Microstructure, crystallization, and coercivity of rare earth-iron-boron amorphous alloy ribbons
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INTRODUCTION
TrIE rar~ earth-iron-boron amorphous
alloy ribbons with small amounts of rare earth (R), 10 to 15 at. pct, show interesting magnetic properties. In the amorphous state, these alloy ribbons are magnetically soft. When crystallized, the ribbons develop very large hysteresis I~] with high coercive force, thus making these alloys suitable for use as permanent magnets. In our study, two series of alloy ribbons were investigated. In one series, series A, the rare earth amount was maintained constant, and the ratio of Fe/B was changed, i.e., RoA(FexBl-x)0.9 (Figure 1). The compositional dependence of the coercive force and microstructure of crystallized Tb(FeB) alloys were investigated before, t2] In a subsequent investigation,t31 the coercivity in isothermally annealed Tbo.05Lao.05(Feo.s2Bo.18)0.90 ribbon was correlated with the microstructure developed on annealing at various temperatures for a specific period of time. The high coercivity of these Tb alloy ribbons seems related to the crystallized compounds and their grain sizes. In the high coercivity alloy ribbons, the crystallized compounds are mainly Fe3B, R6Fe23 and a small amount of both a-Fe and a ternary phase. The grain sizes of the major phases are in the range of 100 to 300/~. Because of the fact that the Fe3B and R6Fe23 compounds with proper grain sizes were the major constituents for the high coercivity alloys, the second series of alloys, series B, constituted the Fe3B and R6Fe23 compounds in varying amounts, i.e., (Rr/29Fe23/29)l_y(Fe3/4Bl/4)y (Figure 1). Surprisingly, the
B.N. DAS, Metallurgist, Materials Physics Branch (Code 6340), P. D'ANTONIO, Research Physicist, Laboratory for Structure of Matter (Code 6030), and N.C. KOON, Research Physicist, Materials Physics Branch (Code 6340), are with the Naval Research Laboratory, Washington, DC 20375-5000. Manuscript submitted May 4, 1988. METALLURGICAL TRANSACTIONS A
alloy with y = 0.27 and R = Tb0.sLa0.5 developed a coercive force of 29.3 kOe, and the magnetization vs temperature showed one magnetic ordering transition, t4j Contrary to the observation in the first series of alloys, the intrinsic coercive force of the second series of alloys did not decline with increased annealing temperature. These properties suggested that the improved magnetic properties were probably related to the presence of an optimum microstructure of a ternary compound not previously reported. In the past five years, several investigators have reported the crystal structure 1~'6'71 and the magnetic properties t~-HJof the ternary compound REFe~4B. This paper presents the results of a study on microstructural parameters, i.e., atomic environment and shortrange ordering in rare earth-iron-boron amorphous ribbons and the microstructural changes during crystallization on nonisothermal heating. The nonisothermal heating method was chosen because it gives the crystallization temperatures of phases and it also provides information on kinetic parameters. The contribution of the crystallized phases on coercivity of the alloy
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