First-principles Study On The Aging Precipitation Behavior Of Cu-Fe Alloys

The Cu–Fe alloys have been widely used as lead-frames,magnet coil andother electronic components due to the low cost and high strength.However,the supersaturated Fe in Cu matrix is known to reduce the electrical conductivity of the composite severely due to the relatively high solution of Fe in Cu at high temperature,coupled with sluggish kinetics of iron precipitation at low temperature.For that reason,at present many researchers focus much on how to promote the precipitation of Fe from the Cu matrix and optimizing the strength and electrical conductivity of Cu-Fe system.The multi-alloying method is one of the effective ways to improve the aging precipitation of Cu-Fe alloys,the method can improve the diffusion coefficient of solid solution Fe atoms and decrease solubility of Fe in Cu matrix by the addition of alloying elements in Cu-Fe alloy,and then promote the precipitation of Fe from the Cu matrix.In order to choose alloy elements more reasonably and reveal the physical essence of alloying elements in multi-alloying,in this thesis,a first-principle atomistic calculation based on density functional theory was employed to investigate interaction of vacancy and solute atoms in the Cu matrix,the effect of alloying elements on the diffusion coefficient of solid solution Fe atoms in Cu matrix and its mechanism,and the effect of alloying elements on the equilibrium solid solubility of Fe atoms and its mechanism.The main work and conclusions are described as the following:Study on the interaction of vacancies and solute atoms in the Cu matrix.The binding energy of solute atoms and vacancies were calculated for the first time in the Cu matrix,the mechanism of interaction between vacancy and solute atoms in Cu matrix was clarified by the investigation of the nearest-neighbor and second nearest-neighbor solute-vacancy binding energies,and the diffusion kinetics of the alloy elements in the copper base were revealed.This work lays a foundation for further research on Fe atomic diffusion coefficient and equilibrium solid solubility.The study found that the solute-vacancy binding energies mainly were determined by the size of the solute,a modest correlation lies between binding energy and solute size shows,i.e.the larger solute atoms are more favorably binding with neighboring vacancies except for the transition metals?Sc–Zn?,Nb and Ta,which is due to vacancies next to the large solute will allow the atoms to relax towards the vacancy.Strong Cu-solute bonds due to sp–d hybridization between Cu and 3d solutes hindered 3d solutes diffusion in Cu.Five solutes V,Cr,Mn,Fe and Co have magnetic moments,due to the magnetic effect,NN binding energies of the magnetic atoms increased,the increment was proportional to the magnetic moment of solute atoms.The calculated solute-vacancy binding energies show that 14 alloying elements B,Si,P,Al,Ge,S,As,Mg,Ag,Mg,Sn,In,Sb and Bi may accelerate the diffusion of Fe atoms in multi-alloying.The effect of alloying elements on the diffusion coefficient of solid solution Fe atoms in Cu matrix was investigated.Based on the above 14alloying elements,the eight frequency model was first constructed to calculate the diffusion coefficients of Fe atoms in Cu-X-Fe alloy.The effect of alloying elements on the diffusion coefficient of solid solution Fe atoms in Cu matrix and its mechanism were revealed.The diffusion coefficient calculation is divided into two parts:The first is the diffusion pre-factor D₀,which is obtained by calculating the phonon frequency.The second is activation energy Q^Fe,which is obtained by calculating migration enthalpy and vacancy formation enthalpy.The position of the saddle-point in the migration process was found accurately by the CINEB method.The research of activation energyQ^Fe found that there was a positive correlation between the size of the third element and the activation energy.The results of diffusion coefficient D^Fe showed that 9 potential elements B,P,S,Mg,Ag,Cd,In,Sb and Zr may accelerate significantly iron diffusion in copper.The interaction between third alloy elements and Fe atoms and the size of third elements play a dominant role in the change of Fe diffusion coefficient.The effect of alloy element on equilibrium solubility of Fe in Cu matrix was investigated.The empirical formula from first principles was employed to determine the solubility of iron in dilute Cu-Fe-X alloys.The empirical formula of solid solubility was effectively combined with electronic structure analysis,the charge density,difference charge density,local density of states and partial density of states were employed to investigate the binding between Fe and its surrounding Cu atoms after addition of the third element.The effect of alloy element on equilibrium solubility of Fe in Cu matrix and its mechanism were revealed.The calculation showed that 5elements with weak Fe-X binding energy?less than 0.1eV?,Ag,Mg,Bi,In and Sn can decrease the solubility of Fe.This is because the favorable binding mainly dominated by hybridization of 3d and 4s states of Fe with 2p or 3p states of the alloying elements will lead to the calculation results are higher than the real value.The investigation of charge density and difference charge density indicate that the Fe-Cu bonding is characterized by covalent and will not be changed by the addition of the third elements.However,the binding strength between Fe and Cu was significantly changed by the addition of the third elements,which is mainly attributed to the charge transferring.To investigate the electron population and bonding strength,the local density of states?LDOS?and partial density of states?LDOS and PDOS?of binary Cu-Fe and ternary Cu-Fe-Ag alloy were investigated.The difference between the LDOS of the pure Cu and that of binary the Cu-Fe revealed clearly the Fe-Cu bonding and anti-bonding states,and the bonding strength of Fe-Cu was studied by charge number of Fe-Cu binding states.The results of DOS shown that four elements Ag,Mg,Al and S can significantly reduce the ratio of electrons in bonding states while the bonding peak shift to the higher energy states,which means they can effectively decrease Fe solubility in Cu.In summary,the mechanism of interaction between vacancy and solute atoms in Cu matrix was clarified by the investigation of the solute-vacancy binding energies,and the diffusion kinetics of the alloy elements in the copper base were revealed.14 alloying elements may accelerate the diffusion of Fe atoms in multi-alloying.The eight frequency model was first constructed to calculate the diffusion coefficients of Fe atoms in Cu-X-Fe alloy.The effect of alloying elements on the diffusion coefficient of solid solution Fe atoms in Cu matrix and its mechanism were revealed.The empirical formula from first principles and electron population were employed to determine the solubility of iron in dilute Cu-Fe-X alloys.The effect of alloy element on equilibrium solubility of Fe in Cu matrix and its mechanism were revealed.The research laid a theoretical foundation for the follow-up work of high strength and high conductivity Cu-Fe alloy.