Three-dimensional atom probe analysis and magnetic properties of Fe85Cu1Si2B8P4 melt spun ribbons

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Highlights

  • We studied magnetic properties and microstructure of Fe85Cu1Si2B8P4 ribbons.

  • In as-spun sample, nano clusters of α-Fe were detected within amorphous matrix.

  • B and P partitioned in amorphous phase that led to the grain growth hindrance.

  • Increasing the Ta led to the rise of B and contents in remaining amorphous phase.

  • The highest BS and smallest Hc were obtained for the samples annealed above 445 °C.

Abstract

The effect of phosphorous on the microstructure and magnetic properties of as-spun and flash annealed (389–535 °C for 7 s) Fe85Cu1Si2B8P4 melt spun ribbons were investigated by three-dimensional atom probe (3DAP) and high resolution transmission electron microscopy (HRTEM) techniques. The formation of quasi-amorphous α-Fe clusters of 3–5 nm size in an amorphous matrix were detected by HRTEM, despite the high quenching rate applied by high wheel speed used. Flash annealing of the as-spun ribbons gave rise to the formation of nanocrystalline α-Fe (Si) phase in amorphous matrix containing Fe, Si, B and P elements as detected by 3DAP. Comparing 3DAP analysis of the samples annealed at 445 °C and 535 °C revealed that the concentration of P and B in amorphous matrix were increased for the latter. Further, it was shown that P hardly solidified into nanocrystalline phase and partitioned in amorphous phase alongside B atoms leading to the further stabilization of amorphous matrix as confirmed by 3DAP analysis. The highest magnitude of saturation magnetic induction (Bs~1.85 T) and the lowest coercive field (~10–20 A/m) were obtained for the samples annealed above 445 °C, for which noticeable reduction of saturation magnetostriction (λs) were also detected.

Introduction

Nanocrystalline Fe-based soft magnetic alloys obtained by crystallizing melt-spun amorphous ribbons have attracted great attention due to their excellent soft magnetic properties such as low coercivity (Hc), high permeability (µ) and very low saturation magnetostriction (λS) [1], [2], [3], [4], [5], [6], [7], [8]. Considerable attempts have been made to improve the magnetic properties of this group of soft magnetic materials to make them competitive with Si steels. This has led to the development of new alloys with the aim to obtain higher saturation magnetic induction (Bs) as well as low core loss giving rise to the increase of efficiency of electrical machines and higher energy saving. The Fe-based nanocrystalline alloys normally contains rather large amounts of glass making elements to attain an amorphous matrix in which nanocrystalline phase/s could be crystallized by appropriate subsequent heat treatment. The incorporation of large amounts of such non-magnetic elements in such alloy system causes remarkable decrease in Bs. The highest Bs (1.7 T) already reported for Fe-based nanocrystalline alloys is about 80% of that of Silicon steels [4], [5], [9], [10], [11]. The substitution of cobalt for iron in Fe-based nanocrystalline alloys has led to the increase of Ms (1.8 T) [12]. However, using cobalt in Fe-based soft magnetic materials normally increases both the cost and the magnitude of coercivity [3]. Recently Fe-based soft magnetic alloys containing phosphorous (Fe–Cu–Si–B–P) have been developed [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. This alloy system brought some significant advantages. The high amount of iron in these alloys (>80 at%) resulted in increasing Bs up to 1.9 T which was almost comparable to that of electrical steels [13], [14], [15]. Furthermore, the simultaneous additions of phosphorous and copper led to the decrease of α-Fe grain sizes within the remaining ferromagnetic amorphous phase. Despite the valuable works which have been done to introduce the superior magnetic properties of these alloy systems [13], [17], [23], [24] alongside comprehensive TEM studies [25], [26], [27], unfortunately, there are not much detailed studies on the microstructural analysis of Fe–Cu–Si–B–P soft magnetic alloys using 3DAP. Recently authors presented the detail microstructural analysis of the Fe84.3Si4B8P3Cu0.7 melt spun ribbons and discussed on the effect of phosphorous on magnetic properties of the flash annealed ribbons based on 3DAP analysis [28]. 3DAP and HRTEM analyses of Fe84.3Cu0.7Si4B8P3 revealed that boron and phosphorous were rejected from α-Fe (Si) phase and enriched in the residual amorphous phase resulting in further stabilization of amorphous matrix leading to improve the magnetic properties of the ribbons [28]. In the work presented here, attempt is made to study the as-spun and annealed nanocrystalline ribbons of Fe85Si2B8P4Cu1 alloy using HRTEM and 3DAP with the aim to understand the phase evolution of the nanocrystalline phase/s of the as-spun and rapidly annealed ribbons. Further, structural/microstructural–magnetic properties correlations are also discussed.

Section snippets

Material and methods

Fe85Si2B8P4Cu1 ingot was prepared by induction melting the mixture of Fe (>99.99 wt%), Cu (>99.99 wt%), Si (>99.99 wt%) and master alloys of Fe3P (>99.9 wt%) and Fe–B (>99.9 wt%) in an argon atmosphere. Amorphous ribbons of Fe84.3Si4B8P3Cu0.7 composition (hereafter referred as P4 as-spun alloy) with the thickness of ~18 μm and ~10 mm width were prepared by rapid quenching from the melt via melt spinning at 40 m/s wheel speed. Thermal analysis of as-spun ribbons was performed by differential scanning

Structural/microstructural analysis

Fig. 1 shows the XRD patterns of the as-spun and annealed samples. As can be realized from these patterns, only one broad peak around 2θ=45° could be noticed for as-spun sample indicating that this sample was amorphous, within the detection accuracy of XRD technique. This was also confirmed by our TEM results.

Fig. 2(a) demonstrates TEM micrograph of the as-spun sample along with the respective selected area electron diffraction (SAED) pattern. As can be realized from this figure, the classic

Conclusions

In this work, microstructural and magnetic properties study of as-spun and flash annealed ribbons of Fe85Si2B8P4Cu1 alloys were carried out. In this respect, the following conclusions were obtained:

  • 1.

    For the as-spun sample, the existence of small clusters (~3–5 nm) of α-Fe phase could be detected within amorphous matrix based on the HRTEM analysis suggesting the existence of quasi-amorphous structure, despite the high speed wheel used during rapid quenching.

  • 2.

    In the present alloy system studied

Acknowledgments

S.J. acknowledges National Institute for Materials Science (NIMS), Japan for the provision of the NIMS Junior Research Assistantship for her Ph.D. research.

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