Atomistic Study Of Solid-liquid Interfacial Properties Of The Cu-Ni Binary System

The properties of crystal-melt interfaces have great influence on many physical phenonmena and processes,such as nucleation and growth during solidification.The interfaces however locate in the interior of condensed phases,making direct experimental observation or investigation rather difficult.Fortunately,atomic scale simulations can be and were already employed in such studies,and great advances have been made during the past years.Nevertheless,an overwhelming majority of the previous studies concentrated on ideal systems or single element systems,while alloy systems which are of great theoretical and practical significance were rarely explored.To meet this gap,in this work we take the Cu-Ni binary system as an example,to examine the microstructure,thermodynamics,kinetics properties and their anisortropies of the solid-liquid interfaces by using molecular dynamic simulations,so as to deepen our understanding on the characteristics of the crystal-melt interfaces and to promote the development and improvement of related theory.The main results are as follows:The profiles of local order parameter,atomic number density,diffusion coefficients and Lindemann indices were evaluated and the interface widths were determined based on them.The widths are found to depend on the orientation of the underlying solid,while the orientation dependence for widths based on different criteria differs.The ones based on the local order parameter,given as the difference between an actual local atomic configuration and a referential ideal crystalline one,are found to be able to reflect the transitions across the interface in terms of both geometry and atomic motility,and are therefore most reliable.The interfacial layers are found to be composed of mixtures of both solid and liquid phases,the tempo-spatial variations of their relative amounts play an important role in determining the interfacial properties,including the interfacial widths.The solid-liquid interfacial free energy?and its anisotropy in the Cu-Ni binary system were measured by using the capillary fluctuation method?CFM?based on an embedded atom method potential.It is found that both the interfacial energy and its anisotropy are enhanced with the increasing of the coexisting temperature,significant variations are however observed in the anisotropy parameters.Nonetheless,the anisotropy relationship is hardly modified,and the inequality?₁₀₀>?₁₁₀>?₁₁₁ holds for all temperatures studied.By projecting the calculated anisotropy parameter and onto the dendrite growth direction selection map,one finds that a<100>dendrite is generally favored for the Cu-Ni alloys.Upon alloying,the preferred dendrite growth direction shifts to the vicinity of the boundary between the<100>and hyper-branched regions,indicating the possibility of a transition of solidification morphology from a<100>dendrite dominated to a hyper-branched one.These predictions agree well with and explain the experimental observations during the equilibrium and non-equilibrium solidifications in the Cu-Ni alloy systems.The kinetics of crystallization and melting are studied for the Cu-Ni binary system by employing molecular dynamic simulations based on an embedded atom method potential.These simulations form the basis for calculations of the magnitude and anisortropies of the interfacial kinetic coefficient?.It is found that?is firstly decreased and then enhanced with the increasing of the solute concentration,significant variations are however observed in the anisotropies.Nonetheless,the anisotropy relationship is hardly modified,and the inequality?₁₀₀>?₁₁₀>?₁₁₁ holds for all temperatures studied.Upon alloying,the anisortropies of the interfacial kinetic coefficient decrease about 15%in the interfacial kinetic coefficient,indicating the possibility of a transition of solidification morphology from a<100>dendrite dominated to a hyper-branched one.These predictions agree well with and explain the experimental observations during the equilibrium and non-equilibrium solidifications in the Cu-Ni alloy systems.