Effects Of Gradient Magnetic Fields On Liquid-liquid Phase Separation Behavior And Solidification Kinetics Of Cu-Co Alloys

Liquid-liquid phase separation and dendritic growth are common phenomena during solidification of liquid-liquid phase-separated alloys, and they have a great influence on micro structure and properties. Hence, studies on liquid-liquid phase separation behavior and dendritic growth kinetics are of great importance both theoretically and technically. Convection exists in metallic melts inevitably, and affects the solidification process and microstructure of materials. A gradient magnetic field imposes different Lorentz forces and a magnetizing force to affect convective flow, and thus can be used to control the liquid-liquid phase separation behavior and solidification processes of the liquid-liquid phase separated alloys. It is necessary to perform investigations of liquid-liquid phase separation behavior and solidification kinetics under different gradient magnetic fields.In the present thesis, bulk melts of Co70Cu30 and Co80Cu20 alloys were undercooled by the glass fluxing technique with and without the imposition of a gradident magnetic field of 2 T. The effects of convection, undercooling and alloy composition on liquid-liquid phase separation behavior and dendritic growth kinetics of Cu-Co alloys were investiaged by means of a single-color pyrometer, a high-speed camera and an opitical microscope. The main conclusions are drawn as follows.(1) Compared with the gradient magnetic fields of 0 T, the gradient magnetic fields suppressed the forced convection so that the effects of weak convection such as floating convection and marangoni convection played a more obvious role, the Cu-rich droplets of Cu-Co alloys were changed from a bimodal size distribution to a multimodal size distribution, and the size of peak centers became smaller as well as the segregation of the Cu-rich phase on the sample surface became serious.(2) The droplets of the Cu-rich phase resulting from liquid-liquid phase separation, the dendritic growth velocities occurred two mutations. The gradient magnetic fields suppressed the convection, slowed down solutal and thermal diffusion, and then refined the droplet size as well as decreased the dendritic growth velocities, resulting in the increase of the critical undercooling of the first mutation.(3) Under high undercooling conditions, the droplets of the Cu-rich phase, especially the small droplets, impeded dendritic solidification. This impeditive effect was enhanced with increasing undercooling first, and then reduced. It was because the gradient magnetic fields refined the Cu-rich droplets, resulting in a more impeditive effect on the dendritic growth.(4) The lower the Cu concentration of the bulk melts, the smaller of the Cu-rich droplets and the fewer segregation of the Cu-rich phase on the surface of the samples as well as the faster of the dendritic growth.