Dendrite Growth And Grain Refinement In Undercooled Melts Of Ni₇₀Cu₃₀ Alloy Under Gradient Magnetic Fields

Dendrites are the most common crystal occurring during solidification of metallic metals. Microstructures of the alloy are determined by their dendrite growth. With increasing the initial melt undercooling, grain refinement occurred in Ni7oCu3o alloy twice. Previous research showed that, the convection will not only change the dendritic growth kinetics of undercooled melt, but also change the microstructure. Convection will be controlled by gradient magnetic field very effectively. Then we can use gradient magnetic field to introduce different Lorentz forces to affect convection flow, which can be used to get controlled convection conditions. It is thus necessary to perform investigations of dendritic growth kinetics and grain refinement in undercooled Ni7oCu3o alloy under different gradient magnetic fields, which may tell us the regularity of rapid solidification process in undercooled Ni7oCu3o alloy under different gradient magnetic fields.In the present thesis, bulk melts of Ni7oCu3o alloy was used as the model alloy to study the effect of convection on dendrite growth and grain refinement. In experiment the Ni7oCu3o alloy was undercooled using a combination of glass fluxing with repeated overheating, and their rapid solidification process were in-situ observed using a high-speed camera and a single-color pyrometer to measure the dendrite growth in undercooled Ni7oCu3o alloy. The microstructures of Ni7oCu3o alloy at different undercooling and gradient magnetic field were observed by metallographic microscope. The dendrite growth velocity which measured in experiment was analyzed by using dendrite growth theory, while the evolution of microstructure was analyzed by using Karma model. The main conclusions are as follows.1. The gradient magnetic field did not have significant effects on the cooling curves. The dendrite growth velocity of Ni70Cu30 alloy is in the wake of the increase of undercooling. The beginning of the solute trapping occurred in Ni70Cu30 alloy and the dendrite growth velocity became slow when the undercooling reached 70K, at this time the dendrite growth mainly controlled by solute diffusion. The critical undercooling is about 190K in Ni7oCu3o alloy. The solute trapping occurred when the undercooling is more than 190K. Then the dendrite growth according to a linear law. The dendrite growth mainly controlled by thermal diffusion at this time.2. The dendrite growth velocities are analyzed by using Galenko’s model. The results show that, the gradient magnetic fields did not have any significant effect on the dendrite growth velocity of Ni70Cu30 under large undercooling. While under small and medium undercooling conditions, the dendrite growth velocity of Ni7oCu3o alloy decreased firstly with increasing magnetic field intensity, and then increased after reaching a minimum at a critical magnetic field of 3T. It is suggested that a suitable gradient magnetic field can damp convection in the melts of Ni70Cu30 effectively, which rendered the dendritic growth velocity reduced. If the magnetic field intensity exceeded 3T, the thermoelectromagnetic convection would be enhanced, resulting in the recovery of the dendritic growth velocity.3. With the increasing of undercooling, the microstructures were changed from equiaxed grains to coarse dendrite and then equiaxed grains in different magnetic fields. We can find spherical crystal and equiaxed grain under small undercooling and large undercooling. Some parts are still connected with dendrite trunk. It was suggested that the grain refinement was caused by dendrite remelting.4. The comparison of Karma model and experimental results suggested that the absolute error are smaller than 6K. This is easily understood by the fact that in large undercooling the dendrite growth were fast than the melt flow. While in small and medium undercooling, the absolute error decreased firstly with increasing magnetic field intensity, and then increased after reaching a minimum at a critical magnetic field of 3T. This is because that at small undercoolings the growth velocity is smaller and fluid flow has a stronger effect on solute and themal diffusion, the dendrite growth is influenced by convection significantly.