Processing of Mn–Al nanostructured magnets by spark plasma sintering and subsequent rapid thermal annealing
Introduction
There has been considerable interest in the development of rare earth (RE) free permanent magnets (PMs) owing to the increasing cost and decreasing reserve of RE metal resources [1], [2]. Among the RE-free PMs, Mn–Al alloys represent a topic of growing interest in the field of magnetism mainly because of the fact that these alloys exhibit PM properties and unlike AlNiCo, they do not contain strategic and expensive elements such as Ni and Co. In addition, Mn–Al magnets can have many practical and commercial uses; as they surpass the performance of conventional hard ferrites and also prove to be more cost effective than the RE based PMs [3], [4], [5], [6]. The attractive ferromagnetic properties of the Mn–Al system are principally derived from the metastable τ-phase, in which the Mn atoms exist at eight apexes in L10-superstructure [7], [8]. Although neither Mn nor Al is ferromagnetic, the Mn–Al -based alloys with nearly equiatomic composition exhibit ferromagnetism. These alloys crystallize into the L10-ordered τ-phase (tetragonal) on quenching the parent high-temperature ε-phase (hexagonal) followed by a suitable heat treatment (annealing) or by cooling the ε-phase at a controlled rate. The τ-phase is reported to be resistant to corrosion and possesses attractive ferromagnetic characteristics such as robust magneto-crystalline anisotropy field (3023.9 kA/m), moderate saturation magnetization (0.96 T) and Curie temperature (523 K) and favorable energy product values (100.6 kJ/m3) [9], [10]. The magnetic properties of the Mn–Al alloys are therefore mainly determined by the extent of formation of τ-phase, which is in turn is strongly influenced by the starting composition (Mn:Al ratio), preparation methods and subsequent annealing treatments.
During the last few years, non-equilibrium processing techniques such as melt spinning [11], [12], splat quenching [13], mechanical alloying [14], [15], gas atomization [16] and plasma arc discharge [17] have been adopted for the Mn–Al based alloys. All these techniques were essentially attempted to stabilize the L10-type τ-MnAl phase with an intention to realize high coercivity (Hc) and saturation magnetization (Ms). The magnetic properties of Mn–Al system are sensitive to the microstructure and the presence of defects developed during the τ-phase formation [8] and these attributes are strongly determined by the nature of production route employed. In Table 1, we present a brief summary of the magnetic properties achieved vis-à-vis fabrication methods employed for the Mn–Al alloys. As far as Mn–Al alloys are concerned, the processing routes which produce a refined microstructure with small ε-crystallites are found to yield enhanced surface-to-volume ratios that promote the formation of τ-phase. Accordingly, excellent magnetic properties such as Hc of 382 kA/m and Ms of 87 A/m2/kg were achieved for the τ-MnAl nanocrystalline powders obtained by mechanical milling and subsequent annealing [18]. However, the realization of maximum achievable hard magnetic properties (close to the theoretical limits) in the bulk form of Mn–Al magnets still poses challenges owing to the difficulties encountered during consolidation such as excessive grain-growth and lack of stabilization of τ-phase. In this context, studies on the development of synthesis strategies for processing Mn–Al bulk magnets with required τ-phase and fine-grained morphology have gained greater importance.
As far as consolidation of nanocrystalline powders is concerned, spark plasma sintering (SPS) technique has emerged in recent times as an efficient method for ceramics, metals, alloys and composites with a capability of achieving fast densification and minimal grain growth in a shorter sintering time. The potential of SPS technique in the processing of RE based PMs [19], [20], [21] and other types of intermetallics [22], [23] has been well demonstrated; however, its applicability in the processing of Mn–Al bulk magnets has not yet been systematically investigated. Very recently, for the first time, the SPS process was introduced in producing Mn–Al–C bulk samples by Pasko et al. [24]. Subsequent to this effort, herein we further exploit the potential of SPS technique in the compaction of Mn–Al magnets. The present study can be distinguished from the earlier work on the processing of Mn–Al–C magnets by SPS, wherein Mn–Al and carbon doped Mn–Al alloys obtained by induction melting were processed in the form of melt-spun ribbons and then the crushed ribbons were subjected to SPS. In contrast, in this study, Mn–Al alloy powders were produced by mechanical milling using Mn and Al metal elements and the milled ingredients were consolidated by SPS. The SPSed Mn–Al bulk samples were then subjected to rapid thermal annealing (RTA) in order to attain the desired τ-phase and hard magnetic properties in the Mn–Al bulk magnets. Through this study we intend to demonstrate that a combination of two rapid processing techniques such as SPS and RTA could be an appropriate method for fabricating Mn–Al nanostructured magnets.
Section snippets
Experimental
A mixture of Mn (56 at%) and Al (44 at%) elemental powders having purity of 99.9% was weighed and subjected to milling in presence of toluene. Milling was performed in a planetary ball mill (FRITSCH pulverisette) with milling vial and balls made of tungsten carbide. The milling time was varied typically from 2 to 25 h at a constant speed of 200 rpm with ball to powder ratio of 10:1. The Mn–Al powders obtained after 25 h of milling were considered for consolidation as they have very good tendency to
Results and discussion
The evolution of particle morphology as a function of milling time for the Mn54Al46 powders is depicted in Fig. 2a–d. From the XRD patterns, the transformation of irregular/angular shape of the starting Mn–Al powders into small aggregates can be noticed due to progressive milling. After 25 h of milling, the powder morphology became more agglomerative (Fig. 2d). For this reason, the actual grain size values for the Mn–Al powders could not be estimated using SEM. The XRD patterns of Mn54Al46
Conclusions
In this study, SPS in combination with RTA was employed to produce Mn–Al nanostructured magnets from mechanically alloyed α-Mn(Al) powders. The application of SPS process did not result in the formation of τ-phase in the Mn–Al magnets and as a result, the SPSed samples showed a weak ferromagnetic behavior. The precipitates of fine-grained τ-phase at the grain boundaries of parent ε-phase were subsequently formed in the SPSed samples upon subjecting them to RTA at 1100 K. This has resulted in
Acknowledgments
The keen interest shown by the Director, DMRL in this work is gratefully acknowledged. The authors PS and SVK acknowledge DRDO for funding this work. The authors VTPV and MC acknowledge the Ministry of Education, Youth and Sports in the framework of the targeted support of the ‘National Programme for Sustainability I’, OPR&DI Project (LO1201) and the OP VaVpI of the Centre for Nanomaterials, Advanced Technologies and Innovation—CZ.1.05/2.1.00/01.0005 through the ‘Project Development of Research
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