| In this paper,the classification and basic properties of magnetic materials,the basic theory of solidification,the research status of FeNiCo alloys,and the research content and significance of this work are briefly introduced.The molecular dynamics（MD）simulation methods and microstructure analysis methods are also described in detail.The solidification of FeNiCo melt under different conditions was simulated by MD,the characteristics of the microstructural evolution and the crystallization are studied in detail.It enriches the solidification theory of alloy melt,and is of great significance for improving soft magnetic properties and application of FeNiCo alloy.First,the rapid solidification of Fe63Ni33Co4 melt under 0 Gpa at different cooling rates(5×1010 K/s,1×1011 K/s,2×1011 K/s,5×1011 K/s,1×1012 K/s,5×1012 K/s,1×1013 K/s,2×1013 K/s,and 5×1013 K/s)were studied by MD simulation.The results show that the good solid solution of Fe63Ni33Co4 alloy can obtain at a higher cooling rate(5×1013 K/s),and the elements are uniformly dispersed in the solid.The crystallization ability of Fe63Ni33Co4 alloy is very strong(the critical cooling rate is about 2×1013 K/s).The face-centred cubic（FCC）,hexagonally close-packed（HCP）,and body-centred cubic（BCC）structures are formed simultaneously at the initial stage of crystallization,and the final solid was FCC crystal or mixed crystals dominated by FCC structure.The HCP structure formed due to the influence of kinetic factors can be transformed into the FCC structure by the slight sliding of the hexagonal close-packed planes,thereby eliminating the stacking fault.The kinetic factors may cause the potential energy of the system solidified at lower cooling rate to be higher than that solidified at the higher cooling rate(In Chapter 3,the potential energy of the system solidified at 1×1011 K/s is higher than that solidified at 2×1011K/s).Then,the solidification of Fe63Ni33Co4 alloy at the subcritical cooling rate(1×1013 K/s)has been investigated by MD simulation,in terms of the potential energy and the largest standard cluster analysis（La SCA）.It was found that the final solid at subcritical cooling rate is composed of many crystalline regions,however the energy of the system decreases continuously and slowly.Further analysis revealed that the solidification experiences liquid-liquid transition,liquid-solid transition,and solid-solid transition,which are respectively dominated by amorphous local structures with3-,4-,and 5-fold symmetries,BCC structures and FCC/HCP clusters.There are a series of competitions:local dense packing vs.global dense packing,rotational symmetries vs.transitional symmetries,and vitrification vs.crystallization;even the crystallization still includes BCC-FCC/HCP competition followed by FCC-HCP competition.The complicated competition between multiple features and different structures caused by the continuous supercooling of the ternary alloy deprives the stable crystal phase of the conditions for uniform nucleation,resulting the FCC crystal is formed through heterogeneous nucleation at lower temperatures.Finally,the solidification of Fe90-xNi10Cox（x=10,20,30,40,50,60,70,80）alloy was simulated under 2×1011 K/s and P=0 Gpa.The results show that there is a typical first-order phase transition during solidification,and the system finally solidified into FCC or HCP crystals.The initial temperature Ts of phase transition of the Fe90-xNi10Cox melts increases monotonically with the content of Fe atom increasing.It was found that the composition is crucial to the solidification of Fe90-xNi10Cox alloys:the less content of Fe atom,the more complex paths of phase transition;therefore,Fe atoms may play a role in stabilizing the structural transformation of the system. |
