Rapidly Solidified Alloys for Permanent Magnets
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RAPIDLY SOLIDIFIED ALLOYS FOR PERMANENT MAGNETS G.C. HADJIPANAYIS Department of Physics, Kansas State University, Manhattan, KS
66605
ABSTRACT The magnetic and structural properties of melt-spun Fe-R-B alloys are reviewed. The hard magnetic properties of these alloys are attributed to the presence of a highly anisotropic tetragonal phase Fe 4 R B which in the case of ribbons occurs in very fine grains. The interachoot of domain walls with this fine microstructure leads to the observed high coercivities. The effects of partial substitution of iron, rare-earth and boron on the formation of Fe 1 4 R2 B and on its magnetic properties are also discussed. INTRODUCTION 1
Research on iron-rare-earth magnets begun in early 70's when Rhyne and his cmowrkers produced amorphous TbFe alloys by rapid dc sputtering. Later Clark observed large coercivities gn the same materials at cryogenic temperatures. Subsequently he succeeded in producing large coercivities at room temperature in crystallized TbFe samples demonstrating thus the potential of these alloys for permaneAt magnet development. Large coercive fields were Ilgo obtained in other rare-earth based systems (amorphous or crystallized ) but with energy products not exceeding 8 MGOe. In 1982 a major breakthrough took place when Hadjipanayis et al. were able for the first time to obtain coercivities in excess of 10 kOe and an energy product of around 12 MGOe in a cobalt-free melt-spun Fe-Pr-BSi sample. The hard magnetic properties of the samples were attributed to the formation of a h~ghly anisotropic tetragonal Fe nPr B phase which is now properly indexed as FeI R B. Later it was Jho tMat the Fe R B phase is formed with most of the aie-earth elements. 1 3 The same pýAsi was found to exist in as-cast alloys and Sagawa et al. produced the fif{t Fe-Nd-B magnets with energy products up to 35 MGOe. Narasimhan et al. were able to increase the energy product up to 45 MGOe and at present these magnets are made commercially by several companies. The objectives of this report are: (i) to discuss briefly our early studies of rare-earth-iron based alloys that had led to the discovery of Fe-R-B magnets, (ii) to summarize the magnetic and structural properties of melt-spun Fe-R-B alloys, (iii) to discuss the effects of iron substitution with Co,Mn,Ni and neodymium substitutions with other rare-earths on the magnetic properties, (iv) to present some preliminary data on the effects of metalloid substitutions, and (v) to discuss the origin of magnetic hardening in the melt-spun alloys. DISCOVERY OF Fe-R-B ALLOYS Our early studies 6 on iron-rare-earth alloys were f~gused on the ternary systems Fe-Rf-Ga , Fe(R R ) and Fe-(R R2 ) alloys with R=--Tb and R =Pr,Nd,Sm. Rapid •oligification ;tymeans of melt-spinning was uied togeth r with a subsequent heat treatment of the as-quenched samples to produce metastable phases and special microstructures that could not be produced otherwise. The coercivity, H , of those samples was relatively high (up to 4 kOe) but the magnetizati&n was relatively small
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