Role of alloying elements in phase decomposition in alnico magnet alloys
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I.
ALNICOmagnets were originally manufactured from Fe, Ni, and A1, l and were later improved by major additions of Co and minor additions of Cu, Ti, and other elements.Z'3 The high coercivity in these alloys is attributed to the shape anisotropy of the oriented particles of the Fe-rich ferromagnetic cr phase in the NiAl-rich a2 matrix phase. 4'5'6 However, the role of individual elements in Alnico alloys is still obscure because of the complexity in both the chemical composition and the microstructure. In a previous work, 7 the authors examined the phase boundaries of al and cr phases in Fe-Ni-A1 and Fe-Ni-A1Co systems by the diffusion couple technique and revealed the whole aspect of the miscibility gap between these phases. In the present work, we have determined the distribution of alloying elements between the oh and a2 phases in Fe-Ni-A1 base quaternary alloys by the same technique, and studied the role of alloying elements in Alnico magnets in connection with the phase diagrams. II.
III.
INTRODUCTION
RESULTS
Typical examples of the microstructure in the vicinity of in Figures 1 and 2, where the second phases formed inside the a l phase are Cu-rich e and Mn-rich y phases, respectively. The concentration profiles obtained for the diffusion couples containing Ti, Mo, and Cu are given in Figure 3. It is clear that Ti and Cu tend to concentrate into the a : phase, while Mo is mostly distributed into the a l phase. In other words, the a2 phase is stabilized by additions of Ti and Cu, and hence, these elements are regarded as the a2-formers, while Mo is the a l-former. The distribution coefficients k~ for individual elements are summarized in Table I and Figure 4, where the value for Co is taken from the previous paper. 7 It should be noted in Figure 3(c) that the diffusion in a2 phase in the Fe-Ni-AI-Cu system is much faster than that in the other systems. This is consistent with the empirical fact that the rate of decomposition in Alnico alloys is markedly increased by the addition of Cu. 3"~ Since the melting temperature of the bcc CuAI is much lower than that of the bcc NiAI, the reason for the above-mentioned result is explained by a general rule that the atomic mobility in some phases is
al/a2 interface in diffusion couples are shown
EXPERIMENTAL PROCEDURE
The preparation of materials and the examination of phase equilibria were performed in almost the same way as in the previous work. 7 Fe-5 at. pct A1-X ternary alloys (X: Cu, Ti, Mn, V, Cr, Si, Mo, and Nb) were made by induction melting under argon. The amounts of alloy additions were 1 at. pct for Nb, 3 at. pct for Mo, and 5 at. pct for other elements. These alloys were cut into specimens about 5 by 5 by 5 mm, and joined with NiA1 compound specimens for composing Fe-Ni-A1-X quaternary diffusion couples. The diffusion couples were sealed in transparent quartz capsules under vacuum and annealed at 900 ~ for 400 hours. After examining the microstructure, the concentration-penetration curves for each element were determined by microanalysis, and the equ
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