Crystal Growth Velocities And Micro Structure Evolution Of Undercooled Ni₅Si₂-Ni₂Si Eutectic Alloys Under Uniform Magnetic Fields

Compound-compound type eutectic alloys have attracted more and more attention due to their potential applications. But little has been known about their microstructural evolution during solidification. Knowledge of crystal growth kinetics may help to solve this problem. Considering magnetic field can control the convection inside the alloy melt, thus affecting the crystal growth velocity and the solidified structure, therefore this study will be conducted with and without magnetic field.In the present thesis, Ni69.7Si30.3 eutectic alloys, Ni7o.7Si29.3 hypoeutectic alloys and Ni68.7Si31.3 hypereutectic alloys were chosen as a model material and undercooled using the glass fluxing technique in the uniform magnetic field. And the uniform magnetic field intensity ranged from 0-6T. Electromagnetic suspension method was adopted to realize the three alloys quenching experiments. Using a single-color pyrometer to measure surface temperature of the alloy samples, so that to obtain alloy undercoolings. Using a high-speed camera to record the recalescence processes of the alloys in the mean time. Crystal growth velocities in undercooled melts were determined using the three-dimensional computer animation technique. The microstructures of the solidified alloys were also examined. The main conclusions are as follows.(1) Under low and medium undercooling conditions, the growth velocities of Ni69.7Si30.3 eutectic alloys and Ni68.7Si31.3 hypereutectic alloys increased slowly without magnetic field. Due to obvious solute trapping phenomenon, the growth velocity increased rapidly nearly exponential function when undercooling increased to a critical ΔT=80K. The growth velocities of Ni7o,7Si29.3 hypoeutectic alloys increased with rising undercooling. But the rising tendency didn’t appear a sudden change.(2) Under the condition of no magnetic field, the solidification microstructure of the Ni69.7Si3o.3 eutectic alloys, Ni70.7Si29.3 hypoeutectic alloys and Ni68.7Si31.3 hypereutectic alloys changed with rising undercooling. Ni69.7Si30.3 eutectic alloys consisted of cellular regular eutectic for low undercooling, and transformed into anomalous eutectic structure for high undercooling. Its crystal growth mode was coupled growth of two phases. Ni70.7Si29.3 hypoeutectic alloys consisted of equilibrium solidification of hypoeutectic structure for low undercooling, and transformed into non-equilibrium solidification of hypoeutectic structure for high undercooling. Its crystal growth mode was uncoupled dendritic growth of γ-Ni5Si2 phase. Ni68.7Si31.3 hypereutectic consisted of equilibrium solidification of hypereutectic structure for low undercooling, and transformed into anomalous eutectic structure at medium undercooling, then into non-equilibrium solidification of hypereutectic structure at last. Its crystal growth mode was uncoupled dendritic growth of 8-Ni2Si phase for small undercooling, and transformed into coupled growth of two phases for medium undercooling, then into uncoupled dendritic growth of 8-Ni2Si phase for large undercooling.(3) Under the condition of 0～6 T magnetic fields, within the scope of low and medium undercooling, the uniform magnetic fields have a significant influence on the crystal growth velocities of Ni69.7Si30.3 eutectic alloys, Ni70.7Si29.3 hypoeutectic alloys and Ni68.7Si31.3 hypereutectic alloys. With increasing magnetic field intensity, the crystal growth velocity decreased firstly and then increased after reaching a minimum at a critical magnetic field of 3T. And crystal growth velocity of the hypoeutectic was the largest when eutectic’s was minimal. Under large undercooling conditions, the uniform magnetic fields had no obvious influence on the crystal growth velocities of Ni69.7Si30.3 eutectic alloys, Ni70.7Si29.3 hypoeutectic alloys and Ni68.7Si31.3 hypereutectic alloys. Magnetic fields had no obvious effect on the critical undercooling when obvious solute trapping occurred.(4) Under uniform magnetic fields, the microstructure of Ni69.7Si30.3 eutectic alloys consisted of anomalous eutectics and regular eutectics had yet to be broken off completely for medium undercooling, and transformed into anomalous eutectics completely for large undercooling. Ni70.7Si29.3 hypoeutectic alloys consisted of gross dendrites and eutectics between the dendrites for medium undercooling, and transformed into non-equilibrium solidification of hypoeutectic structure for high undercooling. Ni68.7Si31.3 hypereutectic alloys consisted of coarse anomalous eutectics and equilibrium solidification of hypereutectic structure for low undercooling, and transformed into anomalous eutectic structure regular eutectics had yet to be broken off completely at medium undercooling, then into anomalous eutectics completely.(5) Analysis indicated that when magnetic field intensity was less than 3T, the uniform magnetic field could restrain convection in Ni69.7Si30.3 eutectic alloys, Ni70.7Si29.3 hypoeutectic alloys and Ni68.7Si31.3 hypereutectic alloys 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. By controlling convection with magnetic fields, the eutectic structure was restrained and promoted formation of eutectic structure between dendrite structure.