Rapid Dendrite Growth Of Undercooled Multicomponent Ni-based Alloys

The rapid dendrite growth of multicomponent alloys is an important research subject in the field of materials science. The present thesis has performed detailed investigations on the high undercooling and rapid dendrite growth of multicomponent Ni-based alloys.Using the electromagnetic levitation and glass fluxing techniques, Ni-10%Cu-10%Si ternary alloy as well as Ni-10%Cu-10%Co-10%Fe, Ni-40%Cu-10%Co-10%Fe and Ni-5%Cu-5%Co-5%Si quaternary alloys were successfully undercooled. The maximum undercoolings obtained in the experiments are 236K(0.17TL), 276K(0.16TL), 296K(0.18TL) and 312K(0.19TL), respectively. The growth velocities of a-Ni dendrite in these alloys are measured experimentally as a function of undercooing by a photodiode device.The dendrite growth velocity of Ni-10%Cu-10%Si alloy is an exponential function of undercooling, which is V=1.6×10^-13×△T^5.7. Within the experimental undercooling regime, the a-Ni dendrite growth is always controlled by solute diffusion, in which the effect of the solute Si is more significant than that of Cu. In Ni-10%Cu-10%Co-10%Fe and Ni-40%Cu-10%Co-10%Fe alloys, the relationships between the dendrite growth velocity and undercooling can be expressed as V=8×10^-2×△T^1.2 and V=1.4×10^-3×△T^1.8, respectively, which indicate that the dendrite growth velocity in the former alloy is larger than that in the latter alloy at the same undercooling. The dendrite growth velocity of Ni-5%Cu-5%Co-5%Si alloy increases with the increase of undercooling, sharply at small undercooling and slowly at large undercooling. During the dendrite growth of a-Ni phase, the effect of Si is significant, but the effects of Cu and Co are negligible.Remarkable solute trapping takes place due to the rapid solidification of Ni-10%Cu-10%Co-10%Fe alloy. The solute contents of Cu, Fe and Co increase with the increase of undercooling. At the undercooling of 200K, the complete trapping of Fe and Cu solutes takes place on the solutes, consequently their contents reach the original alloy concentration. In equiaxed grains, the distributions of all the three solutes are rather homogeneous and the microsegregation can be hardly observed.Both the experimental measurement and theoretical calculation indicate that the crystalline lattice parameter of Ni-10%Cu-10%Co-10%Fe alloy is larger than that ofNi-40%Cu-10%Co-10%Fe alloy. The lattice parameter increases with undercooling by the linear relations as a=3.55+1.77×10^-5△T and a=3.57+3.8×10^-5△T, respectively. Nucleation analysis shows that heterogenous nucleation would take place in the two quaternary Ni-Cu-Co-Fe alloys even at the maximum undercooling obtained in the experiments.With the increase of undercooling, the transition of the microstructure of a-Ni phase from dendrite to equiaxed grain, accompanied by the grain refinement was discovered in all the four Ni-based multicomponent alloys.