Molecular Dynamics Simulation Study Of Chemically Heterogeneous Al-Pb Solid-liquid Interface

The solid-liquid interfaces exist widely in the manufacturing processes of many national key demand products such as high-quality alloys,chips and renewable energy devices et al.For instance,the thermodynamic properties and microscopic structure information of alloy solid-liquid interfaces are key factors in understanding many physical processes of fundamental importance in metallurgical manufacturing,including nucleation,grain growth,inclusion transport,etc.With the rapid development of electron microscopy,theoretical and computation/simulation technology,more and more interfacial structural phase transitions(premelting,etc.)and in-plane multiphase coexistence(complexion phase within grain boundaries)have been discovered.A complete thermodynamics theory for understanding such novel structure phases within the material interfaces is still missing.To construct the thermodynamics theory,in addition to the microscopic structural information,a complete set of thermodynamic quantities describing the interfacial phases is needed.However,the state-of-the-art experimental techniques cannot provide entire lateral atomistic structure for the interfacial layers,and it is nearly impossible to measure the key thermodynamic quantities(such as local pressure)directly.Methodologies which are based on atomistic simulations have the potential to explore key interfacial thermodynamic quantities.Unfortunately,existing studies rarely focused on determining core thermodynamic quantities which are required to construct the thermodynamic theory.Therefore,the fundamental research on the structural phase transition as well as the in-plane multiphase coexistence within the interfaces remains at a stage in which the data is incomplete,and the theory is lacking.In this thesis,we choose a representative heterogeneous alloy solid-liquid interface system(Al-Pb solid-liquid interface)as our primary research subject.We have developed a “simulation-characterization-calculation”integrated methodology.This methodology is based on molecular dynamics simulations and is applied to the realization of construction of well equilibrated Al(solid)-Pb(liquid)interfaces,yielding comprehensive study of the structural and thermodynamic properties,and high-precision calculation of the key interfacial parameters(i.e.,step free energy and pressure tensor components).Significant findings include: 1)A systematic investigation of the similarities and differences between the two pressure calculation algorithms in dealing with the local pressure of the heterogeneous alloy solid-liquid interfaces.The two local pressure calculation algorithms employed in the current thesis are: a)an atomic virial stress based calculation algorithm,b)an algorithm based on the Irving–Kirkwood statistical mechanical definition of local pressure.Major disadvantages of the atomic virial stress based algorithm in the description of the local interfacial pressure have been found.We demonstrate,for the first time,the power spectrum of the equilibrium step fluctuations obeys the capillary wave theory,and the calculated step free energy is consistent with experimental measurement.A new level of understanding of the thermodynamical and mechanical conditions for the solid/liquid interface step is obtained.The properties calculated are essential to the development of a detailed thermodynamic theory for faceted heterogeneous solid/liquid interfaces.3)At the lower temperatures,a large number of transient disorder transitions accompanying the capillary fluctuations of the step are found.Their physical mechanism is closely related to the Kosterlitz–Thouless transition of the two-dimensional system,the transition from the two-dimensional hexagonal phase to the two-dimensional liquid phase,and the interfacial(pre)roughening transformation interface premelting transition et al..We also found that such localized transient disorder transitions have unique properties different from the phase as mentioned earlier transformation process,and may universally exist in metal surface or interface steps.The systematic research carried out in this thesis aims to provide essential data and research paradigm to facilitate the new advancements of the interfacial thermodynamics.The ”simulation-characterization-calculation” integrated methodology proposed in this thesis can be extendable to the exploration of many current hot research frontiers,such as multi-complexion equilibria at grain-grain-boundaries.The new findings in the thesis will bring new fundamental insights into the field of simulation and computation study of material interfaces.