Liquid Structural Evolution On Medium-range Scale And The Correlation Between Structure And Magnetic Properties In Fe-based Alloys By First Principles Simulations

Since the physical properties are determined by structure,the liquid and amorphous structures are considered to be one of basic issues in the study of Fe-based amorphous alloys.In this thesis,we performed ab initio molecular dynamics simulation to investigate the structure evolution and the correlation between structure and properties in liquid Fe-Si and Fe-Al binary alloys.Similarly,ab initio molecular dynamics simulation was used to study the correlation between atomic magnetic moments and local structure of amorphous Fe7sSi9Bis alloy and amorphous pure Fe.We reassessed the structural transition regions along the liquidus of Fe-Si alloys by using ab initio molecular dynamics simulation.Structural transition points are found at both 30 at.%Si and 67 at.%Si(FeSi2)which are the eutectic points.We demonstrated that the liquid structure in the sub-region of 0-30 at.%Si is close-packed,and in the sub-region of 67-100 at.%Si liquid alloys have very open structure.From 30 at.%Si to 67 at.%Si,the close-packed structure gradually changes into open one.These structure transition sub-regions are also supported by the formation enthalpy of liquid alloys.Furthermore,the predicted enthalpy change between 1585 K and 1823 K is so large that there is probably a liquid-liquid transition with temperature at FeSi2 alloy which is an important thermoelectric material.'Further,the liquid structures can be modeled as the Si bonded networks surrounding Fe atoms.The results for the dynamical properties show that DFe is actually limited by Si bonded networks,which confirms the reliability of the model.It is found that the magnetic moment of a Fe atom is affected by both atomic distance and local structure,and the maximum value is near 30 at.%Si.We observed the transition of medium-range orders(MROs)and physical properties in liquid Fe-Al alloys by using ab initio molecular dynamics simulation.The structural transition point of liquid Fe-Al alloy is 66 at.%Al.Above 66 at.%Al,the FeAln(n?9)clusters are very popular and stable,whose packing in space bridged by A1 atoms forms Fe-Fe MRO,while in other binary Fe-Al alloys studied Fe-Fe and Al-Al MROs coexist,and below 50 at.%Al,Al-Al MRO turns to be dominant.The curve of short-range order around Fe atoms versus Al content also shows a juncture at 66 at.%A1 above which<0,3,6,0>polyhedron type increase significantly,correspondingly a significant decrease of the average magnetic moment per Fe atom is observed.In addition,the self-diffusion coefficient of Al atom increases sharply above 66 at.%Al.The structural and magnetic properties of amorphous Fe78Si9Bi3 alloy at different pressures up to 282.1 GPa were investigated by ab initio molecular dynamics simulations.With the pressure increasing,the structural evolution of amorphous Fe78Si9B13 alloy can be approximately divided into three regions:0.0?20.1 GPa,20.1?60.0 GPa,60.0?282.1 GPa.The reduction of the interatomic distance leads to the exponential decrease of the average magnetic moment per Fe atom.Further,to establish the relationship between structure and atomic magnetic moment at a constant pressure,we focused on two types of polyhedra centered by Fe atoms in amorphous Fe78Si9B13 alloy at 0.0 GPa and proposed the minimum coordination polyhedron model.The result shows that the magnetic moment of central Fe atom is also influenced by the chemical composition besides the interatomic distance.For central Fe atom,more surrounding Fe atoms correspond to a larger magnetic moment,while the surrounding Si atoms are not conducive to the magnetic moment.Other than Fe-based amorphous alloys,the atomic configurations of amorphous pure Fe exhibit abundant topological local structures and exclude chemical effects on the local magnetic moment of a Fe atom.Our results show that the decreased volume leads to a reduction of absolute values of both positive and negative magnetic moments,which agrees with earlier investigations.Further,we employed the minimum coordination polyhedrons with the coordination number of 5?10 to depict the local structures,and found that there is a strong correlation between the minimum coordination polyhedrons and local magnetic moments:that the local magnetic moments increase with the coordination number of the minimum coordination polyhedron;and that the larger distortion of the minimum coordination polyhedron corresponds to larger average magnetic moments.