Study On The Structure Of Clusters For Pure Metal And Binary Alloy Melts

Metallic melt is the necessary state for the manufacturing process of most metallic materials.The microstructure and physical/chemical properties of metallic materials are determined by the compositions and structures of their melts.A deeper understanding of the structures of metal melts are essential for the design and production of many metallic materials.In this paper,a new cluster structure model Wulff was established to describe the thermodynamic stable state of metallic melt.Based on this model,the high temperature liquid environment with complex melt structure can be greatly simplified.This model can accurately describe the structural characteristics of metallic melt and study the changes of cluster structure and size before nucleation,which provided a theoretical basis for revealing the nucleation mechanism at the atomic level.The specific contents are as follows:Firstly,a new atomic cluster structure model was proposed to describe the thermodynamic stability of melt structure for pure metals Al and Cu at high temperatures,which was determined by Wulff construction with the crystal structure inside.The shapes of the structures were determined by surface energies of different crystal plane groups,calculated from density functional theory(DFT),while the size was given by the pair distribution function(PDF)of the experimental high-temperature X-ray diffraction(HTXRD).We demonstrated that the simulated XRD curves from present models match the experimental results quite well,not only the position and width of the peaks but also the relative intensity of the first and second peaks.These results indicate a successful model to describe the structure of metallic melt.The model also explained a main peak deviation phenomenon between the HTXRD and the solid ones of pure metal Al.Secondly,the Wulff cluster model was extended to the binary homogeneous alloy melt(Cu-Ni and Ag-Au)by proposing the restrictive conditions(1.We assumed that the cluster sizes of the two types of metals in binary alloy are similar.2.Under the condition of the thermodynamic equilibrium state,the clusters of the two metals were studied separately.3.The quantity ratio of two metallic melt clusters in binary alloy melts should keep up with their composition ratio).The simulated XRD curves from present models matched the experimental results quite well at high temperatures above liquid-solid two-phase region,including not only the position and width of the peaks but also the relative intensity of the first and second peaks.Moreover,when the temperature near the liquid-solid two-phase region,our model also fitted the experimental strength curve well after modification using solid XRD pattern of relatively high melting point metal instead of its’ nano-particle.The agreement indicates the nucleation processes in homogeneous alloy melts.Thirdly,the Wulff cluster model was applied to the binary eutectic alloy melt(Ag-Cu and Al-Si),and the relevant restrictive conditions are also proposed(1.We assumed that the cluster sizes of the two types of metals in binary alloy are similar.2.The two elements were assumed to be completely immiscible,and their clusters were studied separately.3.The quantity ratio of two metallic melt clusters in binary alloy melts should keep up with their composition ratio).The main results are as follows:the simulation results of both eutectic alloys in the high temperature region were in good agreement with the experimental results,which proved that the application of Wulff cluster model in the high temperature region is successful.However,for Al-Si alloys,there was a deviation of intensity and position of the second peak in the low temperature region near the eutectic temperature.The simulated results after structure and composition modification corresponded to experimental ones,which showed the growth process of Si clusters before nucleation.For Ag-Cu alloys,there was no deviation of the intensity and position in the low temperature region near the eutectic temperature,which was supposed to be caused by the mutual inhibition of nucleation between Ag and Cu clusters.Finally,this model was applied to the FeAl intermetallic compounds to study the structures and the growth process before nucleation.In order to understand the B2 type intermetallic compounds with high vacancy rate,we studied the relationship between vacancy rate,electronic structures and mechanical properties,which obtained the corresponding the microstructure and macroscopic properties.Then,we determined their equilibrium shape using the Wulff construction scheme.The surface energies of the low-index FeAl facets were determined from first-principles calculations.For non-stoichiometric surface terminations,the chemical environment was taken into account through the chemical potential of the constituents.Thus different cluster shapes become stable as a function of the chemical environment.In order to explore the growth process in more detail,we proposed a quasi-static growth model based on the sequential addition of(sub-)monolayers in the most favorable surface directions.Thus a sequence of different Wulff shapes led to the growth process.This model is proved preliminarily by calculating the concentration trend of Al/Fe atom on both Al-terminated and Fe-terminated surfaces,and by simulating the most stable layer adsorbed on these two surfaces.This model might be helpful in analyzing cluster shapes observed during cluster growth processes.