Effect of neodymium content and niobium addition on grain growth of Nd-Fe-B powders produced by gas atomization
Graphical abstract
Introduction
Since the discovery in 1984 of neodymium‑iron‑boron (Nd-Fe-B) alloy by Sagawa et al. [1] and Croat et al. [2], this material has become the basis of the strongest permanent magnet thanks to an excellent combination of remanence and coercivity [[3], [4], [5]]. Indeed, during the last decades, there has been a sustained research to improve Nd-Fe-B magnets, particularly since the crisis of rare earth prices in 2011 [6]. More recently, innovation has been driven by the rise of electromobility, sensor industry, robotics or green energy production. Studies have focused on cheaper compositions, improvement of processing, and tailoring the characteristics of the magnet to the application. One main objective has been reducing the consumption, or even eliminating, the heavy rare earths typically utilized in high temperature grades [7]. Among the developed strategies, it is noteworthy to mention the deposition of Dy at the grain boundaries by diffusion [8] and the reduction of the average particle size of the powder up to ~1 μm by milling [9]. The use of consolidation technologies (e.g. spark plasma sintering or die upsetting) involving plastic deformation has been revisited [10,11]. In order to reduce the cost of magnets, new compositions with cheaper rare earth elements have been developed [[11], [12], [13]]. New shaping technologies, as additive manufacturing, that allow making complex geometries are being investigated [6].
The conventional method to produce Nd-Fe-B powder for sintered magnets involves melting and alloying the raw materials; ingot, strip or book mold casting; ingot crushing and / or hydrogen decrepitation, and jet milling under hydrogen or inert atmosphere. The resulting powder is monocrystalline and irregular with a particle size <10 μm. Powder for bonded magnets is mostly produced by either melt spinning or hydrogenation-disproportionation-desorption-recombination (HDDR). Melt spinning results in a flake powder with an isotropic nanocrystalline structure. HDDR yields an irregular powder with an anisotropic ultrafine microstructure [15].
Gas atomization has attracted the attention of researchers as an alternative technology to produce Nd-Fe-B powders, especially for bonded magnets [[14], [16], [17], [18], [19], [20], [21], [22]]. In gas atomization, a homogenous melt of the desired alloy is driven through a feeding tube from the furnace to the atomization chamber [25]. At the exit of the tube, the melt stream is impinged by a high velocity gas flow that breaks the liquid alloy into microscopic droplets. The small size of the droplets and the high velocity of the gas flow enhance the heat transfer between both phases, producing the rapid solidification of the melt. Due to the high reactivity of rare earths, the process is typically carried out under inert atmosphere and using an inert gas as atomizing fluid. Powders obtained by inert gas atomization are spherical [[14], [16], [20], [22]]. Its microstructure depends on the cooling rate, which in turn depends on the particle size and atomizing gas.
Yamamoto et al. [14] reported that particles bigger than 30 μm were dendritic, particles below 30 μm were formed of equiaxial grains, and there was a certain amount of amorphous phase in particles <25 μm. Sakaguchi et al. [16] fixed the limit between dendritic and equiaxial grains in a particle size of 2–3 μm, but also remarked the presence of amorphous phase in fine particles. Sokolowski et al. [20] observed that particles below 25 μm were formed by an amorphous matrix with embedded nanocrystallites and that the fraction of amorphous phase increased with decreasing particle size.
Owing to the peritectic nature of the Nd2Fe14B phase, alloys of stoichiometric and near-stoichiometric composition contain large amounts of α-Fe dendrites [[23], [24]], which is an undesirable soft magnetic phase. The amount of α-Fe can be reduced by increasing the Nd concentration over the stoichiometric one, increasing the cooling rate during solidification and / or adding Nb [26]. Nb additions have many positive effects on Nd-Fe-B magnet properties [[26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. These effects do not only depend on the composition, but also on the processing method. In rare earth lean Nd-Fe-B / α-Fe nanocomposites produced by melt spinning, Nb increases the glass forming ability (GFA) of the alloy, hinders crystallite growth during hot consolidation, and induces anisotropy by hot deformation when introduced at the expense of Fe [27]. The addition of 1 at.% of Nb to HDDR processed near-stoichiometric Nd-Fe-B alloys produces a very fine grain size of <0.2 μm after recombination [33]. Besides, the partial substitution of niobium for iron reduces the homogenization time considerably [23,34]. In sintered magnets with an excess of rare earths, Nb reduces grain growth and improves coercivity, impact toughness, corrosion strength, and the squareness of the demagnetization curve [29,31,32,34]. The benefits linked to niobium are often explained by the formation of Nb-rich phases. Nb-rich precipitates reduce the final grain size in Nd-Fe-B magnets, eliminate free Fe without consuming Nd, and may have a pinning effect on domain walls, being these the reasons for improved coercivity [31,32,35,37]. However, the specific mechanism by which Nb-rich compounds impede grain growth is not usually explained.
The magnetic properties of Nd-Fe-B alloys depend on several microstructural factors, such as the volume fraction of phase Nd2Fe14B, grain size, distribution of the intergranular Nd-rich phase, and the presence of other secondary phases [[38], [39]]. These features vary depending on the thermal history.
