Solidification of tetragonal Ni₂B from the undercooled melt

Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt (DLR) 51170 Köln, Germany

^a sven.binder@dlr.de

Received: 22 September 2011
Accepted: 19 December 2011

Abstract

The dendrite growth velocity during non-equilibrium solidification of tetragonal Ni₂B crystals is measured as a function of undercooling on electrostatically levitated droplets. High-speed video imaging is applied to measure in situ the propagation of the solid-liquid interface apparent on the sample surface. To analyze the raw data, a three-dimensional model is developed to determine the growth velocity within the volume of the melt. Microstructure analysis is performed to resolve the pattern formation mechanism of tetragonal Ni₂B crystals which differs significantly from the cubic case. The dendrite tips are found to grow perpendicular to the crystallographic {111} planes. As a consequence, the propagation of the solidification front on a macroscopic scale is determined by the lattice parameters of the tetragonal crystal structure. The dendrite growth velocities along the {111} normal directions are analyzed on a mesoscopic scale within a sharp-interface model.