Corrosion behaviour of hot-pressed and die-upset nanocrystalline NdFeB-based magnets

Journal of Magnetism and Magnetic Materials 248 (2002) 121–133

Corrosion behaviour of hot-pressed and die-upset nanocrystalline NdFeB-based magnets A.A. El-Moneima,c, O. Gutfleischa, A. Plotnikovb, A. Geberta,* a

Institute for Solid State and Materials Research, IFW Dresden, P.O. Box 270016, D-01171 Dresden, Germany b Institute for Solid State Analysis and Structural Research, IFW Dresden, Germany c Physical Chemistry Department, National Research Center, Cairo, Egypt Received 13 December 2001; received in revised form 14 March 2002

Abstract Isotropic and anisotropic nanocrystalline Nd14Fe80B6 and Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 magnets have been produced from melt-spun materials by hot pressing and subsequent die-upsetting. The microstructure has been characterized using XRD, scanning electron microscope and energy dispersive X-ray analysis. The corrosion behaviour of die-upset NdFeB-based magnets has been studied in 0.1 M H2SO4 by inductively coupled plasma solution analysis and electrochemical polarization techniques and compared with their hot-pressed counterparts. Texturing of hot-pressed (isotropic) NdFeB-based magnets via die-upsetting significantly modifies their corrosion performance. Textured Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 magnets exhibit the highest corrosion resistance in this study. The low effective diffusivity of corrosion hydrogen inside the bulk magnet and the reduction in the strength of galvanic coupling between magnet phases are the main reasons for the observed improvement in the corrosion resistance. The corrosion behaviour of the magnets in relation to their phase composition and phase distribution is discussed in terms of dissolution, hydrogenation and pulverization. Pulverization trends are correlated with hydrides formation and hydrogen-trapping sites using thermal desorption analysis. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.50.W; 82.30.R; 81.65.K; 82.45 Keywords: Nanocrystalline magnet; NdFeB; Die-upsetting; Hydrogen; Corrosion

1. Introduction At present, NdFeB alloys are commercially used for permanent magnetic materials exhibiting the highest maximum energy density. In order to exploit the excellent potential of the Nd2Fe14B compound (hard magnetic phase), c-axis align*Corresponding author. Tel.: +49-351-4659-275; fax: +49351-4659-320. E-mail address: [email protected] (A. Gebert).

ment (i.e. texturing) is needed. The two most common processes used for producing anisotropic or textured NdFeB magnets with high energy density are: (a) conventional powder metallurgy route (sintering) [1] and (b) hot pressing with subsequent hot deformation of nanocrystalline powders [2]. In the former method, crystal alignment is achieved by applying a magnetic field during the cold-pressing stage. The latter employs fine-grained powders produced by using nonequilibrium alloying techniques such as melt

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 2 8 6 - X

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spinning [2], mechanical alloying [3,4] or the hydrogenation disproportionation desorption recombination (HDDR) process [5]. In order to obtain textured magnets from the nanocrystalline powders, it is necessary to hot press the powder for densification with minimal grain growth. A subsequent hot deformation induces grain alignment along the c-axis by plastic flow [6–8]. For instance, die-upsetting of hot-pressed magnets results in textured magnets with c-axis alignment parallel to the pressing direction. With regard to the dieupsetting process, there are a number of investigations analysing and optimizing the effect of hot deformation parameters on the magnetic properties [9,10]. Although the assessment of the corrosion behaviour of nanocrystalline magnets is one of the most important key parameters for their potential applications, little is known about their corrosion behaviour. Furthermore, the effect of die-upsetting or grain alignment on the corrosion behaviour of nanocrystalline NdFeB magnets is scarcely addressed. NdFeB magnets are generally sensitive to attacks by climatic and corrosive environments. The high corrosion sensitivity is attributed to the coexistence of several phases in the microstructure and the high content of rare-earth element in the magnet composition [11,12]. NdFeB-sintered magnets are typically comprised of three phases, the hard magnetic phase matrix (f-phase), which represents about 87% of the magnet volume and the most corrosion sensitive Ndand B-rich intergranular phases [12]. This means that NdFeB magnets are prone to galvanic corrosion, in particular, intergranular corrosion attack. The corrosion mechanism as well as the hydrogen decrepitation behaviour of microcrystalline (i.e. sintered) magnets has been studied extensively [11–14]. Furthermore, the effect of alloying additions on the magnetic properties and the corrosion behaviour of sintered and nanocrystalline NdFeB magnets have been subject of a few investigations. It is reported that Al, Co, Cu and Ga additions are found to improve the corrosion resistance of NdFeB magnets in many corrosive environments [15–17]. The improvement in the corrosion resis-

