Regensburg 2000 – scientific programme

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M: Metallphysik

M 8: Struktur und Umwandlung III

M 8.7: Talk

Tuesday, March 28, 2000, 17:00–17:15, H16

Phase selection in undercooled melts of NdFeB alloys — •Jianrong Gao^1,2, Wilfried Bender¹, Thomas Volkmann¹, and Dieter M. Herlach¹ — ¹Institut für Raumsimulation, Deutsches Zentrum für Luft- und Raumfahrt (DLR) e.V., D-51170 Köln — ²Department of Applied Physics, Northwestern Polytechnical University, 710072 Xian, P.R. China

Small droplets of NdFeB alloys with a wide range of chemical compositions were undercooled and solidified containerlessly using a 8 m drop tube. Phase selection between γ-Fe and the Nd₂Fe₁₄B compound during the peritectic reaction was then investigated by correlating the observed microstructures either to alloy compositions or to droplet sizes. The results show that the primary crystallization of free iron dominates in large droplets, whereas the metastable primary crystallization of the Nd₂Fe₁₄B compound prevails in droplets with reduced sizes. However, the critical droplet size for such a transition of the solidification path varies significantly with the alloy composition. The closer the alloy composition to the chemical stoichiometry, the smaller the critical size. This could be understood because a larger critical undercooling level is required for the primary crystallization of the Nd₂Fe₁₄B compound in alloys with a lower Nd content in terms of the calculated metastable phase diagram.

Support by the Alexander von Humboldt Foundation is gratefully acknowledged.