Study of Co-Fe-V Permanent Magnet alloys (vicalloys)

Abstract

The historical development of Co-Fe-V permanent magnet alloys (vicalloys), their technological importance and scientific study, are reviewed. The basic, as yet unsolved, problem of the origin of their magnetic hardness is examined. Results of recent work employing Mossbauer spectroscopy and magnetic measurements at different temperatures, as well as some X-ray and electron-optical analysis, are presented and discussed. Mossbauer spectroscopy established the fact that vicalloy in the permanent magnet state is composed of two phases: ordered ferromagnetic α′ (bcc, ferrite) and paramagnetic γ (fcc, austenite). Samples quenched from the high temperature γ field consist almost entirely of retained austenite. However, on cooling to cryogenic temperatures (liquid and solid nitrogen, 60 to 80 ° K), or by cold work, some of it is transformed irreversibly into disordered ferromagnetic α. This is confirmed by magnetic measurements in the cryogenic range; the results were inconsistent with a previously postulated model based on a reversible paramagnetic γ to ferromagnetic γ transition or on some superparamagnetic process. The measurements, when correlated with Mossbauer data, confirm that the phase transition is not accompanied by any change in iron content of the phases. An interesting change in magnetic anisotropy of flat, thin samples following cold-work and heat-treatment for optimum permanent magnet properties was confirmed by Mossbauer spectroscopy. X-ray diffraction confirmed the above findings on the phase structure of vicalloy and no direct evidence was found for a third metastable phase, intermediate between α and γ, which was proposed by Henkel to account for the magnetic hardness of vicalloy. However, one unidentified diffraction line in the X-ray pattern of optimum heat-treated vicalloy calls for more attention to this postulate. Electron microscopic results did not remove the existing ambiguity as to the role of shape anisotropy in promoting magnetic hardness of vicalloy. On the other hand, a possible contribution by crystalline anisotropy is suggested by some electron diffraction patterns.