tance is attributed to the change in the microstructure, especially in the phase composition, by segregation of these kinds of additions into intergranular phase regions [16,17], which subsequently reduces the driving force for galvanic corrosion. The present authors reported that Co and Ga additions exert also their beneficial effects via reducing the binding energy of corrosion product hydrogen in nanocrystalline magnets [18]. In other words, Co and Ga additions lower the tendency to mechanical degradation or pulverization of the magnet. The effect of processing routes on the corrosion behaviour of nanocrystalline magnets has been investigated [17]. Significant differences between the corrosion behaviour of melt-spun and intensively milled powders were found. It was assumed that the significance of this observation may lie in microstructural differences (i.e. grain size, shape, phase composition and distribution). A detailed investigation on the influence of various microstructures produced via a conventional annealing process on the corrosion behaviour of hot-pressed nanocrystalline magnets (i.e. isotropic magnets) has been carried out by the present authors [19]. It was shown that the heterogeneity of the microstructure and the increase in the grain size of the matrix phase upon annealing remarkably improve the corrosion resistance, although both phenomena significantly deteriorate the magnetic properties. In this work, emphasis is given to the corrosion behaviour of nanocrystalline hot-pressed and dieupset NdFeB-based magnets. The influence of both microstructure and surface hydrogenation of textured magnets on their corrosion performance is investigated and compared with that of their hot-pressed precursors.

2. Experimental 2.1. Preparation of magnets and microstructural analysis Nanocrystalline melt-spun powders of the nominal compositions Nd14Fe80B6 and Nd12Dy2Fe73.2Co6.6Ga0.6B5.6, with commercial names MQP-A

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and MQU-G, respectively, were used as starting materials. The powders were hot pressed in vacuum at 7001C applying a pressure of 150 MPa to obtain isotropic precursors with a diameter of 8 mm and a height of 8 mm. The densified magnets are then placed in an over-sized die cavity. Dieupsetting process was carried out in an argon atmosphere under optimized conditions, employing a deformation temperature of 7501C and a strain rate of e ¼ 0:001 s1. Under these conditions, excessive grain growth and the formation of cracks can be minimized. Fig. 1 shows a schematic illustration of the pressing mode. For the die-upset samples two different cross-sectional areas were distinguished: one visible after a cut perpendicular to the pressing direction, assigned as surface (>), and one after a parallel cut, assigned as surface (8). Individual grain sizes and shapes, which are correlated to the corrosion and magnetic properties, were studied on fracture surfaces by scanning electron microscopy (SEM, JEOL JSM-6400). The distribution of the Nd-rich intergranular phase was investigated by SEM in the back-scattering mode, complemented by energy dispersive X-ray analysis (EDX). The structure of the prepared magnets was identified by X-ray diffraction using a Philips PW 1050 diffractometer with Co-Ka

Pressing direction

surface (⊥)

123

radiation. For chemical and electrochemical experiments, a cube of each NdFeB-magnet was embedded in epoxy resin, one surface of which was then mechanically polished down to 1 mm and ultrasonically cleaned with ethanol. All measurements were carried out using thermally demagnetized materials. 2.2. Chemical and electrochemical investigations The corrosion behaviour of the magnets was assessed in N2-purged 0.1 M H2SO4 solution at 251C. Solution analytical measurements were carried out in 100 ml of the test electrolyte by using inductively coupled plasma (ICP) analysis, which permits in situ and patch measurements of the concentration of dissolved species of magnets components. The hydrogen desorption behaviour of samples after treatment in 0.1 M H2SO4 solution was studied by means of thermal desorption device coupled with mass spectrometer using a desorption rate of 8.4 K/min. All samples were thinned to 2.4 mm, to ensure a comparable diffusion distance for the desorbing species. Details of the electrochemical cell and instrumentation have been already given elsewhere [18,19]. The rotation speed of the disc electrode was 720 rpm in order to avoid surface screening by corrosion products or hydrogen bubbles and to maintain instantaneous chemical homogeneity of the test solution. Potentiodynamic polarization tests were carried out at two different scan rates, 0.2 and 2 mV/s. All potentials in the present study are quoted with reference to a saturated calomel electrode (SCE) (U(SHE)=241 mV).

