Nucleation and Crystal Growth in Undercooled CrB-Structured Alloys

Deep undercooling of metallic melts and consequent solidification from non-equilibrium conditions can unfold previously unobserved growth mechanisms. Containerless processing, using levitation techniques, and high purity elements are used to undercool melts several 100 K below their liquidus temperature. Kobold et al. [1] reported heat fronts of ten-fold symmetry in deeply undercooled NiZr. These fronts were in-situ observed by a high-speed camera, as the locally released heat yields a visible temperature 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. These twin domains are separated by coherent large-angle grain boundaries. These boundaries constitute the stem of every dendrite, with two neighboring grains being inclined 36° to each other. This angle and the observed symmetry depend directly on the a/b ratio of the lattice parameters of NiZr. Hence, a universal growth mechanism for CrB-type compounds may exist. To this end, experiments on several multiple CrB-structured binary alloys were carried out. Samples with diameter of 3-4 mm have been melted, undercooled, and solidified. Single crystallization events have been in-situ monitored with a high-speed camera. The microstructure of solidified samples has been analyzed by electron backscatter diffraction (EBSD). Crystal growth and resulting microstructure of these orthorhombic compounds are reported here. It is shown that the proposed growth mechanism is present in several alloys such as NiB or NiGd. In line with their a/b ratio these alloys show 8-fold and 9-fold symmetry, respectively, and thereby further validate the proposed model. [1] W. Hornfeck, R. Kobold, M. Kolbe, M. Conrad, D.M. Herlach, Nature Comm. 9, 4054 (2018