Time Dependent Microstructural and Magnetic Properties of Nanocrystalline Fe67.6Co16.9Nb5B8.5P2 Metallic Alloy
K Gehlot1, S S Modak2, L K Varga3 and S N Kane1*
1Magnetic Materials Laboratory, School of Physics, D A University, Khandwa Road Campus, Indore, India
2Physics Department, Jaypee University of Eng & Tech, AB Road, Raghogarh, Guna, India
3RISSPO, Hungarian Academy of Sciences, 1525 Budapest, Hungary, India
Submission: March 21, 2018; Published: April 04, 2018
*Corresponding author: Kane SN, Magnetic Materials Laboratory, School of Physics, D A University, Khandwa Road Campus, Indore, Email: kane_sn@yahoo.com
How to cite this article:K Gehlot,S S Modak,L K Varga,S N Kane.Time Dependent Microstructural and Magnetic Properties of Nanocrystalline Fe67.6Co16.9Nb5B8.5P2 Metallic Alloy. JOJ Mater Sci. 2018; 4(2): 555635. DOI: 10.19080/JOJMS.2018.04.555635
Abstract
Effect of thermal treatment on structural, magnetic properties of Fe67.6Co16.9Nb5B8.5P2 partly nanocrystalline alloy has been studied via X-ray diffraction and magnetic measurements. XRD results show that after isothermal annealing treatment at 450°C the grain size range between 11.0 to 25.0 nm, and saturates to 24.5 nm. For the studied samples, the obtained lattice parameter shows the presence of pure bcc-Fe phase (without any presence of Co) which precipitates out from amorphous matrix. Coercivity and saturation magnetization values range respectively between 50 to 83 A/m and 1.8 to 1.5 Tesla. Time evolution of magnetic properties very clearly shows the correlation with structural changes.
Keywords: Metallic glass; Nanostructured grains; X-ray diffraction; Magnetic properties
Introduction
Fe-based bulk metallic glasses possess good glass forming ability, high thermal stability, high strength and excellent soft magnetic behavior [1]. The combination of nano grain size and good soft magnetic properties have many applications [2]. Vital studies e.g. [3,4] concluded that Fe based amorphous alloys appear to improve the saturation magnetization and having low losses, needed for industrial applications. Si free nanoperm alloy displays good soft magnetic properties, and the presence of Co improves Curie temperature of amorphous matrix and precipitating crystalline phase [5]. Fe-Co containing alloys exhibit high saturation magnetization (1.8 T), so the amount of Nb and B needs to be reduced in the alloy. It may be noted that the absence of Zr in the alloys helps them to be casted in air. P addition is very effective in widening the optimum annealing temperature range and refining of bcc-Fe grains size in addition to increasing of nano crystalline grains density [6]. Thermal treatment near crystallization temperature is known to relax the material structurally, thus affecting the microstructure. Therefore, in the present work we report the effect of isothermal annealing on time evolution of structural and magnetic properties of Fe67.6Co16.9Nb5B8.5P2 alloy.
Experimental
Amorphous alloy ribbons of nominal composition Fe67.6Co16.9Nb5B8.5P2 (4 mm wide, 20 μm thick) were prepared by planner flow casting method on a rotating copper wheel. Thermal treatments (between 15 minute to 60 minute at 450°C in an interval of 15 min) were performed in an inert (Ar) atmosphere. X-ray diffraction measurements were done using Brukar D8 Advance X-Ray Diffractometer with Cu-Kα radiation (λ =0.15406 nm) at UGC-DAE-CSR Indore. X-Ray Diffraction data was analyzed with the help of a MATLAB based program that fits the amorphous and crystalline components with pseudo voigt line profile described elsewhere [3]. Using the width of the (110) peak in Scherer's formula average grain diameter D was obtained. Lattice parameter a is calculated using Nelson-Taylor- Sinclair correction in order to take into account the peak shift due to sample offset. Hysteresis loops of as-cast and isothermally annealed specimens were carried out by the conventional induction hysteresis loop tracer at 50 Hz with a maximum applied field: i) ±3000 A/m to obtain saturated magnetization (Ms) values, and ii) ±1000 A/m to get coercivity (Hc).
