DOI: 10.1051/epjconf/20134006008 Hard magnetic properties of melt-spun Mn-Al-C alloys

Structural and magnetic characterization of Mn(x−y)Al(100−x−y)C(2y) (x = {50, 55}; y = {0, 1}) melt-spun ribbons is reported. To obtain the metastable ferromagnetic τ-phase, rapidly solidified alloys were annealed either in a vacuum furnace at 823 K or directly in the vibrating sample magnetometer under applied magnetic field. Optimal magnetic properties were demonstrated by Mn54Al44C2 samples proved to be single-phase with a coercivity of 0.19 T measured in both cases. For this composition the structural e→τ phase transformation has been magnetically detected at 786 K, Curie temperature of τ-phase (Tc = 592 K, Tp = 610 K) has been determined using mean field approximations in ferromagnetic and paramagnetic regions. Rietveld refinement of X-ray diffraction spectra was employed to analyse the phase constitution of annealed alloys, lattice parameters as a function of chemical composition and mean grain size for the phases involved.


Introduction
MnAl alloys are attractive candidates for permanent magnet applications due to good magnetic properties (superior to conventional ferrites), high technological properties (mechanical strength, machinability, corrosion resistance) and low cost (no rare earth elements). The ferromagnetic tetragonal τphase with strong uniaxial magnetocrystalline anisotropy [1,2] forms from the non magnetic hexagonal εphase by annealing a quenched alloy with 5060 % of Mn at approximately 823 K. Both phases are metastable at room temperature and therefore need special thermal treatment. Carbon addition stabilizes τphase and facilitates production of commercial anisotropic materials by hot deformation [3]. Magnetic hysteresis of MnAlC alloys is sensitive to the microstructure and presence of defects developed during τphase formation, comprising martensitic or massive transformation mechanisms [4], and is therefore strongly influenced by the production route.
Various production techniques have been employed for preparation of homogeneous MnAlC alloys: melt spinning [57], levitation melting with splat quenching [8], spark erosion [9], ball milling [10,11], mechanical grinding [12]. Depending on the solidification rate or size reduction degree, finegrained materials with improved functional properties can be obtained. In this paper we report on the structural and magnetic characterization of MnAlC meltspun ribbons. The goal is to investigate a correlation between the production route, phase constitution and resulting hard magnetic properties. Using accurate Xray crystallographic analysis and magnetization measurements in a wide temperature range, we have advanced our understanding of structural and magnetic phase transitions in MnAlC system and particular role of carbon.

Experimental
Binary alloys Mn 50 Al 50 and Mn 55 Al 45 , as well as their ternary counterparts doped with carbon Mn 49 Al 49 C 2 and Mn 54 Al 44 C 2 were selected for the present study. The ingots prepared by induction melting were cast into ~ 3 mm wide and ~ 25 µm thick ribbons using a melt spinning technique. The details of sample preparation can be found in [6]. Final annealing was done in a vacuum furnace at 823 K for 10 min with ramping rate 10 K/min or directly in the magnetometer cell. Crystal structures were examined by a PANalytical X'Pert Pro Xray diffractometer (XRD) in CoK α radiation equipped with an X'Celerator linear detector. Rietveld fullprofile analysis of XRD spectra was performed using MAUD software [13]. Magnetization curves were recorded on a Lake Shore 7400 Series vibrating sample magnetometer (VSM) equipped with a variable temperature assembly.

Phase analysis
The phase composition of rapidly solidified ribbons has been determined by XRD both in the asquenched state (AQ) and after subsequent heat treatment (HT) as described above. According to the phase diagram of Al Mn system [14], formation of two equilibrium phases is expected: disordered cubic βphase (cP20) with excess of Mn and ordered trigonal γ 2 phase (hR26) with excess of Al. In addition, two metastable phases may appear in nonequilibrium conditions: disordered hexagonal ε phase (hP2) and ordered tetragonal τphase (tP2). Stable at higher temperatures, εphase can be preserved on quenching and transformed into ferromagnetic τphase by a compositioninvariant (involving only shortrange diffusion) structural transition. Depending on the initial chemical composition and thermal history, all these phases were observed in different combinations. Figure 1 presents XRD spectra from the asquenched samples, while Figure 2 shows them after annealing at 823 K. The results of quantitative phase analysis based on Rietveld refinement of XRD data are presented in Table 1. The weight fraction and lattice parameters for each observed crystal structure have been determined. Asquenched equiatomic alloy Mn 50 Al 50 is composed entirely of γ 2 phase which remains also after annealing. The presence of carbon in Mn 49 Al 49 C 2 helps to retain some amount of εphase subsequently transforming to τ phase. However, γ 2 phase persists and is not substantially affected by heat treatment. On the other hand, Mnrich composition Mn 55 Al 45 is more favourable for εphase formation. In this case, however, βphase precipitates along with desirable τphase on annealing. Finally, positive influence of carbon allows to obtain a 100 % τ phase in ternary Mn 54 Al 44 C 2 sample. As seen in Table 1, addition of carbon slightly increases both lattice parameters of εphase, while in τ phase it leads to decrease of a and pronounced (~ 1.5 %) increase of c. The unit cell volume also increases. Though C has been reported to occupy (½,½,½) sites together with Al [15], this observation provides a new evidence for presence of carbon atoms in the interstitial positions (0,0,½) and (½,½,0) of tetragonal τphase [10,16] similar to FeC martensite.

