Grain growth effects on the corrosion behavior of nanocrystalline NdFeB magnets
Introduction
NdFeB-magnets find a rapidly growing variety of applications, because of their high energy density (BH)max, which is sensitive to the degree of alignment of crystallographic c-axis of the tetragonal Nd2Fe14B phase. Such a texture can be achieved by the conventional powder metallurgy route including pre-alignment and sintering [1] or by hot deformation [2]. For hot deformation a fine-grained microstructure of NdFeB alloys is required. There are different possibilities for the preparation of sub-micron grain-sized NdFeB powders such as melt-spinning [3], mechanical alloying [4] and the Hydrogenation–Disproportionation–Desorption–Recombination (HDDR) process [5], [6]. Detailed studies on the influence of the microstructure, particularly grain size on the magnetic properties and deformation behavior showed that independent on the preparation process, very small and homogeneous grain sizes are favorable in order to obtain optimum magnetic properties and minimum deformation stresses [7], [8].
On the other hand, corrosion of magnets is one of the key parameters assessing the applicability of NdFeB-type magnets. The corrosion behavior of microcrystalline (sintered) magnets has been studied extensively in different media [9], [10]. Such studies have shown that the low corrosion resistance of NdFeB magnets can be attributed to the high content of neodymium and their complex multiphase microstructure. The microstructure comprises of ferromagnetic phase matrix (Nd2Fe14B) and the most corrosion sensitive Nd- and B-rich phases in the grain boundary region [11]. Bala et al. reported that sintered magnets are subjected at free corrosion conditions in 0.5 M H2SO4 solution to initial rapid intergranular attack, which causes the grains of the ferromagnetic phase to lose contact with the surface of bulk magnets [12]. Thus, structure integrity of the magnet is lost. Furthermore, the magnets exhibited an abnormal dissolution behavior at high cathodic potential [13], [14].
Due to differences in the powder fabrication method, the chemical composition and microstructure, and the corrosion and electrochemical behavior of sintered and nanocrystalline magnets may differ drastically. A few investigations have been devoted for the understanding of the corrosion and electrochemical behavior of nanocrystalline magnets. Most previous investigations have only addressed the evaluation of corrosion resistance in dependence on alloying additions and processing routes [15], [16]. Results of these investigations revealed that the partial replacement of iron with cobalt, aluminium and galium has a positive effect on the corrosion resistance. This improvement in corrosion resistance results mainly from the electrochemical stabilization of the intergranular phases. It was also found that magnets made from HDDR powder exhibited a comparable corrosion resistance in acid solution than magnets made from melt-spun and intensively milled powders [15]. It was assumed that the significance of this result lies in the microstructural differences (i.e. grain size, shape and distribution of phases). However, no direct experimental evidence was presented to substantiate this claim. Consequently, there is a necessity to optimize the microstructure not only with respect to the magnetic properties but also taking into account the corrosion aspects.
The present work was undertaken to understand the inherent corrosion behavior of nanocrystalline melt-spun and hot pressed NdFeB magnets in 0.1 M H2SO4 solution under various conditions such as free corrosion conditions, and anodic and cathodic polarization. The effect of grain growth due to annealing on the microstructure and corrosion behavior is discussed in this paper.
Section snippets
Preparation of magnets and microstructural analysis
Commercial nanocrystalline melt-spun powder of the nominal composition Nd14Fe80B6 (MQP-A) was used as starting materials. The powders were hot pressed in vacuum at 700 °C, applying a pressure of 150 MPa to obtain a highly densified precursor with a diameter of 8 mm and a height of 8 mm. In order to prepare NdFeB magnets with different grain sizes, hot pressed materials were further annealed at 800 °C for varying times from 0.5–6 h in evacuated silica tube. The microstructure of the magnets was
Microstructure characterization
A detailed analysis of the measured XRD patterns revealed that all magnets retained Nd2Fe14B structure and that no evidence for new phases was observed after annealing at 800 °C.
Fig. 1(a)–(d) shows Kerr-effect micrographs of hot pressed (a) and annealed (800 °C for 0.5, 1 and 6 h) NdFeB magnets. The microstructure of hot pressed magnet is homogeneous with extremely fine grains (Fig. 1(a)). Upon annealing, grain growth takes place and flake boundaries act as points for coarsening process (Fig. 1
Conclusions
The corrosion behavior of nanocrystalline NdFeB magnets with various grain sizes prepared from melt-spun hot pressed and annealed Nd14Fe80B6 powders, in 0.1 M H2SO4 under free corrosion conditions and anodic and cathodic polarizations has been studied. It was found that the corrosion behavior of nanocrystalline NdFeB magnets depends on their microstructure, particularly on the grain size of the ferromagnetic phase. Based on the results presented in this study, the following conclusions can be
Acknowledgements
The authors wish to express their thanks to the International Office at the FZ Jülich for financial support. Also the authors are very grateful for A. Kirchner, A. Güth, A. Plotinkov and C. Vogt for their experimental assistance and V. Panchanathan for supplying melt-spun powders.
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Electrochemical behaviour of nanocomposite Ag<inf>x</inf>:TiN thin films for dry biopotential electrodes
2014, Electrochimica ActaCitation Excerpt :However, since Ag is now polycrystalline, hence more reactive [33], its potential may, in fact, be less noble when compared to that obtained for the crystalline Ag control sample, shifting the Ag:TiN mixed potential in the anodic direction. The same OCP/grain size dependence was observed by Cao et al. [34], who studied two Cu-Ni-Cr alloys with the same composition but different grain sizes (in the micron and nanometre regions) in chloride solutions and El-Moneim et al. [35], who studied the corrosion behaviour of NdFeB nanomagnets. On the other hand, nanocrystallinity has also been associated with an increase of corrosion resistance in other cases, when the larger number of grain boundaries serve as short-circuit channels for element diffusion, hence facilitating passivation [36], or because it generates a more homogeneous elements distribution [37].