Study On The Atomic Structure Of Fe-based Liquid And Amorphous Alloys By Molecular Dynamics Simulation

In the present thesis, ab initio and classic molecular dynamics simulations on pure iron and Fe-based binary and ternary alloys have been performed to investigate the atomic structure of liquid and amorphous iron and structural evolution during liquid-solid transition, correlation between liquid structure and phase diagram in Fe-based binary alloys as well as correlation between liquid structure and glass-forming ability in Fe-based ternary alloys.The atomic structure of liquid and amorphous iron and the structural evolution in the course of glass transition and crystallization were investigated by classic molecular dynamics simulation. It is found that the clusters in liquid iron have the preference of fivefold symmetry. During quenching, the stability of clusters is related to their coordination number, degree of fivefold symmetry and degree of saturated bond pairs. The variation tendency of several structural parameters changes at the glass transition temperature of pure iron, which indicates that there is close correlation between glass transition and atomic structure. The second peak in pair distribution function for amorphous iron splits into two subpeaks. According to the decomposition of pair distribution function, it is found that unevenness of connecting style of atomic clusters results in the splitting of the second peak. The two subpeaks are caused by three-atom-shared connection and one-atom-shared connection between atomic clusters. The hollow between the two subpeaks results from two-atom-shared connection. Comparing the structure of amorphous iron with the structures of liquid and crystalline iron, the underlying reason for the second peak splitting is that metallic glasses have higher density than liquid alloys and different connecting style of atomic clusters from crystals. In the atomic structure of amorphous iron, fivefold symmetry dominants in the clusters as well as the cluster packing. The styles of cluster packing are not face-centered-cubic or icosahedral but have rich diversity. The linear correlation between the coordinate number and the average number of adjust clusters might give an indication of the fractal nature of medium range order in metallic glasses. In the course of crystallization of pure iron, based on the trace of cluster evolution and the transferring form of clusters, a model was proposed to describe the transferring route of clusters from icosahedra to bcc.Liquid structures in six iron-based binary alloy systems were investigated by ab initio molecular dynamics simulation. It is found that there is close correlation between liquid structure and phase diagram in these systems. In the Fe-C system, the connection between the eutectic point and chemical short range order is very clear. Moreover, For the Fe-P, Fe-Ti and Fe-Zr systems, close correlation is found between the eutectic point and topological short range order. The situation in Fe-B system is similar to that in Fe-C. A preliminary conclusion can be made as follows:In the systems with quite large size difference, the eutectic point is more correlated to chemical short range order while the eutectic point is more sensitive to topological short range order in the systems with small size difference. In the systems of Fe-Ni and Fe-Cr, there is some correlation between the variations of liquidus and bond pairs. The tendency of saturated bond pairs is similar to that of liquidus while the tendency of unsaturated bond pairs is opposite to that of liquidus, which maybe suggests the close correlation between bond pairs and some properties of liquid alloys such as flowability.Ab initio molecular dynamics simulation were performed on the liquid structures in Fe70EM10B20(EM=early transition metal) and Fe72RE6B22(RE=rare earth element) to investigate the correlation between their structure and glass forming ability. The concentration correlation function of Fe70EM10B20system is close correlated to glass forming ability. The profile of concentration correlation function mainly depends on the atomic size of EM and the chemical bonding of Fe-EM and EM-B also plays an important role. In Fe72RE6B22system, the good glass forming liquids have smaller atomic size of RE that bad glass forming liquids but have stronger chemical bonding of RE-Fe and RE-B.