Computational Study Of The Material-dependent Atomic Structure And Thermodynamic Properties For The Pure Crystal-melt Interfaces

The grasp of the core crystal-melt interfacial thermodynamics properties and relationships are crucial to the advancement of the solidification process control during advanced metal manufacturing.Due to the direct atomic-scale experimental characterization for the crystal-melt interface being a challenge,most of the research studies over the past years employed “computer experiments” to carry out the relating exploration.The existing studies focus on the calculation and measurement of various properties.Yet,the knowledge regarding how do the lattice type,orientation,and interatomic interaction coherently impact the local particle packing remains incomplete,which results in a series of fundamental questions remain unresolved: for example,the structural nature of a crystal-melt interface(diffusive or sharp)remained controversial;the material dependences of the interfacial thermodynamics and the kinetic properties(and their anisotropies)being inconsistency;the absence of the quantitative theory for predicting excess interfacial quantities;development of the quantitative solidification kinetic theory being stagnating,and so on.The studies reported in the current thesis undertake molecular dynamics simulations to explore the body-centered cubic(BCC)and face-centered cubic(FCC)elemental crystal-melt interfacial(CMI)systems.We computed the interfacial pressure components distribution and the excess interfacial stresses,proposed an order parameter method free of any artificial criterion for distinguishing crystal phase particles from those belonging to the melt phase,developed an intrinsic interface capturing process,and accomplished high-precision statistical analysis on the transition of the interfacial layering structure by removing the capillary waves.The detailed works include:(1)We study the equilibrium CMI stresses in FCC Ni and BCC Fe,BCC Nb,and a model BCC soft-sphere elemental system,for three different interface orientations,i.e.,(100),(110),and(111).The sign,magnitude,and anisotropy of the excess interface stresses and their relationships with the corresponding interfacial free energies have been examined.The universality of a few trends regarding the interfacial stresses observed in FCC CMIs has been assessed for the BCC CMIs.The role of the interatomic bonding that affects the shape of the interfacial stress profiles,thus modulating the magnitude or sign of the excess interface stress,has been discussed through inspecting a particular type of CMI over different materials.Besides,we have demonstrated that the Irving-Kirkwood fine-grained algorithm for depicting microscopic pressure components and stresses in the vicinity of the CMI is superior to the previously used per-particle virial stress algorithm(2)We developed an atomic-resolution intrinsic method for capturing the instantaneous shape of the elemental CMI.We utilized the lattice displacement order parameter and the bond orientational order parameter and proposed a self-consistent optimizing method for obtaining the instantaneous intrinsic interfacial morphology according to the correlation between the two order parameters.(3)The intrinsic interface capturing method was employed in the BCC Fe and FCC Ni CMI systems.We analyzed the material-dependences on the intrinsic particle packing structure across the interface.It is found that,as the CMI is traversed towards the melt phase: the decay in bond orientational order parameter in the BCC system is slower than that of the FCC system;the lattice displacement order parameters decay within only two atomic layers;these two decaying fashion varies as the interfacial crystalline orientations,especially obvious for the lattice displacement order parameters decay.Significant anisotropy between the normal and transverse CMI directions is noticed in the impacts from crystal surface layers to the packing geometry in the adjacent interfacial melt phase layers.(4)We examined how the interatomic interaction affects the intrinsic particlepacking scenarios under the environment of two-phase competition by employing the intrinsic interface capturing method to the four BCC(111)CMI systems modeled by four different interatomic potentials.Our high-precision statistical analysis revealed that each interfacial layer's unique relationship between the values of the two order parameters,independent of material type or the interatomic interaction potentials.Beyond such universal relationships,the known material-dependent behaviors for CMI properties are due to the actual population for the particles corresponding to specific bond orientational orders being highly material-dependent.In addition,we found the asynchronous transitions of the two intrinsic order parameter profiles,as the CMI is traversed,akin to the development of the hexatic phase during 2D melting to a certain degree.The findings in the current thesis could: i)Help to clarify the controversies in the structural nature of a crystal-melt interface,quantify the intrinsic width of the CMI,and provide theoretical justification for improving the phase-field crystal modeling method.ii)Disclose the reason for inconsistency lies in the material dependences of the interfacial thermodynamics and the kinetic properties(and their anisotropies).iii)Facilitate the advancements of the quantitative theories of predicting the CMI excess properties and the solidification kinetic properties.The methodology proposed in my thesis could be further extended by introducing the machine learning algorithm and upgraded to adapt the alloy systems and/or non-equilibrium CMI systems to study the mechanism of the solute/vacancy trapping processes.