surface (II) 3. Results and discussion 3.1. Microstructural characterization

Pressing direction Fig. 1. Schematic illustration of the pressing mode during the die-upsetting process.

Figs. 2(a) and (b) show SEM micrographs of fracture surfaces of hot-pressed and die-upset MQP-A magnets, respectively. A SEM micrograph for a die-upset MQU-G magnet is also shown for comparison (Fig. 2(c)). The microstructure of hot-pressed MQP-A magnet (Fig. 2(a)) is

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Fig. 2. SEM micrographs of fracture surfaces of (a) hot-pressed and (b) die-upset MQP-A magnets. SEM micrograph of a die-upset MQU-G magnet (c) is shown for comparison.

generally very fine with homogeneous grain size distribution. The estimated mean grain sizes for hot-pressed MQP-A and MQU-G magnets are about 100 and 60 nm, respectively. The die-upset magnets are characterized by a very strong alignment of these grains with their flat faces perpendicular to the pressing direction and the magnetically easy axis of Nd2Fe14B platelets (the tetragonal c-axis) parallel to the pressing direction (see Figs. 2(b) and (c)). Additionally, significant grain growth is noted after die-upsetting and two types of grains are observed. Large equiaxed grains (1–2 mm) can be found at flake boundaries and, secondly, small platelet-shaped matrix grains. This indicates that the microstructure becomes

more heterogeneous upon die-upsetting. It should, however, be noted that the die-upset MQU-G magnet exhibits a less heterogeneous microstructure or phase distribution and a finer mean grain size (Fig. 2(c)) than the die-upset MQP-A magnet (Fig. 2(b)). Detailed TEM investigations revealed that die-upset magnets prepared by using the same deformation conditions show different average grain sizes depending on the composition, i.e. for MQP-A magnet (600 nm  150 nm) and for MQUG magnet (400 nm  100 nm) [20]. In other words, Co and Ga additions retard grain growth and retain a less heterogeneous microstructure during both the hot-pressing and die-upsetting processes. Figs. 3(a), (b) and (c) show the back-scattered

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Fig. 3. SEM micrographs (back-scattered electron mode) of polished surfaces of a hot-pressed MQP-A magnet (a) and a die-upset MQP-A magnet: (b) surface (>) and (c) surface (8).

SEM images of polished sample surfaces for hotpressed MQP-A (a) and die-upset MQP-A magnets with surfaces (>) (b) and (8) (c), respectively. In all micrographs, clusters (white contrast) of finely dispersed and continuous Nd-rich regions with a thickness of about 1–2 mm are seen at the flake boundaries. The grey regions represent fine particles mainly consisting of ferromagnetic matrix phase. Also, it is clearly seen that the abundance or frequency of Nd-rich clusters on the surface of hot-pressed and on the two surfaces of die-upset specimens is different with the order seeming to be as follows: die-upset (8)>hot-pressed>die-upset (>). More detailed analysis of hot-pressed magnets using TEM revealed that the grains of the matrix phase, with sizes in the range 50–100 nm,

are surrounded by a thin (1–5 nm), continuous and very uniformly distributed layer of Nd-rich grain boundary phase [21,22]. Nevertheless, this continuous layer cannot be identified after excessive grain growth of the matrix phase or after dieupsetting [21,22]. This fact indicates that the volume fractions of the most corrosion sensitive Nd-rich intergranular phase in magnets are redistributed upon die-upsetting. Similar microstructural observations are revealed in the backscattered electron SEM images of hot-pressed and die-upset MQU-G magnets. EDX analysis results for hot-pressed magnets showed that partial substitution of iron with 6.6 at% Co and 0.6 at% Ga additions remarkably reduces the total amount of RE-elements in the

