Papers by Keyword: Constitutional Undercooling

Abstract: 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 ΔT_c when the Al content increases. This reduces the fraction left for kinetic undercooling ΔT_k, which is responsible for interface migration and which determines the growth velocity. Neither of the models can reproduce the second effect.

Abstract: In-situ synchrotron X-ray radiography has been used to study columnar and equiaxed dendritic growth in directional solidification of Al-Cu alloys employing a Bridgman furnace. Nominal spatial and temporal resolutions of 1.5 µm and 150 ms, respectively, were obtained with a 1.3 %1.3 mm2 field of view, and a signal-to-noise above 99.5 %. Dedicated processing software has been developed to allow for quantitative extraction of data such as solid-liquid interface morphology, local propagation velocities and constitutional gradients from the images. The data collected also contain unprecedented in-situ observations on dendrite fragmentation. The limited field of view together with a slight sample position dependency in the heat transfer coefficients made it necessary to impose thermal gradients, G > 10 K/mm, to have reproducible solidification processes. Non-vanishing horizontal G-components contributed to convection that at some occasions resulted in observable effects on growing crystals or on liquid segregates.