Jet milled micrometric powder is usually pressed under an aligning magnetic field and consolidated by liquid phase sintering (LPS) at 1000–1100 °C [3,4]. An ideal liquid for LPS should wet the solid phase. The degree of wetting is assessed by the contact angle (θ), which is the equilibrium angle adopted by a drop placed on a horizontal surface at the liquid-vapor-solid intersection and is given by the following Eq. [40]:
where γSV is the solid-vapor surface energy, γSL is the solid-liquid surface energy, and γLV is the liquid-vapor surface energy. Liquids with a low contact angle spread readily along porosity channels after its formation. Spreading is important to obtain a uniform densification, since the liquid often arises forming isolated pockets that are dispersed through the compact [41]. Moreover, wetting produces an attractive capillary force between particles that leads to densification by rearrangement. Solubility of the solid into the liquid is another requirement for a successful LPS. The liquid provides a fast diffusion path for the transport of atoms from small to large particles, causing grain growth and densification by a solution-reprecipitation mechanism [[42], [43], [44]]. Coalescence is the other mechanism that produces grain growth during LPS of Nd-Fe-B alloys [45]. The development of a solid skeleton due to the growth of necks between particles depends on the dihedral angle and the amount of liquid [[46], [47], [48]]. The dihedral angle (ϕ) is given by the following expression:
where γGB is the grain boundary energy. If the dihedral angle is zero and the amount of liquid high enough, a continuous layer of liquid prevents the formation of interparticle bonds.
Liquid formation starts at around 685 °C in ternary alloys, temperature at which Nd-rich phase melts [24]. The amount of liquid depends on composition and sintering temperature [41,43,44]. As a result, Davies et al. [41] observed that increasing the Nd content resulted in greater densification at lower temperatures. Moreover, Straumal et al. [49] showed that the Nd-rich liquid phase can wet the Nd2Fe14B grain boundaries either completely or partially depending on the temperature. They concluded that all the grain boundaries are wetted at 1150 °C, whilst the covered percentage is only 10% at 700 °C. In order to restore the continuity of the Nd-rich layer and the coverage of Nd2Fe14B grains, sintered magnets are subsequently annealed below the solidus temperature of the alloy [49]. In this way, Nd2Fe14B grains are magnetically uncoupled, which increases sharply the intrinsic coercivity. Finally, it is important to highlight that small concentrations of impurities can greatly modify the wetting behavior and thus the spreading of the liquid [38,42,50]. These variations can change grain boundary properties such as mobility [49].
In order to homogenize the alloy and eliminate α-Fe, as-cast product is annealed between 1000 and 1080 °C for long periods (40 to 300 h) [23,28,34,36]. During annealing, the quantity of Nd2Fe14B increases due to the reaction of the Nd-rich phase with other borides and free iron. Besides, gas atomized powder is annealed between 500 and 750 °C to induces the crystallization of amorphous phases into Nd2Fe14B [14,18,51].
So far, gas atomized powder has been used only to manufacture bonded magnets. However, it could be an alternative raw material for some of the processes outlined above. For example, it could be jet milled to produce submicron monocrystalline particles for sintered magnets. In the European project NEOHIRE, it has been used as a precursor for the preparation of anisotropic powder (https://neohire.eu/results/documents/). Additionally, it could be consolidated into anisotropic magnets with a fine microstructure by hot plastic deformation. Gas atomization exhibits some interesting advantages. It is an industrial process, which should result in cheaper raw material. Solidification rates are high, leading to low chemical segregation and fine microstructures. In order to develop these ideas, it is necessary to understand the effect of high temperature exposure on the microstructure of gas atomized powders, since there are not reports for temperatures above 800 °C. The objective of this work is to study the effect of annealing at high temperature (between 1000 and 1150 °C) on the microstructural evolution of Nd-Fe-B gas atomized powder, analyzing in detail grain growth. The research is mainly focused on the effect of Nd content. As explained above, niobium has several positive effects on the properties of Nd-Fe-B alloys. Consequently, the research includes also one composition with this alloying element.
Section snippets
Experimental procedure
Nd-Fe-B powders were produced by gas atomization using a convergent-divergent, close-coupled atomizer in an atomization unit PSI model Hermiga 75 / 3VI. The raw materials were induction melted and alloyed in a high purity alumina crucible. In order to minimize oxidation, the furnace and the atomization chambers were evacuated (up to 0.1 mbar) and filled back with high purity Ar several times. About 3 kg of powder were produced per batch. Atomization gas was Ar. Table 1 shows additional
Differential scanning calorimetry (DSC)
Fig. 1 displays the DSC traces of the as-atomized powders. The relevant thermal events have been marked in the figures with symbols. The numerical values associated to each symbol are reported in Table 3 for each composition. The meaning of these thermal transitions can be interpreted with the help of the ternary phase diagram [24] and the microstructure of the powders, described in the next section. As a first approximation, main thermal events can be understood neglecting the influence of Nb,
Conclusions
- a)
Gas atomized Nd-Fe-B powders exhibit small amounts of amorphous or metastable phases due to the high cooling rate associated with the process.
- b)
Whereas the solidus temperature of the rapid cooled alloys is between 703 and 793 °C, melting starts between 650 and 688 °C if they are solidified under a slow cooling rate. Melting of NdFe4B4 occurs between 1057 °C and 1089 °C. Melting of Nd2Fe14B takes place between 1162 and 1184 °C. Liquidus temperature ranges between 1251 and 1277 °C.
- c)
Gas atomized
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 720838 (NEOHIRE project).
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2022, Materials CharacterizationCitation Excerpt :%) exhibits more densification. This is explained by its higher volume fraction of liquid, which increases the area of the solid/liquid interface [31]. It is noteworthy to remark that pores are spherical in all the particles sizes, indicating as well a higher degree of sintering.