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Table 1 Concentration of magnets components (at%) determined by EDX analysis at triple points of intergranular phase regions of hotpressed and die-upset MQP-A (Nd14Fe80B6) and MQU-G (Nd12Dy2Fe73.2Co6.6Ga0.6B5.6) magnets Nd (at%) MQP-A (hot-pressed) MQP-A (die-upset) MQU-G (hot-pressed) MQU-G (die-upset)

92.60 93.00 60.10 48.00

Dy (at%)

6.15

intergranular region from about 93 to about 67 at% (see Table 1). These results are consistent with earlier investigations by Yamamoto et al. [23] and by Fidler et al. [24] on microcrystalline (sintered) permanent magnets, for which the formation of new Nd16Fe13Ga, Nd3(Ga,Fe), Nd5(Ga,Fe)3, Nd(FeCo)2 and Nd3Co intergranular-type phases was detected. EDX analysis results for samples after die-upsetting show no significant change in the composition of Nd-rich intergranular regions in MQP-A magnets (see Table 1). Nevertheless, further reduction in the total amount of RE-elements in intergranular regions from about 67 to 48 at% is detected in the intergranular region of MQU-G magnets after die-upsetting (see Table 1). Taking into account the standard redox potentials of the constituent elements of the magnet, these results (i.e. for MQU-G magnets) indicate that the intergranular phase regions transform into a nobler phase. On the other hand, Co and Ga additions and the die-upsetting process do not significantly influence the content of constituents in the matrix phase. In summary, texturing via the die-upsetting process induces a heterogeneity in the microstructure and can lead to the formation of grains in the sub-micron range. However, Co and Ga additions reduce the evidence of both phenomena and lead to a significant modification in the composition of intergranular phases. 3.2. Corrosion characterization Fig. 4 shows typical examples of the corrosion rate curves for the hot-pressed MQP-A and MQUG magnets in N2-purged 0.1 M H2SO4 solution as a function of immersion time. The corrosion rates

Fe (at%)

Co (at%)

Ga (at%)

7.40 7.00 6.40 20.00

14.11 17.00

13.18 15.00

Fig. 4. Corrosion rates measured by means of ICP solution analysis for hot-pressed MQP-A and MQU-G magnets in N2purged 0.1 M H2SO4 at 251C and 720 rpm rotation speed as a function of immersion time. The insets are SEM micrographs for etched surfaces of the hot-pressed MQP-A magnet after 1 and 10 min immersion.

shown here represent the sum of the partial dissolution rates of magnet constituents instantaneously measured during immersion in the test solution using on-line ICP solution analysis. Regardless of the magnet composition and microstructure, these results along with the results of surface characterization by means of SEM (insets of Fig. 4) show that the corrosion process is characterized by three stages. The initial stage (I) of about 1–3 min corresponds to a preferential dissolution of Nd-rich intergranular region. This attack is known to occur predominantly due to galvanic interaction with the more noble matrix

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phase [16]. The second stage (II), characterized by a rapid increase in the corrosion rate, is due to the preferential dissolution of Nd-rich intergranular regions and the partial undermining of matrix phase particles. The corrosion rate reaches its steady state (III) after about 6–7 min, as a consequence of the undermining and breaking out of matrix phase particles that lose their contact with the bulk magnet surface after the initial incubation period of intergranular corrosion (I+II), i.e. the pulverization of the magnet surface proceeds. Fig. 5 summarizes the corrosion rates of hotpressed and die-upset (both surface (8) and surface (>)) MQP-A and MQU-G magnets estimated after 1 and 10 min immersion in 0.1 M H2SO4 solution. As it can be seen, the corrosion rates of die-upset or textured magnets are in general lower than those of the hot-pressed precursors. The degree of discrimination between the corrosion rates of hot-pressed magnets and die-upset magnets for surface (8) after 10 min of immersion is less pronounced, in particular for MQP-A magnets. Such improvement in corrosion resistance can be correlated with the detected grain growth (Fig. 2) and the reduction in the total amount of RE-elements in the intergranular regions (see Table 1) following the die-upsetting process. Furthermore, it is worth mentioning that the

