Dresden 2003 – wissenschaftliches Programm

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M: Metallphysik

M 18: Symposium: Phasenumwandlungen in Schmelzen VII

M 18.4: Vortrag

Dienstag, 25. März 2003, 15:30–15:45, IFW A

Phase selection in undercooled Nd-Fe-B alloy droplets of non-peritectic chemical compositions — •Jianrong Gao^1,2, Thomas Volkmann¹, Sven Reutzel^1,3, and Dieter M. Herlach¹ — ¹Institut für Raumsimulation, Deutsches Zentrum für Luft- und Raumfahrt (DLR), D-51170 Köln — ²Department of Applied Physics, Northwestern Polytechnical University, 710072 Xian, China — ³Institut für Experimentalphysik IV, Ruhr Universität Bochum, D-44780 Bochum

Phase selection during solidification of Nd-Fe-B alloys is of great technical interest. The previous work (1,2) on peritectic Nd-Fe-B compositions has shown that γ-Fe solid solution, Nd₂Fe₁₄B compound and the metastable Nd₂Fe₁₇B_x compound (x ≈ 1) could be primarily solidified in the sequence of increasing bulk undercooling. Using the drop tube technique, the present work studied phase selection in undercooled droplets of non-peritectic compositions Nd₁₆Fe₇₆B₈, Nd₁₇Fe_74.5B_8.5, and Nd₁₈Fe₇₃B₉ alloys, where Nd₂Fe₁₄B is the equilibrium primary phase. The results show that regardless of alloy composition, primary Nd₂Fe₁₇B_x formation occurs in excess of 70% of the droplet population, whereas primary Nd₂Fe₁₄B formation takes place in the rest minor fraction of the droplet population. In the droplets showing primary Nd₂Fe₁₇B_x formation, Nd₂Fe₁₄B is also crystallized but as a secondary phase. During continuous cooling, the metastable Nd₂Fe₁₇B_x compound is subjected to partial or complete decomposition depending on the droplet size. The decomposition is initiated at the interfaces to secondary Nd₂Fe₁₄B grains and proceeded inwards. In the droplets showing primary Nd₂Fe₁₄B formation, there often exist a few dendritic grains of Fe and Nd₂Fe₁₇B_x. However, there are no fixed phase relations between them and the primary Nd₂Fe₁₄B. Such a finding suggests that these three phases can be nucleated simultaneously from undercooled droplets of non-peritectic Nd-Fe-B compositions under certain conditions, though the growth of two of them is obviously limited.

This work is supported by DLR-Bonn under grant No. 50WM9930 (S.R.) and by the Alexander von Humboldt Foundation (J.G.).
(1) J. Gao, T. Volkmann, S. Roth, W. Löser, D.M. Herlach, J. Magn. Magn. Mater. 234 (2002) 313.
(2) J. Gao, T. Volkmann, D.M. Herlach, Acta Mater. 50 (2002) 3003.