Results and Discussion
Figure 1 depicts the XRD patterns of isothermally annealed samples annealed at 450°C for different time, exhibiting structural evolution with time. Perusal of Figure 1 clearly shows that the as cast sample consists of overlapping of two peaks: one corresponds to amorphous matrix (broad amorphous hump), and the second one is the precipitated crystalline phased (sharp peak). As we increase the annealing time, broad hump gradually disappears and crystalline peaks start to appear with prominence suggesting the higher volume fraction of the crystalline phase. The primary crystallization stage creates bcc Fe nanocrystals, dispersed in residual amorphous matrix [7]. The relative intensities match well with the JCPDS database [8]. Choosing different annealing time would lead to various level of structural relaxation, and as a sequence specimen will become more ordered at higher annealing time. Also the presence of Phosphorus forms high density primary nucleation sites and facilitates the precipitation of Fe-Co type crystalline phase [9]. For the duration of annealing at time up to 60 min, no additional crystalline phase develops, as the peak remains at the same 20 position. Lattice parameter for the studied as-cast sample is 0.2871 nm, which increases slightly (0.2877 nm) after 15 min. annealing, is ascribable to the presence of Nb (ionic radius of Fe = 156 pm and Nb = 198 pm) in the crystalline phase. Obtained lattice parameter clearly shows that Co is nearly absent in the precipitating bcc Fe. Grain diameter, and volume fraction of nanograins is calculated from XRD data. Figure 2 shows the evolution of grain size (D) and volume fraction (Vx) of nano grains as a function of annealing temperature, at different annealing time. It can be interpreted that after first isothermal treatment, due to the diffusion of Nb, grain diameter and volume fraction of nanograins increases respectively from 11 nm and 12% (observed for as quenched samples), and saturates ~ 24 nm, 74% for all the samples annealed for higher annealing times (within error bar practically remains same), as was also observed earlier [10]. Effect of isothermal annealing on hysteresis loops of the studied alloy, annealed for 15 min, 30 min, 45 min and 60 min, are represented in Figure 3, and its inset shows inflated coercivity. Magnetic parameters obtained by magnetic measurements are represented in table (inset of Figure 4). After annealing, marginal change were observed in the value of remanance (Br) and the squareness ratio (Br/ Bs) as also reflected in B-H loops. For all annealed specimens, where crystallization process occurs, Hc shows considerable increase (shown in the inset of Figure 4), ascribable to increase in anisotropy due to grain growth [2,11,12]. As here Hc does not follow Random Anisotropy Model (as Hc increases, instead of its reduction), it can be accounted to domain wall pinning on the bcc-Fe nanograins due to local demagnetizing field. Further, increase of H after first heat treatment can be associated to c the stress-relaxation, whereas further increase of H for the c samples annealed for 60 min. is ascribable to domain wall pinning by growing nanostructured grains. Figure 4 depicts the Hc dependence of VxD, while straight line shows linear fit to the data. Perusal of Figure 4 clearly follows the domain wall pinning theory described by Bertotti et al. [11]. According to two dimensional wall bowing [12], an explanation was provided on linear relationship between Hc and VxD (H[aViD) [11], as is also observed in the present work.
Figure 5 depicts the annealing time dependence of lattice parameter, whereas inset shows annealing time dependence of Bs. Perusal of Figure 5 (inset) shows that, as we move on from as-cast to annealed samples (for different times), a noticeable deterioration in Ms was observed. It should be noted that in sample annealed at 450°C/15 min, an increase of lattice parameter with concurrent decrease of Ms is observed (see Figure 5 and its Inset). This observation indicates the presence of non-magnetic element 'Nb' (which has higher ionic radius than Fe) in the first near-neighbour shell of Fe present in the growing bcc-Fe, thus reducing the magnetic moment of Fe, reflected in the reduction of Ms. For higher annealing times, lowering of lattice parameter results in corresponding increase of Ms, indicating the presence of different Nb atoms in the growing bcc Fe-nano grains.
Conclusion
X-ray diffraction, magnetic measurements were used to monitor the time evolution of structural and magnetic properties of Fe67.6Co16.9Nb5B8.5P2 alloy, isothermally annealed at 450°C for different times between 15 - 60 minutes. Isothermal treatment shows:
i) Formation of bcc Fe nanosrystalline phase with lattice parameter ranging between 0.2871-0.2877 nm.
ii) That with increasing annealing time both D and Vx are saturated.
iii) Presence of Nb in the growing nanocrystalline phase (grain diameter ranging between 11-25 nm), leading to concurrent increase of lattice parameter with decrease of saturation magnetization.
iv) Coercivity does not follow Random Anisotropy Model (as Hc increases, instead of its reduction), due to pinning of domain wall in growing bcc-Fe nanograins over 20 nm.
v) Linear dependence of Hc with VxD further clarifies the applicability of domain wall pinning theory described by Bertotti [11] (instead of Random Anisotropy Model) in the studied specimens.
Acknowledgement
SNK is grateful for support in the framework of India- Portugal project No. IND/Portugal/P-07/2013. KG acknowledges the financial support in the framework of CSR-IC-ICBL-25/CRS- 1222014-15/1218 project.
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