Magnetic and mechanical properties of FeSi alloys with high Si content

LU Feng-shuang(卢风双), QIAO Liang(乔 梁), BI Xiao-fang(毕晓昉)

School of Materials Science and Engineering,
Beijing University of Aeronautics and Astronautics, Beijing 100083, China

Received 20 April 2006; accepted 30 June 2006

Abstract: The chemical vapor(CVD) deposition-diffusion method was applied to prepare FeSi alloys with high silicon content up to 6.5%. In spite of various deposition and post-annealing, the sample remains α-Fe bcc structure. The cross section of the composition was analyzed to evaluate the Si content and distribution before and after annealing. The results show that the soft magnetic properties are improved by increasing the silicon content. For the samples containing about 6.5% Si, the coercivity decreases to 60 from 237.3 A/m of the original. It is also obtained that, in addition to the Si content, Si distribution has a large influence on the core loss due to the effect of resistivity. The micro-hardnesses were also evaluated along the cross-section after various annealings.

Key words: FeSi alloy; chemical vapor deposition; diffusion; magnetic properties; micro-hardness

1 Introduction

FeSi alloys attract much attention due to their improved excellent soft magnetic characterizations with increasing Si content up to 6.5% and consequently their applications in magnetic core and transformers. However, it is well known that FeSi alloys become extremely brittle due to the formation of DO₃ order structure. Therefore, conventional FeSi sheets contain Si content of about 3% because of difficulty in fabrication of the FeSi alloys with higher Si content. As mentioned above, common electric steel with the silicon content less than 3.5% is unsatisfactory compared to FeSi alloy with 6.5%Si, because the core loss and noise are not up to the demands. In order to meet the demands of low core loss and low noise, adding silicon content up to 6.5% is feasible [1]. It is well known that soft magnetic properties are improved by increasing Si content of the alloys, and particularly, the magnetostriction reaches zero when the content of Si increases to 6.5% [2-4]. But it is very difficult to roll the high silicon steel (above 3.5%) because of its brittleness [5]. And the rolling process is generally complicated and high cost. Many researchers investigated various methods including the warm-rolling, fast-setting, spray-up and chemical vapor deposition (CVD) to obtain Fe-6.5%Si alloys [6-8]. Therefore, chemical vapor deposition (CVD) 
process is a effective way in the fabrication of the Fe-6.5%Si [9-11]. On the other hand, microstructure has a large influence on the magnetic and mechanical properties of FeSi alloys. In this work, therefore, a combination of CVD and diffusion treatment by post-annealing was applied to prepare the FeSi alloys with high silicon content. In addition to the effect of Si content, the Si distribution and microstructure on the magnetic and mechanical properties were investigated.

A commercial non-oriented FeSi sheet 0.37% Si was used in this work. The sample is 0.5 mm in thickness, which was cut into the shape of EI cores and polished by the sand paper for depositions. At first, the samples were sealed in a pot which was filled with powder (NaF,    Al₂O₃, Si), and then put the pot in  a furnace under a flowing gaseous Ar for chemical vapor deposition. The CVD process was carried out at 1 040 ℃ for 1.5, 2.5, 3.5 and 4.5 h, respectively. Following the deposition, a diffusion treatment by post-annealing was performed in vacuum atmosphere at 1 200 ℃ for 2 h, in an aim of obtaining a uniform distribution of the Si in the samples. Before and after the diffusion treatment, the silicon content along the cross-section of the sheet was analyzed using scanning electron microscopy (SEM) along with energy-dispersive characterized by SEM and X-ray diffraction. The magnetic properties of these samples with the shape of EI cores were measured with the automatic ac B—H curve tracer. The mechanical properties were measured with the micro-hardness tester along the cross-section.

Fig.1 shows XRD patterns of alloys before and after CVD depositions for different times. It can be seen from Fig.1 that all samples remain α-Fe bcc structure after the deposition. The Fe (110) peak exhibits the largest intensity for the original, and decreases to a large extent after the deposition. For the sample after deposition for 4.5 h, all the peaks show the smallest in intensity. This implies that more Si has been introduced into the specimen from its surface. After further post-annealing at 1 200 ℃ for 3 h, the intensity of peaks show an increase trend in spite of different deposition times, as shown in Fig.1 (b). Fig.2 shows microstructures of cross-section of the samples after depositions. It can be observed that there exists two clear boundaries for each sample. When the deposition time is 1.5 h, the boundary is not flat, while they become flat and smooth with the increase of the deposition time. On the other hand, the grain size

Fig.1 XRD patterns before and after deposition for various depositing time (a) and after diffusion by post-annealing (b)

Fig.2 SEM images of cross-section for alloys after CVD deposition for (a) 1.5 h, (b) 2.5 h, (c) 3.5 h and (d) 4.5 h(Arrows in the photos indicate boundaries)

grows bigger with the increase of the deposition time.

