![]() ![]() At first, the solidification of metallic alloys with relatively low melting points-such as Sn-, Al-, and Mg-based alloys-were observed in situ by 2D transmission imaging. In the last twenty years, in-situ observation techniques using synchrotron radiation X-rays have been developed extensively in third-generation synchrotron radiation facilities. 16, 17 ) To the best of our knowledge, though, the preferred growth directions of dendrites in HEAs have not been identified systematically. 9 ) The interface undercooling, Δ T r, owing to curvature is given by The selection of the growth direction has been explained by considering the interface shape modified by anisotropy. The preferred growth direction is determined by the anisotropy of the solid–liquid interfacial energy. When the kinetic undercooling at the solid–liquid interface is negligibly small, dendrites grow along a preferred crystallographic orientation. 8 ) Thus, to understand solidification in the alloys of interest, the preferred growth directions should be measured. ![]() For example, changing the growth direction of the dendrite arms during solidification produces a casting defect called feathery grains. Grain selection during solidification, which determines the solidification structure, is influenced by the preferred growth direction. By combining time-resolved tomography 6 ) and X-ray diffraction (XRD), 7 ) performed at the SPring-8 synchrotron radiation facility (Hyogo, Japan), the microstructure and crystallographic orientation were observed simultaneously.ĭendrite arms grow along a preferred crystallographic orientation, as well known. For comparison, the secondary dendrite arm spacing in the solidification structure was examined. In the present study, we aim to describe the three-dimensional structure of the growing dendrites, determine the preferred growth direction of the dendrite arms, and measure the solid–liquid interfacial area and average curvature in CrMnFeCoNi alloys by using X-ray imaging with synchrotron radiation. Thus, further study on the fundamental properties of dendritic growth in HEAs is required to quantitatively analyze their solidification. In addition, uncertainties remain in understanding the dendritic growth of HEAs, such as their preferred growth direction, dendrite arm spacing, and interfacial area. 4 ) Previous reports 4, 5 ) have disagreed in these results, though, because multicomponent alloys exhibit a complicated equilibrium between liquid and solid phases. 5 ) In this solidified structure, the core of the dendrite arms is rich in Co, Cr, and Fe, while the interdendritic region is rich in Mn and Ni. CrMnFeCoNi alloys are simply described by a pseudo-binary CrFeCo–MnNi system. 4 ) CrMnFeCoNi alloys solidify dendritically, forming a single FCC solid solution.īased on scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), the HEAs have shown an interdendritic region that is poor in Fe and rich in Cr and Mn, indicating that solute partition occurs during solidification. 1 – 3 ) The CrMnFeCoNi system is a typical example of a high-entropy alloy. High-entropy alloys (HEAs) have been studied for various properties, including the effect of their high entropy, the stability of their solid solution (FCC and BCC), and their mechanical properties and corrosion behavior. Overall, it appears that solidification in this high-entropy alloy can be analyzed by using models developed for binary or pseudo-binary alloys.ġ.1 Solidification of high-entropy alloys The secondary arm spacing was on the same order of magnitude as the spacing of conventional alloys. The interfacial area was compatible with the reported values of Al–Cu and Mg–Sn alloys. The interfacial area reached a maximum at a solid fraction of 0.55. ![]() The specific solid–liquid interfacial area, which was normalized by the total volume, was evaluated as a function of solid fraction. The dendrite arms grew preferentially along the ⟨100⟩ direction, corresponding with typical FCC alloys. Simultaneously, the crystallographic orientations of the dendritic grains were measured by XRD. The evolution of the dendritic grains cooling at 0.083 K/s was reconstructed using 200 projections over a 180° rotation every 4 s from 4D-CT and a phase field filter. Time-resolved tomography (4D-CT) and X-ray diffraction (XRD) were combined to observe growing dendrites and to measure their crystallographic orientation in a CrMnFeCoNi high-entropy alloy with an FCC structure. ![]()
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