Effect Of Phase-separated Pattern And Globule Collision On The Core-shell Structure In The Droplet-shaped Immiscible Alloys

Recently,the spherical immiscible alloy plays an important role in the automobile bearing,catalytic field and electrical contact switch because of the special structures.Owing to the potential applications in industry,such materials have drawn considerable atterntion for several decades.Thus,explaining the formation mechanism of the internal constitution has always been one of the hottest topic in immiscible alloys,aiming to providing a theoretical basis for the efficient production of ideal immiscible alloy.However,during the actual solidification of spherical alloy,the harsh experimental conditions often make the solidification process impossible to be observed,which leads to the lack of evolution process and greatly limits the researchers to further explore the formation mechanism.Since the thermodynamic processes between nucleation and spinodal decomposition are different,phase separation is an important factor affecting the product of droplets.Based on this,the migration velocities of droplets in the same temperature gradient field will be different.However,there is little report on this phenomenon,thus the effects of phase-separated pattern on the structure and the evolution progress of"core"formation are of central importance and need to be further explored.For spherical alloys,the internal temperature gradient is related to the spherical size,i.e.the droplets have different migration rates in different-sized alloys.The resulting differences in morphology remain unclear.For spherical alloys,the internal temperature gradient is related to the spherical size,which means that the droplets have different migration rates in spherical alloys of different sizes.The resulting differences in morphology remain unclear.To solve above problems,we select the SCN-H₂O system to study the nucleation and spinodal decomposition by utilizing the in-situ observation,and perform the experiments in the constant,unidirectional and circular temperature fields,respectivelly.The interaction and morphology of droplets were investigated.The influence of phase-separated pattern on the formation path of"core"structure was investigated.In addition,the influence of temperature gradient on the formation ability of"core-shell"structure was also investigated.The Fe-Sn powder was compared and studied to obtain the formation mechanism of particle internal morphology diversity.The main conclusions of this paper are as follows:（1）In a constant temperature field,the effect of phase-separated pattern on the growth scaling law and the following morphology of the droplets was investigated.Also,the mechanism of phase separation on the growth of the second phase droplets was clarified.The nucleation-growth and spinodal decomposition were investigated in SCN-70wt.%H₂O and SCN-50wt.%H₂O mixtures by quenching method.We in situ observed the kinetic behavior of the second phase droplets and morphology evolution characteristics of the droplets in different phase separation pattern.The results show that spinodal decomposition occurs more quickly than the nucleation process and the second phase droplets can be precipitated in a short time when compared with the nucleation.The relationship of the radius and the time satisfies:r～t in spinodal decomposition,whereas the growth rule of droplets during nucleation is mainly:r～t^1/2.At the same time,different phase separation methods in the same system SCN-60wt.%H₂O were carried out when the samples were quenched to the same temperature.The results show that spinodal decomposition in SCN-50wt.%H₂O mixture is different from the SCN-60wt.%H₂O mixture.（2）The morphological evolution of minor-phase droplets were studied in unidirectional and annular temperature gradient fields,respectively,and the relationship between the morphology and phase-separated pattern was revealed.SCN-50wt.%H₂O and SCN-70wt.%H₂O mixtures were used to explore phase separation and the interaction of droplets druing the phase separation and its influence on the macroscopic microstructure in unidirectional and annular temperature gradient fields.When the samples were under a unidirectional temperature gradient fields,it is found that the interaction between the droplets is strong,and the bands happen when individual droplets got together in spinodal decomposition.On the contrary,no obvious collision was observed in the case of nucleation.When the temperature gradient field is circularly distributed,the strong interaction causes the droplets to join in a ring-shaped structure,after which the ring-shaped tissue is further tightened toward the center of the sample,thereby forming a"core-shell"structure,which is different from the conventional wisdom.（3）Effect of temperature gradient on the migration behavior of droplet was studied,and the mechanism of temperature gradient on the"core-shell"structure was revealed.Different temperature gradient fields are established by adjusting the temperature values around the circular sample.The SCN-50wt.%H₂O and SCN-70wt.%H₂O systems were used as the research objects,and two phase separation methods were realized respectively.The migration characteristics of the second phase droplets in different temperature gradients under the same phase separation mode were studied.And the evolution process of the morphology during droplet assembly,the study finds that the higher the temperature gradient is,the more obvious the droplet migration is,the easier it is to concentrate in the center of the sample to form a"core-shell"structure;The ring-like structure formed by spinodal decomposition is independent of the temperature gradient,so the"core-shell"structure is independent of the temperature gradient in this case;the droplets’spacings formed by the nucleation are big and the migration distance is proportional to the temperature gradient.The"core-shell"structure is closely related to the temperature gradient:the higher the temperature gradient is,the stronger the"core-shell"structure formation ability becomes.（4）The movements of minor-phase spheres in different-sized immiscible alloy particles were explored,and the origin for morphology diversity in different particles was revealed.Using the containerless technology of drop tube processing,the various microstructures of different sizes of Fe-58wt.%Sn powder particles were studied.The internal temperature fields of powder particles of two sizes（Radius=200μm and=400μm）were calculated and Marangoni migration rate of the second phase pellet and collision strength inside the particle are compared.Study finds that at the same relative position,the temperature gradient inside the small-sized powder particles is about 1.5 times the large particle’s temperature gradient,and in which the droplet migration rate is faster,whereby the guided droplet collision is more intense.Although the solidification time of the particles is different,the relative positions of the same size pellets inside the particles are independent of the particle size.In addition,the calculation results show that the collision intensity of the droplets inside the small-sized particles is about 10 times that of the large particles.This difference is the root cause of the diversity of morphology.