Solidification of electromagnetically levitated Nd-Fe-B melt droplets Sino-German Workshop on EPM 10-12 October, 2004, Shanghai Solidification of electromagnetically levitated Nd-Fe-B melt droplets J. Gao1,2, T. Volkmann1, J. Strohmenger1, S. Reutzel3, D.M. Herlach1 1 Institute of Space Simulation, German Aerospace Center, Cologne, Germany 2 Key Laboratory of Electromagnetic Processing of Materials, Northeastern University, Shenyang, China 3 Institute of Experimental Physics IV, Ruhr-University of Bochum, Bochum, Germany

Outline Motivation Experimental-Setup Solidification experiments Microstructural examination Conclusions & outlook

Introduction to Nd-Fe-B alloys B H (BH)max Nd-Fe-B Ferrite 10 20 30 40 50 60 Nd2Fe14B (MGOe) Sm2Co17 max (BH) SmCo5 Ba-Ferrite DS-Alnico AlNiCo5 1940 1950 1960 1970 1980 1990 Year Hysteresis loops Development of PMM F-Nd2Fe14B cell

Peritectic formation of f Fabrication of sintered Nd-Fe-B magnets requires casting of bulk ingots for powder precursors. g L f Peritectic formation of f Nd-Fe-B phase diagram Residual Fe dendrites lead to reduced magnetic properties.

Motivation - direct f formation T (K) Fe (at.%) (Nd:B=2:1) 82.3 76.5 L+ g L+ f Undercooled Overheated f TL g L TP Peritectic f-phase might be solidified directly from an undercooled liquid. It is a big challenge to undercool Nd-Fe-B melt droplets because of their high chemical reactivity!

Electromagnetic Levitator To chart recorder Pyrometer Sample (1g, 6mm) R. F. Generator Vac:=10-6 mbar PHe=10-50 mbar Coil Quartz tube He (6N) To vacuum pump EML + low P + T>>TL  large DT Nd2O3 (s)+ Nd (L) NdO (g)

In-situ X-ray diffraction analysis X-ray Synchrotron radiation: highly penetrating Beamtime: ID 15A, ESRF, Grenoble, France

Cooling curves during solidification Recalescence occurs due to rapid latent heat release!

In-situ diffraction analysis MS= c-Nd2Fe17Bx; Th2Zn17-type rhombohedral structure; Decomposition of Nd2Fe17Bx into Fe plus Nd2Fe14B.

Primary phase selection map ●g-Fe ■f-Nd2Fe14B ♦c-Nd2Fe17Bx

Solidification microstructure

Thermomagnetic analysis

Summary Chemically reactive Nd-Fe-B melt droplets could be undercooled by electromagnetic levitation under controlled conditions. Solidification of levitated Nd-Fe-B droplets depends on melt undercooling, and three metastable solidification pathways have been determined for Nd-rich compositions.

Outlook – Fe-rich compositions Preliminary drop tube experiments Fe phase is not harmful any longer! EM-levitation for in-situ studies.