Liquid-liquid Phase Separation And Dendritic Growth Kinetics Of Co-Cu Alloys Under Static Magnetic Fields

Convection exists in the metallic melts inevitably,and affects heat and mass transformation of metallic melts.For the metastable liquid phase separated alloys,the convection not only has an effect on the liquid phase separation behavior,but also influences the dendrite growth process,and ultimately affects the microstructure and properties of the materials.Therefore,it is significant for the study of the liquid phase separation and dendrite growth under the convection conditions.The static magnetic field is an effective method for controlling the convection.So,it is necessary to study the liquid phase separation and dendrite growth kinetics under static magnetic fields.In this paper,Co₇₀Cu₃₀,Co₈₀Cu₂₀ and Co₉₀Cu₁₀ alloys were undercooled by glass fluxing technique with and without the imposition of a static magnetic field.The recalescence of Co-Cu alloys were investigated by means of a single-color pyrometer and a high speed camera.Dendritic growth velocity was calculated by computer simulation and analyzed using LKT/BCT models.Microstructures of Co-Cu alloys with different static magnetic field were analyzed.The main conclusions are drawn follows.（1）The forced convection in the sample of high frequency induction melting is very notable for the size distribution of the dispersed phase drop.Under the condition without magnetic field,cooling off without source,no forced convection,the droplet size distribution is unimodal distribution;Under the condition without magnetic field,cooling off with decompression,there existed forced convection,dispersed phase droplets size distribution is a bimodal distribution.The static magnetic field is very effective for controlling of the convection,and the size distribution of the droplet is a single peak distribution under the static magnetic field of 2T and 1T.When the magnetic field strength is higher than 2T,the static magnetic field has the promotion effect on the liquid phase separation,the liquid droplet nucleation greatly increases,the probability of collision coagulation is also increasing,forming the distribution of bimodal or multimodal distribution.（2）The static magnetic field has obvious effect on dendritic growth velocity of Co₇₀Cu₃₀ alloys under high undercooling（greater than 250K）.When the static magnetic field intensity is greater than 2T,the over cooling degree is greater than 250K,and dendritic growth velocity of Co₇₀Cu₃₀ alloy will be jumped.Cu-rich phase thermal conductivity rate is much higher than that of the Co-rich phase,the static magnetic field prompted Cu-rich phase absorping crystallization heat of Co-rich phase,equivalent to increases the Co-rich phase temperature gradient and improve the undercooling.（3）For Co₈₀Cu₂₀ and Co₉₀Cu₁₀ alloys,dendritic growth velocity without magnetic field is more than dendritic growth velocity with magnetic field under small undercooling.But under high undercooling.There are two sides of the effect of Cu-rich droplets on the dendrite growth of Co-rich.On the one hand,dendritic growth velocity is promoted from the perspective of increasing undercooling of Co-rich phase,droplets promote the dendrite growth,on the other hand,inhibiting dendritic growth velocity growth from the perspective of interface energy.