Effect Of Rare Earth Y On Al-20wt.%Mg₂Si Composites

In recent years,with the rapid development of science and technology,the applications of lightweight alloys have attracted wide attention.Among them,the applications of aluminum and aluminum alloys are extremely remarkable.Al-Mg₂Si composites are typical aluminum matrix composites,which have good specific strength,hardness,thermal expansion coefficient,wear resistance,corrosion resistance,light density and good bite resistance.It is an ideal material for the manufacture of engine pistons.The preparation of Al-Mg₂Si composites by in-situ process is simple,inexpensive and has good stability.However,the primary Mg₂Si particles have a great influence on the mechanical properties of materials due to their coarse grain size,poor microstructure and uneven distribution of composition.Rare earth microalloying is an important means to modify primary Mg₂Si,which is of great significance for improving the mechanical properties of Al-Mg₂Si composites.The first-principles calculation based on density functional theory can be used to simulate the physical and chemical properties of material structures from the electronic and atomic scales,such as surface energy,interfacial energy,etc.By calculating the formation enthalpy,binding energy,elastic constants,density of states and band structure of materials,the structural stability and mechanical properties of materials can be analyzed,and the possible metal phases and their properties in melting experiments can be predicted.In addition,the first-principles calculation is also widely used to study the interfacial properties between the matrix and the nucleus,especially the mechanism of various factors on the grain refinement.In this paper,the effect of rare earth Y on Al-20wt.%Mg₂Si composites was studied by experimental and first-principles calculation.Al-20wt.%Mg₂Si composites with different contents of Y were prepared by in situ process.The effects of Y addition on the microstructure,grain size and mechanical properties of the composites were studied.It is found that rare earth can refine the primary Mg₂Si particles,and the morphology changes from fishbone and sharp angle to smooth,round,and uniform distribution.When the addition of Y is 0.8 wt.%,the refining effect of primary Mg₂Si is the best,and the ultimate tensile strength,elongation and hardness of the composites are ideal.The formation enthalpy,binding energy and elastic constant of Al-Y binary phases were calculated by first principles method.The results show that Al₃Y is a brittle compound,therefore,when the addition of Y exceeds 0.8 wt.%,the mechanical properties of the composites will decrease.The formation enthalpy,binding energy,elastic constant and electronic structure of Mg₂Si and its doped solid solution were calculated.The results show that Y atom preferentially replaces the Mg atom in Mg₂Si cell,and the ductility and stability of Mg₂Si can be improved by doping.In addition,the two-dimensional lattice mismatch and interface model of Y?0001?/Mg₂Si?111?was constructed to study the heterogeneous nucleation mechanism of Y to Mg₂Si.The results show that the surface energy of Mg2Si?111?surface is reduced by Y doping.The triangular chemical bonds of Y,Mg and Si formed at the Y?0001?/Mg₂Si?111?-MgII interface improve the stability of the interface.Therefore,Y can be used as the heterogeneous nucleation of Mg₂Si.