Structural Evolution And Solidification Behavior Of Melt On Wedge Substrate And In Nanoslit

In nature,metallurgical technology and material preparation process,solidification is a very common physical phenomenon.Solidification widely exists in every field,and objects from a liquid into a solid have to go through the process of solidification.Solidification begins with nucleation.The two main types of nucleation are homogeneous nucleation and heterogeneous nucleation.In actual production,most of the nucleation process is heterogeneous nucleation.So it is necessary to study the impact of solid phase nanoparticles on melt solidification process in the heterogeneous nucleation.In this dissertation,molecular dynamics simulations are performed to study the impact of solid-phase particles on structure and solidification behavior of melt,revealing the relevance of the solid phase nanoparticles and solidification structure of melt.These theories are significant to improve the macroscopic properties of materials by controlling of heterogeneous nucleation.Main content of this thesis is as follows:(1)We have explored the microscopic details of how the wedge substrate spreads its structural information to a growing crystal and further affects the solidification process.The simulation results show that,owing to the induced effect from the substrate,the copper atoms become layered at the liquid-solid interface in a"V"-shaped pattern and tend to form a twin crystal.The structural information delivery of the substrate decays with the distance away from the substrate,and the final solidified structure would gradually recover its inherent structure.Interestingly,the wedge angle of 90° seems to be an exception at which the solidified structure exhibits a perfect crystal due to the nearly perfect match with the Cu atoms.Moreover,the cooling rate and the atomic structure of the substrate are also found to have striking correlations with the final structures.These research results are favorable for a better understanding of the inherent relation between the solidified structure and the substrate.(2)Then,we study the solidification of Al melt in confined nanoslits(NSs)constructed by identical or different substrates,as well as on Fe substrates.Compared to the single substrate,the confined NS could promote the crystallization of A1 melt,and its size has a significant impact on the solidified structure.In symmetrical NSs,liquid Al atoms would stack based on the atomic arrangement mode of the substrate,however in asymmetrical confined NSs,the atomic arrangement mode of liquid A1 is governed by the constitution of asymmetrical substrates.Specifically,for the NS formed by Fe(110)and Fe(111)substrates,the induced region from the Fe(110)substrate is much bigger than that from Fe(111).Moreover,the freezing of liquid Al in asymmetrical NSs constructed from copper and iron has also been studied.These results throw light on heterogeneous nucleation in confined space.(3)The layering structure and liquid-liquid transition of liquid water confined between two graphene sheets with a varied distance at different pressures are explored.Both the size of nanoslit and pressure could cause the layering and liquid-liquid transition of the confined water.With increase of pressure and the size of nanoslit,layering in confined water becomes more and more obvious.Furthermore,the neighboring water molecules firstly form chain structure,then will transform into square structure,and finally become triangle with increase of pressure.These results throw light on layering and liquid-liquid transition of water confined between two graphene sheets.