A Study On Dendrite Growth And Thermophysical Properties Of Alloy Melt

Rapid solidification of undercooled melt provides an effective approach for preparing bulk materials. The simulation of dendrite morphology in mesoscale is of great importance for controlling the quality of solidification products. Phase-filed method is an effective approach to simulate the dendrite growth of undercooled melt. However the accuracy of the result depends on the thermophysical properties of the undercooled melt. In the present thesis the dendrite growth of the undercooled melt are investigated by the phase-filed method. Then the thermophysical properties of some undercooled alloys are calculated by molecular dynamic simulations.First, the 2D dendrite growth without sidebranch in undercooled melt of pure substance is simulated by the phase-filed method. Then, the thermal noise is incorporated into the phase-filed model with Langevin formalism. The phenomena such as formation of the secondary branches, the tertiary branches, the growth of dendrite, and the ripening progress are observed during the simulation. The secondary branch spacing and amplitude of the sidebranch are obtained. The relationship between the dendrite growth and the parameters in phase-field, which includes undercooling, anisotropic coefficient, magnitude of thermal noise, has been studied quantitatively. The results show that the undercooling and the anisotropic coefficient have great influence on the secondary branch, but the magnitude of thermal noise has little.Base on the embedded atom method, the specific heats of Ni-30%Fe and Ni-55%Fe alloy have been calculated by molecular dynamics simulations. The specific heat of these alloys increase linearly with the increase of temperature before 1500K. A maximum is observed in the specific heat near 1500k. The results of structure analysis show that the FeNi alloys begin to crystallize near the 1500K, which explains to the unusual behavior of the specific heat. Both the results of the molecular dynamic simulation and the experiment indicate that the specific heat is not sensitive to the concentration of FeNi alloy.The simulation of specific heats and melting temperature of Ni₃Al alloy has beencarried out. The melting temperature is obtained by the crystal-liquid sandwich method and the NVE ensemble method. The results of these two methods are very similar, but both above the result of the experiments. The specific heat of Ni₃Al alloy increases linearly with the increase of temperature.The specific heat and the density of Ni₂TiAl alloy have been simulated by molecular dynamics method. The results indicate that the specific heat and the density increase linearly with the decrease of temperature, and the Neumann-Kopp rule can be applied to estimate the specific heat of undercooled liquid Ni₂TiAl alloy. Meanwhile, the size effects in the simulation domain are proven to be negligible.