Effect Of Sc On Mechanical Properties And Microstructure Of Al-Mg-Si(-Cu) Alloys

The heat-treatable Al-Mg-Si(-Cu)alloys(known as 6xxx series)have been widely used in automobile and aviation industries due to their good formability,high strength-to-weight ratio,and low cost.During proper heat treatment,a large number of nanometer-sized precipitates formed in the alloy effectively hinder the movement of dislocations,which strengthen the materials.In order to improve the properties of Al-Mg-Si(-Cu)alloys to meet the requirements of production applications,other alloying elements are sometimes added.As a typical example of micro-alloying elements for Al alloys,Sc has been reported to be the most effective strengthening element per atomic fraction in the Al-matrix.Studies have found that the addition of Sc in Al-Mg-Si(-Cu)alloy can bring about fine properties such as grain refinement and inhibition of recrystallization.In recent years,the Sc effect on precipitation has been attracting more and more interest.Vickers hardness,transmission electron microscopy,and high Angle annular-dark field scanning imaging techniques were used to investigate the mechanical properties and microstructure of Al-Mg-Si(-Cu)-Sc alloys in this study.In association with first-principles calculations.The findings show that:The Al₃Sc dispersoids still remain in the matrix after solution aging of Al-Mg-Si(-Cu)-Sc alloy.The Al₃Sc dispersoids in the Al-Mg-Si-Sc alloy are less in number and large in size,and the Al₃Sc dispersoids in the Al-Mg-Si-Cu-Sc alloy are more abundant but small in size.Two kinds of heterogeneous nucleation can be observed on the Al₃Sc dispersoids:one is heterogeneous nucleation of the rod phase at the interface of the Al₃Sc dispersoids,and another is heterogeneous nucleation of the lath phase at the interface of the Al₃Sc dispersoids.The former appears in the large Al₃Sc dispersoids,while the latter appears in the small Al₃Sc dispersoids.The effect of Sc on the mechanical properties and microstructure of Al-Mg-Si alloy under 180?aging was investigated.It was found that the micro-alloying element Sc results in a slightly decreased peak hardness and the improvement of thermal stability of Al-Mg-Si alloy,and significantly alters their precipitate microstructures.At the peak-aging stage,most of the precipitates are disordered?"precipitates.During the over-aging period,the disordered regions of the disordered?"precipitates gradually evolve to the?'/B'/U2 sub-units,leading to formation of?"/?'/B'/U2 composite precipitates at the long-time over-aging.Occupation of Sc atoms is found in both precipitates.In the disordered?"precipitates,the Sc atoms are mainly located in the disordered region,and in the?"/?'/B'/U2 composite precipitates,the Sc atoms randomly occupy the Mg or Al-sites of the?'and B'.The formation of the composite precipitates is probably related to the strong binding between Sc and Si atoms,and to the non-preference for Sc atoms to occupy the atomic sites of the?"-structure.Our results reveal that the formation of?"-related composite precipitates is responsible for the improvement of thermal stability of the Sc-containing alloy.The effect of Sc on the mechanical properties and microstructure of Al-Mg-Si-Cu alloy under aging was investigated.It was discovered that adding Sc to an Al-Mg-Si-Cu alloy can improve its thermal stability when aged at 180?and 250?(high temperature).This is due to the fact that adding Sc to the Al-Mg-Si-Cu alloy facilitates the precipitation of a large number of fine lath QP2 phases,which are difficult to coarsen in size during the later aging process,and the strengthening effect is excellent.