Study of the Interaction between Impurity Elements and Segregation Mechanism in Aluminum Molten Alloy

Author:Yang Jian

Supervisor:sun bao de zhang zuo


Degree Year:2018





High purity metal,especially high purity aluminum(>5N)has great application in optoelectronics industry,semiconductor industry and aerospace industry.Segregation and three-layer liquid electrolytic purifying methods are commonly used in fabrication of high purity aluminum.Segregation has many advantages compared with three-layer liquid electrolytic purifying method such as low energy consumption,sample technology and environmentally friendly.Segregation method becomes a sample method to fabricate high purity metal.However,the impurity elements which equilibrium distribution coefficients greater than or close to 1 can’t to removed by segregation,such as Cr,V(k0>1)or Mn,Zn(k0→1)in Al.The equilibrium distribution coefficient of those elements must be changed if the segregation method can be used to remove those impurities effectively.Therefore,purposefully adjustment(decreasing)of equilibrium distribution coefficient of the element becomes the key factor.The equilibrium distribution coefficient is a thermodynamic parameter which can be obtained by chemical potential.k0 can be adjusted by the interaction of elements through alloying.Some studies have verified that the addition of other alloying elements can change the chemical potential of an element by the interaction between elements,thus changing its k0.However,the mechanisms underlying this interaction have rarely been reported.The k0 of all impurity elements varies periodically with the increase of atomic number in Al,which is closely related to the periodicity of the peripheral electronic configuration of impurity elements.In order to study the influence of the electronic configuration of atoms on segregation.The fourth period transitional metals which are the most common alloying elements in Al were selected to study the properties of Al-X(X= the fourth period transitional metals)molten alloy by AIMD.The relationship between periodicity of k0 of solutes and the periodicity of interaction of Al-X was analyzed,and correlation between the local structure and the migration behavior of solutes was also studied.In order to remove the solutes of k0>1,taking V as an example,AIMD was also carried out to study the kinetic mechanism of the strong interaction between the solutes and V in Al-X(X=Fe,Si,Zn and Ga)ternary molten alloy and segregation process of bonded Si-V also illustrates through the simulation of the model of L/S interface.The effect of strong interaction on the segregation behavior of V was investigated combined with directional solidification experiments.An effective method for adjusting the equilibrium distribution coefficient of a target element is proposed.Moreover,the behavior of solute atoms at the front of solid liquid interface is also studied by AIMD.The segregation process can be deduced by studying the influence of solute atoms on the interface of solid-liquid interface.The main results and conclusions are listed as follow:(1)The results of AIMD simulation of Al-X binary molten alloy show that the interactions between Al and solutes have periodical correlation,which is inversely related to the periodic relation of k0 of solutes.The diffusion of solutes atom is strongly related to their local structure.The intensity of the first peak of pair correlation function reperesents the intensity of interaction between Al and solute atoms.The higher the peak intensity,the greater the cluster density,the stronger bonding of atoms in cluster.the fraction of 1551 bonder pairs closely related to the diffusion coefficient of solute atoms which implies 1551 is a key factor to determine the diffusion.(2)The results of high concentration of Al-X binary molten alloy show that the dispersion of different impurity elements in the melt is quite different.The chemical order parameters demonstrate Cr or V prefer to bond with congeneric atoms and make micro-inhomogeneity of the melt,while some solutes such as Fe,Co or Ni atoms are inclined to bond with Al atoms and disperse in the melt.(3)The simulaiton of the growth of L/Sinterface shows that the solutes atoms with different segregation behavior has the various influence on the solid-liquid interface.Fe with smaller equilibrium distribution coefficient obviously inhibit the crystal arrangment of Al atoms at the front of L/S interface.The local structure around Fe is very stable,the Al atoms in the Fe cluster need more energy to separate from Fe atoms,and arrange as crystal structures(Fe clusters retain the original molten structure,and make the Fe atoms more easily aggregated at the front of the interface.In the phase diagram,the higher the Fe concentration is,the lower the melting point of the melt is).Zn with equilibrium distribution coefficient close to 1 has no influence on the interface growth.Ti with equilibrium coefficient greater than 1 will promote the growth of the interface.(4)The results of tenary Al-Si-V melt show that the interaction of Si-V is much stronger than that of Al-Si and Al-V.The Si-V bonds are very stable,the equilibrium length of Si-V bond is about 2.5?,the critical distance of Si-V interaction is about 3.7?.The V-V bonds are broken by atoms Si.(5)The simulation of the model of Si-V atom at the front of solid-liquid interface shows that the V atom is obviously dragged by the Si atom into the liquid phase.The results of directional solidification show that the equilibrium distribution coefficient of V can effective adjusted by Si.The effective distribution coefficients of Si and V elements both decreased.The decline of k0 of V element is different in various Si/V ratio.