Free dendritic tip growth velocities measured in Al-Ge

We determine the tip growth velocities of equiaxed dendrites in an Al-24 at. % Ge alloy using in situ x-ray radiography. The tip growth velocity is a main characteristic of dendritic growth and allows for an accurate comparison against theories and simulations describing the growth of a single dendrite tip or an array of dendrites. The disk-shaped samples are 200 or 350 $\ensuremath{\mu}\mathrm{m}$ thin and are positioned horizontally in a near-isothermal furnace to suppress buoyancy and gravity-driven melt flows. We obtain a large data set of free dendritic tip growth velocities of Al dendrites over one order of magnitude (4--140 $\ensuremath{\mu}\mathrm{m}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$) and over a decade of global undercooling (1--10 K). No indications for a significant deviation from three-dimensional growth kinetics due to the sample confinement are found.