Rapid solidification of undercooled nickel and nickel-based alloys

The current rapid solidification theory is known as the Ivantsov solution with marginal stability arguments or "IMS model," whose result gives the relationship between dendrite growth velocity and total undercooling. Disagreement between experimental results and theoretical predictions of the IMS model has been observed.; In contrast to the theory, the solidification velocity of nickel-based alloys exhibits a decreased dependence on undercooling at intermediate undercoolings. Specifically, velocity "plateaus" occur at approximately 100 to 175 K undercooling due to the presence of solute and are accompanied by a morphological transition and increased solute trapping. The IMS model fails to correctly predict the existence of experimentally observed plateaus for alloy systems with a sufficiently large equilibrium partition coefficient.; The IMS model was analyzed in conjunction with a large volume of experimental solidification velocity data collected at Vanderbilt University. Two assumptions of the model are challenged. Firstly, the assumption that dendrites are isothermal paraboloids of revolution is called into question. Modeling the experimental results via optimization routines shows that a hemispherical shape more closely represents thermal diffusional fields at low Peclet numbers, while solutal diffusional fields approximate a paraboloidal shape. Consequently, experimental solidification velocity results at low undercoolings may be modeled by combining a hemispherical thermal diffusional field and a paraboloidal solutal diffusional field. Secondly, the assumption that the stability parameter of the marginal stability criterion is a constant is examined. Determination of the necessary stability parameters to yield the experimentally observed temperature-velocity relationship shows that the stability parameter decreases with increasing Peclet number in accordance with the optimum stability conjecture.; Other noteworthy findings emerged in the process of the investigation. Phase separation was discovered in Ni-4at.%Sn at high temperature following rapid solidification. In addition, significant hydrogen concentrations were measured in nickel and nickel-based alloys after processing in both electromagnetic and electrostatic levitation. Hydrogen was determined to be responsible for historically observed deviations from theoretical predictions at high undercoolings. These measured hydrogen concentrations were considerably greater than the maximum solubility of hydrogen in liquid nickel as predicted by computational thermodynamics.