Although the production and applications of NdFeB magnets have increased enormously in the last decade because of their outstanding magnetic properties at room temperature, some applications were limited because of poor thermal stability and corrosion resistance. To address these problems, there have been many efforts and much progress. In many instances, the alloy modifications produced an improvement of some characteristics but only at the expense of other characteristics. It is therefore necessary to find a method (or alloy) for improving the thermal stability, coercivity, and corrosion resistance without sacrificing performance. We have found that the proper control of very small amounts of Cu, Co, and O in (Nd, Dy)FeB alloys substantially improves the coercivity, high-temperature capabilities, and corrosion resistance without a reduction of remanence. As a result, a high performance NdFeB magnet with excellent temperature stability and corrosion resistance was developed.
An investigation was conducted comparing the corrosion behavior of NdFeB magnets in flowing hydrogen and in the heat and humidity of an autoclave. The results show that corrosion is both macroscopically and microscopically similar in both environments. In both cases, the corrosion progressed most rapidly in those areas where the magnetic orientation of the Nd2Fe14B matrix grains was perpendicular to the outer surface. A corrosion mechanism involving the reaction of hydrogen—either as a pure gas or as a by-product of the decomposition of water vapor—with the neodymium-rich grain boundary phase is proposed.
We have previously shown that the corrosion behavior of ternary NdFeB magnets is affected by their oxygen, carbon, and nitrogen contents. The corrosion was measured in autoclaves which give accelerated testing environments of high heat and humidity. In this study, we relate this corrosion behavior in both NdFeB and NdFeCoAlB magnets to effects upon their microstructure. When the oxygen and carbon contents are low in ternary NdFeB magnets, a thick Nd-rich phase (α-Nd and/or NdOx) forms along grain boundaries and their triple junctions. As the oxygen and carbon contents increase, the Nd-rich coating along the boundaries becomes thinner and agglomerates into the triple junctions. With thin grain boundaries, the pathways for corrosion propagation are hindered, thus improving corrosion resistance. With increases in oxygen, the α-Nd and unstable NdOx are changed to stable Nd2O3. Nitrogen increases also aid in the process of conversion to Nd2O3 which leads to better corrosion resistance. Two different features are seen in the microstructure of NdFeCoAlB magnets. The grain boundary phase (Nd3Co) is very stable. Any oxygen in the system appears as fully oxidized Nd2O3. Easily corroded α-Nd and NdOx are not detected. All of these factors combine to produce excellent corrosion resistance in this variety of NdFeCoAlB magnets.
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