Surface oxidation of the Fe based amorphous ribbon annealed at temperatures below the glass transition temperature

Surface oxidation of the Fe based amorphous ribbon annealed at temperatures below the glass transition temperature

Journal of Alloys and Compounds 618 (2015) 269–279 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 618 (2015) 269–279

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Surface oxidation of the Fe based amorphous ribbon annealed at temperatures below the glass transition temperature H.M.M.N. Hennayaka, Ho Seong Lee, Seonghoon Yi ⇑ Materials Science and Metallurgical Engineering, Kyungpook National University, 80 Daehakro, Bukgu, Daegu 702-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 2 July 2014 Received in revised form 15 August 2014 Accepted 18 August 2014 Available online 27 August 2014 Keywords: Amorphous alloy Oxidation Melt spinning

a b s t r a c t The Fe based amorphous ribbons with the nominal compositions Fe84.2 xC11.0SixB3.9P0.9 (x = 4.5 and 13.5 at.%) were oxidized at low temperatures of 300 and 400 °C (i.e., well below the crystallization temperature) for 24 h in the air atmosphere. Significantly different oxidation kinetics between surfaces of the wheel side and the air side were observed, respectively. A continuous amorphous Si oxide layer with a few nanometers in thickness was formed on the air side surface of as-spun ribbon and effectively protected the ribbon from oxidation during annealing. However, discontinuity of the amorphous Si oxide layer was observed near casting defects on the wheel side surface. Moreover, due to low cooling rate near the casting defect regions, nanocrystals can be formed resulting in enhanced oxidation kinetics of the wheel side surface. As the annealing temperature is increased to 400 °C, the air side surface exhibited deteriorated oxidation resistance attributed to partial nanocrystallization of a-Fe. Nanocrystals of a-Fe formed on the surface acted as effective nucleation sites for the formation of iron oxides. Increasing annealing temperature leads to the increased thickness of both amorphous and crystalline oxide layers. However, effects of surface casting defects on the oxidation resistance can be significantly reduced by increasing the Si content. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The atomic packing of the amorphous alloys does not exhibit long range ordering which is the main reason for its unique physical and chemical properties compared to the conventional crystalline alloys [1]. Among various amorphous alloys, Fe based amorphous alloys play an important role in the functional applications due to their excellent properties such as high strength, good soft magnetism and homogeneous corrosion on the surface as well as the economic advantages [2–4]. Due to the chemical homogeneity and the absence of the grain boundaries it is predicted that the oxidation of the amorphous alloys should be more uniform compared to that of crystalline. Numerous data regarding the mechanical and magnetic behaviors of Fe-based amorphous alloys based on the atomic structure are reported but, oxidation behavior related literature is very rare. Understanding the oxidation behavior of the Fe based amorphous alloys is useful upon utilizing them in the industrial environment [5,6]. Oxidation behavior of CuZr metallic glassy alloys have been studied thoroughly for its oxidation resistance in the low tempera-

⇑ Corresponding author. Tel.: +82 53 950 5561. E-mail address: [email protected] (S. Yi). http://dx.doi.org/10.1016/j.jallcom.2014.08.160 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

tures [7–10]. Mostly, the oxidation behavior of the Fe based metallic glasses have been studied under long term air exposure at high temperature region such as 600–700 °C [5,6,11]. It is reported that even oxidation at elevated temperatures for long times such as 48 h, the thickness of the oxide layer is extremely small which needs high technology for characterization of the oxide layer. However, in order to enhance the soft magnetic properties such as low coreloss, the Fe based amorphous ribbons should be annealed at temperatures below its crystallization temperature which relieves the residual stresses resulting from the casting process [8]. Therefore, studies concentrated on the oxidation behavior below or in the vicinity of the crystallization temperature is highly necessary. The oxidation behavior of the wheel side of the amorphous ribbons, which is in direct contact with the Cu wheel during the casting process using the melt spinner, is predicted to be quite different from that of the air side. So the scope of this study is to investigate the oxidation behavior of the Fe based amorphous ribbons according to the Si content, when annealed at temperatures below its crystallization temperature and to investigate the effect of surface defects upon the oxidation properties.

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Table 1 Chemical compositions of the pig iron and industrial grade Fe–P alloy. Unit (wt.%).

