and sharp edges,casuing the aluminum alloy matrix was torn severely,which reduces the mechanical properties of Al-Mg₂Si composites.Rare-earth microalloying is an important method for modifying primary Mg₂Si phase,and it has important significance for improving the mechanical properties of the Al-Mg₂Si composites.First-principles calculation can explain the reasons for changes in the macroscopic properties of materials from a microscopic perspective,providing necessary evidence for experiments,and reduces the need for exploration of unnecessary experiments.Therefore,in this work,the effects of rare earth La on Al-25wt.%Mg₂Si composite were studied by a combination of experiments and first-principles calculation.Al-25wt.%Mg₂Si composite was prepared by in-situ endogenous process,and the microstructure of composites was improved by adding different contents of rare earth element La and heat treatment to achieve the purpose of improving the overall mechanical properties of composites.The specific research results are as follows:The rare earth element La improves the microstructure and mechanical properties of Al-25wt.%Mg₂Si composite.The experiments found that the addition of rare earth La makes the morphology of the primary Mg₂Si phase regular,and the distribution is uniform and the grain size becomes smaller.When the addition of La is 1.0wt.%,the structure refinement effect of is the best and the mechanical properties are also optimal.By changing the time of solid solution treatment and aging treatment,the morphology and distribution of the primary Mg₂Si phase and eutectic structure were improved,and the optimal heat treatment process for the composite material was determined.The results show that the best heat treatment process for the Al-25wt.%Mg₂Si-1.0wt.%La composite is solid solution at 545?C for12h,and aging treatment at 175?C for 7h.At this time,the microstructure and mechanical properties of the Al-25wt.%Mg₂Si composite material reached the best.The first-principles method was used to calculate the crystal structure,elastic properties,and electronic structure of Mg₂Si and its doped solid solution.The calculation results show that the doping of La atoms and Al atoms reduces the bonding strength and quantities of the covalent bonds in the Mg₂Si system.The weakening improves the plasticity of Mg₂Si and reduces the influence of the intrinsic brittleness of the reinforcing phase Mg₂Si on the plasticity of the Al-Mg₂Si composites.The first-principles calculation of the crystal structure,the density of states,and the elastic constants of the binary phase Al₄La shows that Al₄La can form and exist stably in the system,which is manifested as a plastic characteristic.The presence of Al₄La can improve the plasticity of the composites.