Heterogeneous Nucleation Behavior Of Pure Iron And Its Atomic Level Structure Origin

Heterogeneous nucleation behavior exists widely in practical production,such as casting and welding of metal components,because in the actual production process,the metal melt inevitably contains some heterogeneous phases,such as oxides,intermetallic compounds or other high melting-point particles with different sizes and shapes.The existence of heterogeneous phases greatly affects the nucleation and growth behavior of solid phase in the melt,determines the distribution of solidified structure and,thus,its properties.As the precursor of solidification,the evolution of melt structure determines the formation and development direction of solidification.To understand the process of heterogeneous nucleation,it is of great significance to study the evolution of melt structure and its regulating mechanism during melt cooling and its influence on the behavior of heterogeneous nucleation.Therefore,in order to discuss the nucleation process of iron melt and the evolution of atomic level structure,we took pure iron as the research object in this paper.We studied the influence of endogenous oxides in pure iron melt on the evolution of melt structure by using the synchrotron radiation high-energy x-ray diffraction?HE-XRD?,explored its atomic level structure origin during nucleation process,and revealed the regulating mechanism of heterogeneous on melt structure.We also discussed the efficiency of heterogeneous phase in promoting nucleation during heterogeneous nucleation process by high resolution transmission electron microscope?HRTEM?,and investigated the heterogeneous nucleation behavior of pure iron on the surface of oxide particles.The main findings are as follows:?1?Two kinds of oxide particles produced by oxygen-dissolved liquid iron during thermal cycling,which are FeO and Fe₃O₄.The exposed crystal planes of FeO and Fe₃O₄are?002??_FeO,?1?11??_FeO and?2?02?_Fe3O4,?2?22??_Fe3O4,which is detected by HRTEM.Among them,?002??_FeO,and?2?02?_Fe3O4 were assumed as the potential nucleation substrate surfaces with the calculated Bramfitt planar misfits as 15.4%for{110}_?-Fe and 9.0%for{112}_?-Fe,respectively.?2?Tracking the melt evolution process by HE-XRD during the solidification process of the melt containing heterogeneous phases.The results show that the first peak intensity of S?Q?increases with the decrease of temperature,its peak shifts to higher Q value;a shoulder exists on the right side of the second peak indicates that the icosahedron short range ordered?ISRO?structure appears obviously in the solidification process of pure iron.?3?The analyses of the nearest neighbor distance?r₁?and coordination number?Z?of melt structure show that the existence of heterogeneous phases do not obviously change the topological type of clusters in the melt,but r₁ exhibits obvious difference and depends on the type of heterogeneous substrate.?4?The r₁ in different liquids and oxide crystals are ranged in the following order at the similar temperature:r₁?liquid iron??r₁?FeO-contained liquid?>r₁?Fe₃O₄-contained liquid?>r₁?Al₂O₃-contained liquid?>r₁?solid iron?>r₁?FeO crystal?>r₁?Fe₃O₄ crystal?with the undercooling sequence of?T_{pure liquid}?304^oC?>?T_{FeO-contained liquid iron}?223^oC?>?TFe3O4-contained liquid iron?75^oC?>?TAl2O3-contained liquid iron?48 ^oC?.?5?The observed relationship between r₁ and the nucleation undercooling of the nucleation system shows that there is a positive correlation between them.The smaller the difference between the r₁ of melt and the corresponding solidified crystal,the smaller the nucleation undercooling??T?would be.It is further verified that the difference between the ordered structure of the melt and the solid crystal structure is the atomic level structure origin structure of nucleation undercooling.?6?The misfit between primary phase and heterogeneous substrate also reflects the efficiency of liquid/solid transition to some extent.The results are helpful to further clarify the influence mechanism of melt structure,new phase and substrate atomic structure on the nucleation process in heterogeneous nucleation process.