Pig irona Fe–Pa

Fe

C

Si

P

94.351 73.161

3.984 –

1.559 0.009

0.106 26.83

a Both the pig iron and Fe–P might contain extremely small amount of impurities such as Mn, S and Ti, with a total concentration below 0.6 wt.%. Therefore, both of the designed alloy may contain Mn < 0.24 at.%, S < 0.01 at.% Ti < 0.03 at.%. The total concentration of the impurities in the designed alloys is below 0.28 at.%.

Fig. 1. DSC analysis of the amorphous ribbons with various Si content analyzed by SDT with a heating rate of 10 C/min and Ar flow rate 100 mL.

2. Experimental details In this experiment, the oxidation behavior of melt spun amorphous ribbons with respect to the Si content was investigated at the low temperature region, upon heat treatment at air atmosphere. The alloy ingots with a composition of

Fig. 3. Asymmetric XRD results of the air and wheel sides of the amorphous ribbons with 4.5 and 13.5 at.% Si (a) as-quenched and (b) 300 °C annealed for 24 h.

Fig. 2. OM and FE-SEM images of the (a) air side, (b) wheel side of as-quenched ribbon with 4.5 at.% Si, and (c) air side and (d) wheel side of the as-quenched ribbon containing 13.5 at.% Si respectively (inset are the FE-SEM images of the same ribbons).

H.M.M.N. Hennayaka et al. / Journal of Alloys and Compounds 618 (2015) 269–279 Fe84.2 xC11.0SixB3.9P0.9 (x = 4.5, 13.5 at.%) were prepared by arc melting using pig iron and industrial grade Fe–P alloy of known composition (refer to Table 1), and pure commercial grade elements; Fe, B, Si (purity >99.5%). The amorphous ribbons were synthesized with melt spinning technique using a Cu wheel rotating at a 38 ms 1 linear velocity. The heat treatment temperature was fixed at 300 and 400 °C which are near the crystallization temperature according to the differential scanning calorimeter (DSC) results analyzed by simultaneous TGA/DSC (SDT) (Q600, TA Instruments, United States). Annealing of the amorphous ribbons was carried out for 24 h in the preheated tube furnace in air atmosphere. The surface morphology of the oxidized ribbons were investigated through optical microscopy (OM) (EPIPHOT 200, Nikon, Japan) and field emission scanning electron microscopy (FE-SEM) (S-4300, Hitachi, Japan) with an electron beam energy fixed at 15 keV and X-ray diffraction (XRD) analysis (D/MAX-2500, Rigaku, Japan) with Cu ka radiation (k = 1.54 Å). The structural analysis of the oxide layer formed on the ribbons was carried out using transmission electron microscopy (TEM) (Titan G2 ChemiSTEM Cs probe, FEI, United States) analysis equipped with energy dispersive X-ray spectroscopy (EDS) (Super-X). The cross section samples containing the oxide layer and the interface between the oxide layer and the amorphous matrix for TEM analysis were prepared using the focused ion beam milling (FIB) equipment (Versa 3D LoVac, FEI, United States) and a Pt layer

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was deposited using electron beam and thereafter using ion beam, prior to milling process with the purpose of protecting the oxide layer and to minimize the damage during the ion milling process.

3. Results and discussion 3.1. Comparison between the as-quenched and the 300 °C annealed ribbons 3.1.1. Thermal analysis In order to determine the onset crystallization temperature (Tx) of the amorphous ribbons of both compositions, the DSC analysis was performed at 100 mL/min Ar flow rate and 10 °C/min heating rate. The results are shown in Fig. 1. The crystallization temperature of the ribbon containing 4.5 at.% Si is measured to be 477 °C while the ribbon containing 13.5 at.% Si exhibits a higher crystallization

Fig. 4. (a) BF, (b) HR-TEM, (c) HAADF and (d–f) EDS mapping images of the air side of the as-quenched ribbon with 4.5 at.% Si.

