Dendritic growth rate in undercooled, dilute TiNi melts

The dendritic growth rate in undercooled, dilute TiNi melts was measured by optical means. At a critical value of the undercooling, which is correlated with the difference between the liquidus and the T0 temperature: ΔT* ≈ 1.5·(T1 − T0), two Ti-rich alloys with 5 and 10 at.% Ni showed an abrupt increase in the growth rate. In a third Ti-rich alloy with 15 at.% Ni a comparable undercooling level was not reached and only low growth rates were observed. On the other hand a Ni-rich alloy showed only high growth rates of the order of pure nickel. For this alloy the T0 temperature lies only a few Kelvin below the liquidus, thus a possible jump in the growth rate may be expected in a very low undercooling regime, which was experimentally not accessible. The discontinuous growth behavior can be explained qualitatively by the growth of the supersaturated solid solution phase when the undercooling level exceeds the T0 temperature by a certain amount. Then the growth mechanism changes abruptly from slow, diffusion-controlled growth to rapid, purely thermally controlled growth of the supersaturated solid solution phase. For diffusionless, purely thermally controlled growth the equilibrium liquidus temperature has to be replaced by the T0 temperature, the ‘melting’ temperature of the supersaturated solid solution phase. Taking into account some modifications in current dendritic growth theory, basically a modified ‘melting’ temperature, the calculated growth rate fits the experimental data quite well without thermodynamic inconsistency, as is shown for the example of the alloy with 10 at.% Ni.