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corrosion rates of the hot-pressed and die-upset nanocrystalline MQU-G magnets are significantly lower than those of MQP-A magnets (see Figs. 4 and 5). This can be attributed to the beneficial effects of the Co and Ga additions [18–24]. Comparing the examined sides of the die-upset samples, it is clearly seen in Fig. 5 that the corrosion rate of surface (>) is signifcantly lower than that of surface (8) for both types of magnets. The larger size of the flat faces of the platelet matrix grains, in addition to the lower frequency of Nd-rich clusters at the flake boundaries, which offer initiation sites for intergranular corrosion and subsequent pulverization of the magnet, are likely explanations for the high corrosion resistance of the surface (>). The above quantitative results are further correlated with SEM micrographs shown in Figs. 6(a)–(f) for hot-pressed and textured MQPA ((a), (c) and (e)) and MQU-G ((b), (d) and (f)) magnets, immersed in 0.1 M H2SO4 for 10 min. Regardless of the magnet composition, it is obvious from these micrographs that the mode and intensity of corrosion damage are primarily determined by the microstructural features of the examined magnets. Intergranular corrosion attack (IGC) and undermining of grain particles are generally the most prominent features for the hotpressed and die-upset magnet surfaces. It should

Fig. 5. Corrosion rates of hot-pressed and die-upset MQP-A and MQU-G magnets measured after immersion in N2-purged 0.1 M H2SO4 for 1 and 10 min at 251C and 720 rpm by means of ICP solution analysis.

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Fig. 6. SEM micrographs for MQP-A magnets as hot-pressed (a) and die-upset with surface (8) (c) and surface (>) (e), after etching in N2-purged in 0.1 M H2SO4 at 251C and 720 rpm for 10 min. Micrographs for MQU-G magnets as hot-pressed (b) and die-upset with surface (8) (d) and surface (>) (g) are shown for comparison.

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be noted, however, that MQU-G magnets generally exhibit much less corrosion damage after the same immersion time, particularly the die-upset MQU-G magnet for the surface (>) (Fig. 6(f)). 3.3. Electrochemical characterization Figs. 7(a) and (b) show typical examples of Tafel polarization plots recorded for hot-pressed and

Fig. 7. Potentiodynamic polarization curves of hot-pressed and die-upset MQP-A (a) and MQU-G (b) magnets in N2-purged H2SO4 at 251C and 720 rpm rotation recorded at a scan rate of 0.2 mV/s.

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die-upset MQP-A and MQU-G magnets, respectively, in 0.1 M H2SO4 at a scan rate of 0.2 mV/s, which gives the best definition for polarization plots and maintains quasi-steady conditions. From both figures it is obvious that a distinct reduction in the corrosion current density (icorr: ) of about one order of magnitude is reached after die-upsetting, particularly for the surface (>). The estimated values of corrosion current densities (icorr: ) for hot-pressed and die-upset MQP-A and MQU-G magnets are presented in Table 2. The corrosion current density (icorr: ) is determined from the intersection of extrapolated cathodic and anodic Tafel lines at the corrosion potential (Ecorr: ). The values of icorr: presented in Table 2 confirm that the partial substitution of Fe by Co and Ga, along with texturing, particularly for the surface (>), positively influence the corrosion performance of nanocrystalline NdFeB magnets. Qualitatively, the icorr: values presented are in good agreement with the analytical investigations shown in Figs. 4 and 5. Nevertheless, the difference in the corrosion rates noted from the comparison between analytical and electrochemical measurements for the examined materials is not clearly understood yet. Potentiodynamic polarization curves over a wider potential range are also recorded in order to characterize the effect of texture on the anodic polarization behaviour in more detail. The polarization curves show generally an activation-controlled region of about three decades of the measured current density, prior to the onset of the diffusion controlled region at anodic overpotential of about 0.15 V (SCE) [18]. A clear difference in the electrochemical behaviour between the examined magnets occurs mainly in the vicinity of the corrosion potential, and diminishes with progressing anodic polarization, where all

Table 2 Corrosion current densities (icorr: ) of hot pressed and die-upset of MQP-A and MQU-G magnets in 0.1 M HsSO4 determined from Tafel polarization curves (as shown in Fig. 7) Magnet type