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temperature of 523 °C. However, it should be noted that this crystallization temperature is only valid for the above conditions and could be slightly changed when the heating rate is varied as the characteristic temperatures of metallic glasses such as glass transition temperature, onset crystallization temperature and crystallization peak temperature are strongly dependent on the heating rate [12]. The annealing temperatures were decided as 300 and 400 °C, which are below the crystallization temperatures of the both compositions. 3.1.2. Surface morphology analysis The surface morphology of the as-quenched ribbons are shown in Fig. 2 which are observed by the OM. The air sides of the ribbons displayed in Fig. 2(a) and (c), irrespective of the composition exhibit a defect free, uniform surface. In contrast, the wheel sides of the ribbons shown in Fig. 2(b) and (d) exhibit rough surfaces with

defects and inhomogeneity which might accelerate the oxidation. As the wheel side is in direct contact with the rotating Cu wheel upon melt spinning, these surface defects are unavoidable. Due to the defects such as air pockets present in this wheel side, it is predicted that the cooling rate could be slow in the vicinity of them. As a result, it is assumed that the nanocrystals could be occurred sparsely on the wheel side. It is very interesting to study the oxide layer formation mechanism on the smooth surfaces (air side) and the surfaces with defects (wheel side) upon annealing. The OM and FE-SEM images for the annealed ribbons are attached in Supplementary data. It should be noted that the oxide layer formation is enhanced as the annealing temperature is increased. 3.1.3. Structural analysis From the XRD results shown in Fig. 3(a), it is confirmed that the as-quenched ribbons exhibit amorphous structure irrespective of

Fig. 5. (a) BF, (b) HR-TEM, (c) HAADF and (d–f) EDS mapping images of the wheel side of the as-quenched ribbon with 4.5 at.% Si.

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Si composition. The nanocrystals that have formed on the wheel side as described in Section 3.1.2, are predicted to be too small to be detected through XRD analysis. According to the XRD results for the 300 °C annealed ribbons shown in Fig. 3(b), it is worth noting that the (1 1 0) peak of a-Fe is visible on the wheel side of 4.5 at.% Si containing ribbon. It is possible to assume that the nanocrystals that are formed on the wheel side during the production of the amorphous ribbons are subjected to substantial grain growth due to the annealing process. Hence, the a-Fe peak is visible on the XRD result of the wheel side of the ribbon with 4.5 at.% Si. Existence of the crystalline peak may be ascribed by the change in alloy composition locally occurred due to annealing process [13]. No evidence of a-Fe is observed on the air side of the same ribbon as well as the both sides of the 13.5 at.% Si containing ribbons annealed at 300 °C. It is concluded that the Si addition has stabilized the amorphous phase and delays the crystallization which is in accordance with the DSC data shown in Fig. 1.

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3.1.4. TEM analysis The oxide layers formed on the amorphous ribbons were hard to characterize only using the XRD results and the FE-SEM images, as the oxide layers formed were extremely thin. Therefore, the TEM analysis was carried out in order to understand the oxidation mechanism. Prior to the annealing experiment, the as-quenched ribbon with 4.5 at.% Si was characterized using the TEM and the images of the air side and the wheel side are shown in Figs. 4 and 5 respectively. According to the bright field (BF) TEM image shown in Fig. 4(a) and high resolution (HR) TEM image in Fig. 4(b), it is investigated that the air side of the as-quenched ribbon containing 4.5 at.% Si exhibit fully amorphous structure. A very thin oxide layer of around 2 nm thickness is observed and it is predicted to be formed during the ribbon production. Fig. 4(c) represents the high angle annular dark field (HAADF) image of the corresponding sample. The EDS mapping results for Si, O and Fe are shown in

Fig. 6. (a) BF, (b) HR-TEM, (c) HAADF and (d–f) EDS mapping images of the air side of the ribbon with 4.5 at.% Si annealed at 300 °C for 24 h.

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Fig. 7. (a) BF, (b) DF (c) HR-TEM (d) HAADF and (e and f) EDS mapping images of the wheel side of the ribbon with 4.5 at.% Si annealed at 300 °C for 24 h.

Fig. 4(d)–(f) respectively. No evidence regarding segregation of elements during the production of the ribbons is observed. It should be taken into account that the thickness of this thin oxide layer is extremely small which makes it hard to obtain precise information such as chemical composition regarding this oxide layer.

