Solidification behaviour in undercooled Intermetallics with CrB-structure

Metallic melts can be undercooled significantly below their melting temperatures when containerless processed using electrostatic levitation. At these far from equilibrium conditions the system can exhibit dendritic growth. Kobold et.al. [1] reported heat fronts of ten-fold symmetry for NiZr at high undercoolings. These fronts were in-situ observed by a high-speed camera, as locally released latent heat yields a visible emissivity contrast on the samples surface. The heat front symmetry correlated to the microstructure of the solidified sample. Cross-sections showed that the polar axis is aligned with the rotation axis of a tenfold twinned structure. Kobold et al. explained their results by homogeneous nucleation of a quasi-crystalline core, which eventually transitions into the mesoscopic 10-fold twinned structure. The twin domains are separated by coherent large-angle grain boundaries. These boundaries constitute the stem of every dendrite, with the angle between two neighboring grains being 36°. This angle and the observed symmetry depends directly on the b/a ratio of the lattice parameters of NiZr. Hence, a universal growth mechanism for CrB-type compounds may exist. To this end, ESL experiments on NiB and NiGd were carried out. ESL has been used to melt, undercool and solidify samples with a diameter of about 3-4 mm. Single crystallization events have been monitored in-situ with a high-speed camera. The microstructure of the solidified sample has been analyzed by electron backscatter diffraction (EBSD). The crystal growth and resulting microstructure of these orthorhombic compounds are reported here. These indicate twinned symmetric structures with expected grain boundary angles. These can be observed even at relatively low undercoolings. NiB shows an 8-fold symmetry in the microstructure. Distorted twinned structures in NiGd indicate, an altered mechanism due to the expected uneven 9-fold symmetry. [1] W. Hornfeck, R. Kobold, M. Kolbe, M. Conrad, D.M. Herlach, Nature Comm. 9, 4054 (2018