2

icorr: (mA cm )

MQP-A (Nd14Fe80B6)

MQU-G (Nd12Dy2Fe73.2Co6.6Ga0.6B5.6)

Hot-pressed

Die-upset (8)

Die-upset (>)

Hot-pressed

Die-upset (8)

Die-upset (>)

10.00

2.90

1.00

8.00

2.10

0.60

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magnets exhibit no tendency for passivation in this strongly acidic solution. 3.4. Hydrogenation behaviour Our previous study has shown that the sensitivity of magnets surfaces to the hydrogen released during the acid corrosion process is an additional factor playing a crucial role in assessing their corrosion performance, particularly during the steady state of corrosion [18,19]. The sensitivity of the magnet surfaces to the hydrogen formed during the corrosion was also found to be related to the microstructure and the alloying additions [18,19]. Thus, for a better understanding of the effect of die-upsetting on the complex corrosion degradation process for the nanocrystalline NdFeB magnets, information about hydrogen absorption and hydride formation is desirable. With this, a more accurate assessment of the effect of absorbed hydrogen on the pulverization of dieupset magnets can be achieved. Fig. 8 shows thermal desorption spectra for dieupset MQP-A and MQU-G magnets, measured after treatment in 0.1 M H2SO4 solution for 20 min. For this measurement, it has to be mentioned that the recorded spectra are due to

the hydrogen desorbed from simultaneously treated surfaces (8) and (>) of die-upset magnets. The spectra for hot-pressed magnets are also shown for comparison in Fig. 8. The spectra obtained are in principle consistent with those presented in the literature [25–27], where it was shown that the process of vacuum desorption of hydrided powders of NdFeB magnets occurs in three stages. The three characteristic desorption peaks, shown in Fig. 8, with peak maxima in the temperature ranges corresponding to those reported in literature, can be assigned to: I. Complete desorption of hydrogen from the ferromagnetic phase (room temperature to 3001C): Nd2 Fe14 BHB2:9 -Nd2 Fe14 B þ 1:45H2 : ð1Þ II. Transformation of Nd ‘‘trihydride’’ (NdHB2.7) to Nd ‘‘dihydride’’ (NdHB1.9) (200–4001C): NdHB2:7 -NdHB1:9 þ 0:4H2 :

ð2Þ

III. Complete decomposition of Nd ‘‘dihydride’’ and desorption of hydrogen (550–6501C): NdHB1:9 -Nd þ 0:95H2 :

ð3Þ

Fig. 8. Thermal desorption spectra for hot-pressed and die-upset (all sides) MQP-A and MQU-G magnets recorded after immersion in N2-purged H2SO4 for 20 min.

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In general, the amount of hydrogen desorbed from the overall sample surface is directly related to the integrated intensity of the area under the curve. Therefore, from Fig. 8, it is clear that the amounts of hydrogen desorbed from the treated MQU-G magnets are much smaller than those desorbed from MQP-A magnets after a similar treatment. Additionally, die-upset magnets (all surfaces) exhibit a lower affinity for corrosion hydrogen than their hot-pressed precursors. It has also to be noted that the desorption maximum (III) shifts to lower temperatures with Co and Ga additions and texturing, indicating that textured MQU-G magnets have lower affinity for the corrosion hydrogen. The effective diffusivity of hydrogen in NdFeB magnets is reduced with Co and Ga additions and as a result of the texture induced in the microstructure via die-upsetting process. In addition to the results shown in Fig. 8, the thermal desorption behaviour related to the hydrogenation of a die-upset MQU-G magnet is different for the two orthogonal (8) and (>) surfaces. It is clearly seen in Fig. 9 that the amount of hydrogen desorbed from surface [>] is significantly lower than that for surface [8] after a similar treatment. The previous observations can be ascribed to the microstructural modifications discussed with regard to the results shown Figs. 2 and 3. This refers

Fig. 9. Thermal desorption spectra of a die-upset MQU-G magnet after immersion in N2-purged H2SO4 with surfaces (8) and (>) for 20 min.

