Dendritic Growth Kinetics Of Undercooled Melts Of Ni-Si Alloys Under Uniform Magnetic Fields

In the field of metal solidification, dendrites are common tissue morphologies of metal castings. Microstructures and mechanical properties of the solidified material are determined by its dendrite growth. Hence, studies on dendritic growth kinetics are of great importance. Previous studies showed that dendrite growth would be influenced by convection in undercooled melt. In the present thesis, convection environment in undercooled melt was changed by forcing different uniform magnetic fields, so that to study the dendritic growth kinetics in undercooled solid solution melt by experimental research as well as to analyze it by using theory models.In the present thesis, bulk melts of Ni95Si5 alloy and Ni90Si10 alloy were undercooled and solidified using glass fluxing method firstly. The recalescence processing of the alloy were using a high-speed camera to record and the temperatures of the sample surfaces were using a single-color pyrometer to measure at the same time. Dendrite growth velocities in undercooled melts were determined using the three-dimensional computer animation technique. The dendrite growth kinetics of solid solution under non-magnetic field and different static magnetic fields were analyzed by the modified LKT/BCT model. The main conclusions are summarized as follows.1. The dendritic growth velocities of the Ni95Si5 alloy and the Ni90Si10 alloy both increased with undercooling, and solute trapping phenomenon occurred in both melts. The increasing of dendritic growth velocity slowed down when solute trapping occurred apparently. For the time being, dendrite growth is mainly controlled by diffusion. When the undercooling is more than the critical undercooling, fully solute trapping occurring. And the dendritic growth velocity is linear growth by thermally controlled. The critical undercooling of fully solute trapping are about 196K and 206K in the two alloys respectively. And magnetic fields had no obvious effect on them.2. When magnetic fields intensity B ranging between 0-6T, The dendritic growth velocity of Ni90Si10 alloy is smaller than the dendritic growth velocity of Ni90Si5 alloy under different conditions of uniform magnetic fields. Under low and medium undercooling conditions, the uniform magnetic fields have a significant influence on the dendritic growth velocities of Ni95Si5 alloy and Ni90Si10 alloy. With increasing magnetic field intensity, the dendritic growth velocity decreased firstly and then increased after reaching a minimum at a critical magnetic field of 3T. Under large undercooling conditions, the uniform magnetic fields have no obvious influence on the dendritic growth velocities of Ni95Si5 alloy and Ni90Si10 alloy.3. Analysis indicated that when magnetic field intensity was less than 3T, the uniform magnetic field could restrain convection in NigsSis alloy and Ni90Si10 alloy melts effectively and decreased dendritic growth velocities of the alloys. When magnetic field intensity was larger than 3T, dendritic growth velocities of the alloys would increase slowly because of the thermo-electromagnetic convection effect being higher. Comparing the two alloys, magnetic field had a bigger effect on growth velocity of Ni90Si10 alloy.4. Fitting the measured dendritic growth velocities by using the modified LKT/BCT model showed that the thermal diffusivity and the diffusivity of Ni95Si5 alloy and Ni90Si10 alloy decreased firstly with increasing the uniform magnetic field intensity and then increased after reaching a minimum at a critical magnetic field of 3T, the same as their dendritic growth velocities did.