Effects Of Uniform And Gradient Magnetic Fields On Liquid Phase Separation Behavior Of Undercooled Cu-Fe Alloys

Immiscible alloys behave in a liquid phase separation behavior in a thermodynamic equilibrium or metastable state.However,gravity segregation often occurs inside these alloys during solidification under the ground conditions due to the different densities of the two liquid phases.Recent studies have shown that the applications of strong magnetic fields can partially or completely reduce the effects of gravitational segregation on immiscible alloys.However,the mechanisms of the influence of strong magnetic fields on the liquid phase separation behavior of undercooled alloy melts are not well understood in the literature.Therefore,in the present experiment,Cu-Fe alloys were taken as the model alloy.In the present experiment,two kinds of Cu84Fe16 and Cu75Fe25 alloys were prepared by electric arc melting method.Then,the deep undercooling and rapid solidification of the two alloys were achieved by the glass fluxing method under the uniform and gradient magnetic fields.The solidification micro structure was quantitatively analyzed by optical microscopy and CAD software.The effects of compositions,undercooling,magnetic field strength and magnetic field gradient on the phase separation behavior were investigated.The main results obtained are as follows:(1)Under the magnetic fields,the size distribution of Fe-rich droplets in the Cu-Fe alloy melts change from unimodal distribution to bimodal distribution or multimodal distribution.The number of Fe-rich droplets in the solidified structure decreases obviously,the sizes of the average and maximum droplets increase significantly.The analysis showed that convection inside the melt perpendicular to the direction of magnetic induction line is restrained by the Lorentz force and that the effects of the Marangoni convection and other weak convection are enhanced.As a result,the second peak of the Fe-rich droplet size distribution appears.(2)Under the gradient magnetic fields,the droplet size of the second peak of the Fe-rich droplet size distribution in the negative gradient magnetic fields is greater than the droplet size of the second peak under the positive gradient magnetic fields,and the droplet size of the positive gradient magnetic fields is greater than the droplet size of the positive gradient magnetic fields.The Fe-rich droplets set on the lower surface of the sample.The analysis showed that the direction of magnetization force in the Fe-rich phase under the negative gradient magnetic fields is opposite to the direction of the buoyancy,restricting the vertical and horizontal convection in the melt and the influence of Marangoni convection is more obvious than that under positive gradient magnetic fields.There are Lorentz force and magnetization force under positive gradient magnetic fields,so the influence of Marangoni convection under positive gradient magnetic fields is more obvious than that under uniform magnetic fields.(3)For the alloys with different alloy compositions,the droplet size of the peak of the droplet size distribution of the Cu75Fe25 alloys sample is larger than the droplet size in Cu84Fe16 alloys.This is because the critical undercooling in the liquid phase separation of the Cu-Fe alloys decreases with increasing Fe content and the influence of the magnetic fields on the sizes of the Fe-rich droplets in the Cu75Fe25 alloys is more obvious.