Calculation Of Electronic Work Function Of The Second Phase And Study On Corrosion Mechanism Of Mg-Zn-Y-Nd Alloy

With the increasing number of vascular diseases,magnesium alloy stents have become one of the research hotspots in the field of vascular intervention therapy because of their degradability,no need for secondary surgical removal and less trauma.In spite of their good biocompatibility and mechanical properties,magnesium alloy stents have a very quickly degradation rate,and thus is easy to lose effective support after implantation.Therefore,researchers have studied the corrosion behavior and intrinsic corrosion mechanism of magnesium alloys in order to improve the corrosion resistance.The prosperity of computational materials science provides an effective way to explore the corrosion mechanism.The electrochemical corrosion process is accompanied by the gain and loss of electrons,oxidation and reduction.The electronic work function is closely related to these phenomena,reflecting the electrochemical properties of materials.In this paper,the Mg-Zn-Y-Nd magnesium alloy for vascular stent was selected as the research object.The structural stability,electronic work function,intrinsic potential difference and segregation behavior of the alloy were studied by the first principles calculations,and the electrochemical trend and cause of micro-galvanic corrosion were analyzed.By combining with experimental methods,the influence of Zn,Y,Nd alloying elements and second phases on the corrosion resistance of Mg alloy was explored.The main research results are as follows:The stability of the doping model of magnesium alloy was studied by calculating the formation energy and binding energy.It was found that the alloy system doped with Zn,Y and Nd show good structural stability.Among them,Mg alloy doped with Y has the lowest formation energy and binding energy.The calculation result of the segregation energy showed that three elements have no obvious segregation behavior and would not cause a large number of defects.Zn can increase the electronic work function of doped surface.Single-doing of Nd and Y and co-doping of Zn,Nd and Y would reduce the work function.The internal doping has little effect on work function.The work function of different crystal surfaces varies with doping concentration.The work function increases with the increase of Zn concentration,while decreases with the concentration of rare earth elements Y and Nd.Due to the existence of a potential difference between second phases and the matrix,the severe local galvanic corrosion of magnesium alloy would occur.Thus,the work functions of several typical crystal planes of more than 20 second phases including Mg-Zn phases,Mg-Y phases,Mg-Nd phases,ternary phases and other second phases without Mg element were calculated.It was found that the work function is affected by the types of second phase and crystal orientation.The types and arrangement of atoms on the surface also have different confinement capabilities to electrons.From the perspective of electron gain and loss,the cause of galvanic corrosion was analyzed.The result showed that the work function of Mg-Zn phases is generally higher than that of Mg matrix,and the intrinsic potential difference of Mg-Zn phases with?-Mg is positive.Thus,Mg-Zn phases are much easier to gain electrons than?-Mg.In the electrochemical environment,Mg-Zn phase would act as the cathode phase,leading to the preferential dissolution of the matrix.On the other hand,Mg-Y and Mg-Nd binary phases and some ternary phases act as an anode and were corroded earlier than Mg matrix because of a relatively lower work function.In addition,the work function values of Mg₃Nd and Mg₁₂Nd phases are very close to that of?-Mg.They have little effect on the micro-galvanic corrosion,which is conducive to the homogeneous degradation of biomedical Mg alloy.Furthermore,several typical second phases were selected as research objects to study the micro-galvanic corrosion behavior between them and?-Mg matrix,and experimental means were carried out to verify the reliability of the calculation results.For example,a model alloy Mg?30wt.%Nd only containing Mg₁₂Nd phase and?-Mg was fabricated.The EBSD results showed that the outer surfaces of?-Mg are those with low indices,i.e.(0001)and(10-10).The scanning Kelvin probe force microscope testing suggested that the{0001}plane of?-Mg has a higher Volta potential than Mg₁₂Nd,which also means that Mg₁₂Nd tends to lose electrons when contacting with?-Mg and corrode preferentially.In the simulated body fluids,the Mg₁₂Nd phase corroded before the?-Mg phase.When contacted with Mg matrix,Mg₇Zn₃ and W-Mg₃Y₂Zn₃ phases act as the cathode and anode phases,respectively.Compared to Mg₇Zn₃or W-Mg₃Y₂Zn₃ phases.