Concentration Dependent Growth Velocities in Undercooled Al-Rich Al-Ni Alloy Systems

Dendrite growth velocity V as a function of undercooling on the Al-rich side of the Al-Ni system has recently been measured by electromagnetic levitation. The results have shown an anomalous behaviour, which so far cannot be theoretically described. The present work uses two simplified qualitative models of sharp interface theory, one of them treating the forming AlNi (B2) phase as a solid-solution, one treating it as an intermetallic phase, to investigate the influence of the phase diagram on the growth velocities. The results imply that the concentration dependent growth behaviour is a superposition of at least two effects: 1) A strong decrease of the total growth velocitiy level for increasing Al concentrations. 2) An increase of growth velocities with increase of Al concentration at medium and low undercoolings. The present work is able to explain the first effect, namely the concentration dependent reduction of velocities. Results are compared to experimental data. In both models the properties of the phase diagram lead to an increase of the constitutional undercooling ΔTc when the Al content increases. This reduces the fraction left for kinetic undercooling ΔTk, which is responsible for interface migration and which determines the growth velocity. Neither of the models can reproduce the second effect.