Metastable solidification of NdFeB alloys by drop-tube processing
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Th. Volkmann Institute of Space Simulation, DLR, D-51170 Cologne, Germany Institute of Experimental Physics, Ruhr University of Bochum, D-44780 Bochum, Germany
D.M. Herlach Institute of Space Simulation, DLR, D-51170 Cologne, Germany (Received 17 July 2000; accepted 18 June 2001)
Metastable solidification of small droplets of Nd11.8Fe82.3B5.9 and Nd14Fe79B7 alloys was performed in an 8-m drop tube filled with helium. The results showed that the solidification path of the droplets depends on the alloy composition and on the droplet size. For Nd11.8Fe82.3B5.9 alloy, larger droplets are solidified by primary iron formation and subsequent Nd2Fe14B crystallization from the residual liquid phase, whereas smaller ones tend to be frozen by metastable primary Nd2Fe17Bx growth (x ⳱ 0–1). A similar transition of the solidification path from primary iron formation to primary Nd2Fe17Bx formation occurred in Nd14Fe79B7 alloy with reducing droplet size. However, metastable primary growth of Nd2Fe14B was also observed within a wide droplet size range prior to the appearance of the metastable Nd2Fe17Bx phase. Nucleation and growth of different phases were considered to produce an explanation of the observed phase selection phenomena in these two alloys.
I. INTRODUCTION
Nd2Fe14B-based alloys have been of great technical interest since their discovery in 1983. Commercial magnets with high performance have been successfully prepared on the basis of their use as starting material.1,2 It is well-known that the formation of a correct microstructure in final magnets is very critical to the attainment of excellent magnetic properties. Some fundamental principles have been established so far with respect to this.3 To achieve high-energy products, the volume fraction of the magnetically hard Nd2Fe14B phase in sintered magnets should be maximized by optimizing chemical compositions and processing conditions. Due to nonequilibrium solidification conditions encountered in the production of commercial magnets, however, properitectic primary iron phase in considerable amounts is often preserved in as-cast ingots, which has proven detrimental to magnetic properties of magnets. As a result, an additional homogenization treatment at elevated temperatures has to be adopted before the powder preparation so as to reduce the amount of primary iron.4 Unfortunately, this time-consuming treatment has led to a prolonged a)
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J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 22 Mar 2015
production period and, therefore, is unfavorable from the economic point of view. An alternative solution is in demand now. The previous work5 on drop-tube-processed NdFeZrB alloy particles has shown that large undercooling is able to solidify small NdFeB alloy droplets with nearstoichiometric compositions into a primary iron free structure, hence providing a promising solution to the above problem. The physical mechanism underlying this process is that primary iron formation is suppressed
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