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The mean size of reflecting crystallites determined from line broadening is 100150 nm for most phases. Exception is εphase having large crystallites of ~ 600 nm and a noticeable (0 0 1) preferred orientation in as quenched samples which may be attributed to a columnar microstructure. Further refinement of grain size can be achieved by increase of the cooling rate in the melt spinning setup.

Magnetic properties
Magnetic measurements were performed on rapidly solidified ribbons in the asquenched state, after heat treatment in the furnace, and also during insitu heating/cooling in VSM.   (Table 1) as τ phase responsible for hard magnetism is observed after annealing only in Mn 55 Al 45 , Mn 54 Al 44 C 2 and Mn 49 Al 49 C 2 . Moreover, there is a correlation between the weight fraction of τphase determined from XRD and the saturation magnetization extracted from VSM measurements. Singlephase alloy Mn 54 Al 44 C 2 has the highest magnetization and better coercivity of 0.19 T compared to binary alloy Mn 55 Al 45 . Twophase alloy Mn 49 Al 49 C 2 displays lower magnetization and higher coercivity of 0.23 T. Our coercivity values are higher than 0.17 T [6] and 0.15 T [7] reported for other rapid solidification attempts, but lower compared to 0.48 T [10] achieved in a ball milled powder. However, pure τphase can be difficult to obtain in mechanically activated nano structured material [10]. Figure 4 shows the magnetization of asquenched Mn 54 Al 44 C 2 sample as a function of temperature under a constant magnetic field of 1.4 T. The ramps were performed between 300 K and 900 K at 5 K/min rate. Precursor εphase is antiferromagnetic below 97 K [17] and paramagnetic at initial room temperature. On heating, εphase transforms to τphase which is also paramagnetic at the transition temperature ~ 786 K (see the inset). The structural transition is detected due to a difference in magnetic susceptibility of the phases. On cooling, magnetization is increased drastically below Curie temperature of τphase as the reverse transition to εphase does not occur, probably, due to kinetic reasons.    used follow from the temperature dependence of magnetic susceptibility in the mean field theory: (T − T p ) −1 in paramagnetic region and (T c − T) 1∕2 in ferromagnetic region. Calculated temperatures T p and T c correspond well, satisfying usual inequality T p > T c . Thus, the variable temperature VSM experiment allows to observe ε→τ phase transition, receive τphase (under magnetic field) for further investigation and measure its Curie temperature by two methods. In MnAlC alloys Curie temperature of τphase decreases with carbon doping, available data largely vary [6,7,10]. Taking into account this dependence, we find good agreement of our T c measurements with results of Zeng et al. [10]. The coercivity of Mn 54 Al 44 C 2 sample subjected to in situ annealing is 0.19 T, i.e. equals to that after furnace annealing, and increases to 0.23 T on cooling down to 100 K.

Conclusions
Meltspinning technique enabled us to produce homogeneous singlephase precursors for MnAlC permanent magnets, though higher cooling rate is desirable for reducing the grain size. The metastable ferromagnetic τphase has been obtained by annealing nonmagnetic εphase samples either in a vacuum furnace at 823 K for 10 min or directly in the vibrating sample magnetometer under applied magnetic field. Among the studied alloys, best magnetic properties are demonstrated by Mn 54 Al 44 C 2 samples due to the positive effect of carbon on the stability of τphase and domain wall pinning by fine carbide precipitates. The coercivity decreases with temperature from 0.23 T at 100 K to 0.19 T at 300 K and is independent on the annealing method (with or without field). On heating in VSM Mn 54 Al 44 C 2 alloy undergoes the structural ε→τ phase transformation at 786 K, on cooling Curie temperature of τphase (T c = 592 K, T p = 610 K) is determined using mean field approximations in ferromagnetic and paramagnetic regions. The results of magnetization measurements are in good agreement with XRD quantitative phase analysis based on Rietveld refinement. The tetragonal distortion of τphase unit cell has been confirmed to increase with carbon content, which suggests a possible presence of carbon atoms in the interstitial positions similar to FeC martensite.