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Investigation of solidification in undercooled Al-rich Al-Ni alloy systems

Dendrite growth velocities as a function of undercooling on the Al-rich side of the Al-Ni system have recently been measured by electromagnetic levitation. They have shown an anomalous behaviour, which so far cannot be theoretically described. The present work investigates whether properties of the phase diagram could explain this effect. For this purpose two simplified qualitative models of sharp interface theory, one treating the forming AlNi (B2) phase as a solid-solution, one treating it as an intermetallic phase, have been developed. To treat the concentration range between 50 and 70 at% Al, sets of thermodynamical data for six different compositions have been created. Results are compared to the experimental data. In both models low undercoolings at low Ni concentrations lead to very limited velocities, because the constitutional undercooling ΔTC overrides the kinetic undercooling ΔTk. The experimental results for low undercoolings seem to contain an effect which cannot be described by a model of this type.