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in particular to the significant reduction in the volume fractions of Nd-rich intergranular phase regions, which have a comparatively high tendency for hydride formation. Furthermore, the wellknown property of Co in enhancing the rate of hydrogen atom recombination at a magnet/solution interface, which is the competitive surface reaction to the hydrogen absorption during the cathodic hydrogen reduction reaction [18,28], and the ability of Ga to reduce the diffusion of hydrogen into rare-earth-containing magnets [29] will be contributing factors. Further analysis of the desorption spectra (Figs. 8 and 9) along with XRD investigations (not shown) reveal the following: (1) The higher temperature peak (III) is very large, not symmetrical in shape and broadened over a wider temperature range. This indicates that the amount of hydrogen desorbed is very high and not so well correlated with those desorbed from the lower temperature peaks (I and II), as it was observed, for example, for air-exposed sintered magnets [25]. (2) The changes in the microstructural features by alloying additions and after the die-upsetting process lead to an appreciable change in the amount of hydrogen desorbed for all events, particularly at higher temperatures (peak III). (3) Part of the corrosion hydrogen dissolves in the normal interstitial lattice sites in the magnets, as indicated from the recorded distinct shift in the XRD reflection peaks of the hard magnetic phase towards lower diffraction angles. These facts indicate the contribution of interstitial sites with low binding energy for hydrogen, as well as conventional structural heterogeneities (e.g. grain boundaries, interfaces and dislocations) with high binding energies in the hydrogen absorption process, particularly in the case of desorption peak III. In other words, interstitial sites within magnet phases and structural heterogeneities cannot be ruled out as the principal hydrogen trapping sites governing magnet pulverization during the acid corrosion process, in addition to hydride formation. Thus, the distinct lack of hydrogen transport sites such as dislocations, as reported earlier [2,7,9], apart from the reduction in total volume fraction of the most hydrogen sensitive Nd-rich intergranular phases and

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interfaces in bulk magnets after the die-upsetting process, can be cited as additional factors responsible for the higher corrosion resistance of textured magnets compared with that of hot-pressed precursors. In summary, the formation of hydrides and lattice expansion due to hydrogen absorption, in addition to the preferential dissolution of the Ndrich phase, generally contribute to the decrease in the corrosion resistance and pulverization of the NdFeB magnets. More importantly, Co and Ga additions and texturing render NdFeB magnets less susceptible to pulverization by reducing the effective diffusivity of the corrosion hydrogen in magnets. In other words, optimizing the magnet composition and microstructure, via alloying additions and hot deformation, provide an advantageous means for improving both magnetic and corrosion properties of nanocrystalline NdFeB magnets.

4. Conclusions A comparison between the corrosion behaviour of hot-pressed isotropic and die-upset anisotropic (textured) nanocrystalline NdFeB-base magnets, with and without Co and Ga additions, in 0.1 M H2SO4 solution has been performed by means of electrochemical polarization methods, chemical solution analysis and thermal desorption analysis. From this, the following conclusions can be drawn: 1. Textured magnets, particularly those with Co and Ga addition, generally offer better corrosion resistance compared with isotropic hotpressed magnets. Furthermore, for the textured MQU-G magnets, surface (>) exhibited better corrosion resistance than surface (8). 2. The higher corrosion resistance of textured MQU-G magnets can be explained on the basis of microstructural and electrochemical aspects, as follows: I. Texturing generally provides magnets with lower microstructural defect concentrations such as interfaces, grain boundaries, dislocations. Further, the magnets have smaller

fractions of Nd-rich intergranular phase regions, due to grain growth and hence reduced numbers of effective paths for hydrogen penetration. II. Co and Ga additions generate new, more noble intergranular phases and this reduces the driving force for galvanic corrosion between magnet phases. Also, these alloying additions render the surface hydrogenation of magnets more difficult, though magnets with Co and Ga exhibit a finer grained microstructure than those without additives. Co and Ga additions reduce the effective diffusivity of hydrogen in the magnet by enhancing the rate of H-atom recombination at the magnet/ solution interface as competitive reaction to hydrogen absorption.

Acknowledgements A.A. El-Moneim wishes to express his thanks to the International Office of the BMBF (FZ Juelich, Germany) for financial support. The authors are very grateful to A. Guth . and K. Hennig for their experimental assistance and Magnequench Int. for supplying melt-spun powders.

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