| Al-Si alloys are widely used in aviation and machinery industry for their excellent castability. However, coarse and large silicon phases in solidification structure formed in conventional casting process, with its sharp and irregular shapes, will dissever matrix structure and deeply reduce the alloy's mechanical properties. Chemical modification such as addition of Sr element can affect morphology, size and distribution of silicon phase and enhance comprehensive mechanical properties, nevertheless Sr modification interpreted in various perspectives in academia, has no uniform verdict until now. In this paper, theoretical computing and experimental study were conducted on the mechanism of Sr-modification of hypoeutectic Al-Si alloy, studying the influence of Sr-adsorption to the growth of silicon and revealing the essence of Sr-modifying silicon phase in electron force level by modeling the Sr-adsorption on growth step of silicon.First principle calculation results indicate, Sr-adsorption on Si(100) surface has reduced clean Si(100) surface energy, while the behavior of Si atoms stacking on Sr-adsorbed Si(100) surface has increased the surface energy, due to Si dangling bonds on Si(100) surface saturated by Sr atoms, but the radius of Sr is larger than that of Si, which leads lattice distortion of Si(100) surface with the generating of distortional energy. Found in analysis of electron structure, Sr atom will lose its electrons, in which some captured by silicon atom though less amount, and others transmitted to the space between Sr and Si atoms, so the atomic force between Sr and Si atoms is composed of covalent bond and electrovalent bond while the covalent bond is weaker than the electrovalent one. Furthermore, the extent of electron hybridization of Sr-s and 3d orbits is enhanced,as to increase the Sr-absorption force on the Si(100) surface. Andthe stability of adsorption surface is weaker than silicon bulk and existing that structure and properties is gradually changed from silicon bulk to surface. The interatomic forces between Sr and Si atoms on the Si(100) surface are contributed by Sr-4s4p5 s and Si-3s3 p electrons, in which the s and p electron orbits will perform electronic hybridization, moreover the hybridization of adsorption surface is stronger than the clean surface, which indicates the interatomic forces of Sr-Si on adsorption surface is more stronger than Si-Si on clean surface, namely the structure of Sr-Si is more stable than Si-Si, which ensures detachement of Sr from Si(100) surface be difficult. The interatomic forces between Sr and Si include ionic bond and covalent bond,but the covalent bond will be weaken even to disappear when under a high Sr concentration, thus causes the forces between Sr and Si to be weaken, which phenomenon is related to the excessive concentration of Sr atoms on Si(100) surface, due to the repellent force between Sr and Sr atoms increases with the concentration of Sr atoms, which can increase the distance between Sr and Si on the surface and the covalent bond will disappears when the atomic distance surpassing the value of Sr-Si covalent bond(2.92?),so as to weaken the attraction between Sr and Si atoms on the Si(100) surface. With analysis of eletronic structure, the hybridization and coupling of Si-Si 3s and 3p electronic orbits decreased by interact of Sr-s,3d electrons and Si-3s,3p electrons, which weakens Si-Si binding force and makes the Si stacking on adsorbed surface more difficult, as the electronic mechanism of Sr element modification to silicon phase.The un-modified eutectic silicon phase with its flake-like morphology distributes in the alloy obtained using optical micrography, electron micrography and three-dimensional construction, inserting in primary aluminum in some angle between the viewing surface and its normal direction. The Sr-modified silicon phase changed into approximately ball-like with the reducing of its size. Studies finding,Sr can modifies the large and irregular silicon phases in Al-Si alloys when Sr occupy 0.02% of weight in the alloys, so as to obtain enough small silicon equiaxed grains which diminish the degree of stress concentration. In the best content of Sr as 0.02%wt,the effect of Sr-modification become more and more obvious following the content of Sr element.Adsorption-poison mechanism of Sr-modification silicon is confirmed by theoretic computing and experimental study. Surface energy of the stacking Si(100) surface increased by the Sr-adsorption. Moreover, as to the interaction between Sr-s?3d and Si-3s?3p, the atomic bonds between silicon atoms of stacking atomic layer and absorbed surface weakened, as well as silicon atoms of the surface and silicon atoms below the surface, when lowering stability of absorbed surface, thus silicon stacking on absorbed Si(100) surface hindered by Sr-adsorption, while the silicon growing on stages in the side of Si(100) hasn't affected, so the stages around Si(100) probably transform as an new preferential growing plane, then change growing style and morphology of silicon phase. |
PDF Full Text Request