In order to investigate the Si distribution on the cross section after the deposition, the compositions were analyzed by EDX. Fig.3 shows Si mapping image and line analysis across the cross section for the sample after deposited for 4.5 h. It can be found that Si content near the surface is much higher than that in the inner. In combination of their microstructures shown above, it can be obtained that the boundaries on the cross section distinguish the areas containing higher Si content due to the deposition from those with the same Si content increasing as the original.

Fig.3 Silicon distribution along cross-section of alloy after CVD deposition for 4.5 h: (a) Si mapping image; (b) Line analysis curves of Si and Fe

The former area is called deposition layer below. It can be further seen that the deposition layer is largest in thickness for the alloy after deposited for 4.5 h.

To investigate the influence of deposition time on the Si content and its distribution in the deposition layer, the compositions were also analyzed. The results are listed in Table 1. The Si contents at three spots for sample in the layer, where a-c indicate areas near surface, in the middle and near the boundary, respectively. Because all the samples have various deposition times, the Si content is reduced from surface to the boundaries. It can be seen from Table 1 that the Si content near surface is 14.71%, which is much higher than the others. In addition, when the deposition time is less than 4.5 h, the deposition layer contains the Si of only less than 2.75% on average. In contrast to this, the average Si content in the deposition reaches up to 6.52% when the deposition time is 4.5 h. It can be obtained that there exists a critical deposition time necessary to acquire a certain amount of Si content in the layer. On the other hand, it is obvious that the Si content is not uniform along the cross-section. In order to solve the problem, an post-annealing at 1 200 ℃ is carried out. Fig.4 shows the Si mapping image and line analysis after a post- annealing following the deposition for 4.5 h. It reveals that the distribution of Si becomes uniform on the cross-section. The improvement of uniformity of the Si distribution is attributed to the diffusion of Si from the surface to the boundary during the post annealing.

Table 1 Si contents at different positions after CVD deposition processes

Fig.5 shows the ac hysteresis curves obtained by the automatic ac B-H curve tracer. It can be seen from Fig.5 that the original sample shows the largest magnetization and the sample after deposition for a longer time has smaller magnetization. The results also indicate that Si has been introduced in the sample. Obviously, on the other hand, the addition of Si to the samples improves its soft magnetic properties to different extents. As displayed in Table 2, it can be seen that the soft magnetic properties are dependent not only on the Si content and on its distribution. The alloy containing 6.5% Si shows a smaller coercivity than those with lower Si content, which is due mainly to the very low magnetostriction. It is also observed that the core loss of the alloy containing lower Si but with a uniform distribution along the cross-section exhibits a little lower than the one con-

Fig.4 Silicon distribution along the cross-section of alloy after annealing at 1 200 ℃ for 2 h following CVD deposition for 4.5 h: (a) Si mapping image; (b) Line analysis curves for Si and Fe

Fig.5 Hysteresis curves of alloys with different deposition time after diffusion process by post-annealing

Table 2 Magnetic properties of alloys before and after different depositions and post-annealings

taining 6.5% but without uniform distribution. The higher core loss is suggested to be due to the lower resistivity in the areas lack of Si. The results indicate that the Si distribution is somewhat significant to the ac magnetic properties.

Fig.6 shows the change of micro-hardness along the cross-section with different treatments. It can be seen from Fig.6 that the micro-hardnesses for the as-deposited are greatly enhanced at the areas near the surface. After diffusion treatment by post-annealing, the hardnesses exhibit constant along the section. In combination with the above results, it can be obtained that the change of the hardnesses is caused by the change of the Si distribution with the post-annealing. In addition, it can be also observed that the hardness of the sample with 3.5 h deposition is increased only a little after the diffusion compared to before. This indicates that a small amount of Si is introduced in the alloy due to a short time of deposition, which is consistent with the above com- position analysis results.

Fig.6 Change curves of micro-hardness along cross-sections for as-deposited alloys and after diffusion treatments: (a) 3.5 h; (b) 4.5 h

The Si content reaching to 6.5% depends greatly on the deposition time when the sample is deposited for 4.5 h. After being subjected to the diffusion treatment by post-annealing at 1 200 ℃ for 3 h, the samples show  uniform Si distribution along their cross-section. The coercivity reduces with the increase of the Si content, and the minimum value of coercivity for the sample with 6.5%Si is 60 A/m. It is also obtained that, in addition to the Si content, the Si distribution has a large influence on the core loss due to its effect of resistivity. The micro- hardnesses are greatly enhanced with the increase of the Si content, and exhibit constant along the cross-section when the Si distribution is uniform.

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(Edited by LI Yan-hong)

Corresponding author: BI Xiao-fang; Tel: +86-10-82315999; E-mail: bixf@buaa.edu.cn