Table 2 The Gibbs free energies of formation (DGf ) of possible oxides formed on Fe84.2 C11.0SixB3.9P0.9 amorphous alloy. Unit (kJ/mol O2). 300 °C Fe2O3 Fe3O4 B2O3 P2O5 SiO2

445.7 461.6 746.6 490.5 806.3

400 °C 428.4 455.6 729.0 472.1 788.2

x-

Fig. 5 represents the TEM images for the wheel side of the asquenched ribbon of same composition. It is interesting to see that nanocrystallites exist in the interface between the amorphous matrix and the thin oxide layer. The nanocrystallites which exhibit distinct lattice arrangement of around 5 nm are highlighted by blue1 circles in the HR-TEM image in Fig. 5(b). The formation of these nanocrystallites is discussed in preceding sections. There was no evidence of element segregation according to the EDS images. Fig. 6(a)–(c) are BF-TEM image, HR-TEM image and HAADF image of the air side of the 300 °C annealed ribbon containing 4.5 at.% Si respectively. According to these images, it can be concluded that the oxide layer which formed uniformly on the air side surface is amorphous with a thickness of around 4–5 nm. This could be the same oxide layer which exists on the as-quenched 1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.

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Fig. 8. (a) BF and (b) HR-TEM image of the air side of the ribbon with 13.5 at.% Si annealed at 300 °C for 24 h.

Fig. 9. (a) BF and (b) HR-TEM image of the wheel side of the ribbon with 13.5 at.% Si annealed at 300 °C for 24 h.

ribbon thus increased in thickness due to the enhanced diffusion rates during annealing. According to the BF image, this oxide layer exhibits a very dense, continuous and homogeneous nature when compared to the as-quenched ribbon. No evidence of the crystalline oxide layer or oxide particles is visible on this ribbon. Fig. 6(d)–(f) shows the EDS mapping results of Si, Fe and O component respectively. According to these results it is evident that the Fe distribution in the oxide layer is relatively low and the Si distribution is relatively high than that in the amorphous matrix.

Fig. 10. Asymmetric XRD results of the air and wheel sides of the amorphous ribbon with 4.5 at.% Si annealed at 400 °C for 24 h.

In literature, it has been reported that in the Fe based amorphous alloys containing smaller atoms like B and Si, which diffuse faster than the bigger atoms, are dominant in the thin oxide layer [14– 17]. Based on the above references, it can be assumed that the thin oxide layer formed on the air side of the Si 4.5 at.% containing amorphous ribbon in the present study could be boron or silicon oxide. According to the EDS mapping results, it was confirmed that the thin oxide layer consists of Si. The BF, dark field (DF) and HR-TEM images of the wheel side of the 300 °C annealed ribbon with 4.5 at.% Si are shown in Fig. 7. It is obvious that the oxide layer is formed in two layers according to the BF image which is significantly different from the air side. The inner oxide layer is confirmed to be amorphous and exhibits a thickness varying from 4 nm to 6 nm which is similar to the air side of the 300 °C annealed ribbons of same composition. Furthermore, it is possible to see that this amorphous oxide layer contains discontinuities, which is in contrast to the uniform oxide layer observed for the air side. The difference in cooling rate attributed to the relatively high surface roughness on the wheel side seems to cause this non-uniform nature of the thin oxide layer. These discontinuities give way to the diffusion of Fe and O and results in the formation of iron oxide layer. Also, it is worth noting that the substrate now exhibit crystallites embedded in the amorphous matrix. It is predicted that these crystallites are originated from the nanocrystals that exist on the wheel side. During long term annealing, these nanocrystals may subject to grain growth. These crystallites may provide effective nucleation sites for the formation of iron oxides during further annealing. The crystalline oxide layer formed above the amorphous oxide is non-uniform and the thickness is varied from 20 to 50 nm. According to the EDS mapping results

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shown in Fig. 7(e) and (f) for Si, Fe and O, it can be concluded that the inner oxide layer is composed of Si and O while the outer crystalline layer is composed of Fe and O. The Gibbs free energies for the formation of possible oxides on the alloy considered on this study are given in Table 2 [18]. According to these values it is expected that SiO2, B2O3 and P2O5 which have more negative values of Gibbs free energy of formation could be formed preferentially on the surface. The formation of SiO2 in the inner oxide layer and the Fe2O3 in the outer layer can be described by the above Gibbs free energy values. It should be noted that, due to the low activity of P corresponding to the low concentration of 0.9 at.%, on the substrate when compared to Fe and Si as well as the preferential consumption of oxygen by the Fe or Si, P2O5 is not observed in the oxide layer. Figs. 8 and 9 are the BF and HR-TEM images of both air side and wheel side of the 300 °C annealed ribbons containing 13.5 at.% Si respectively. It is possible to see that there is no evidence of crystalline oxide layer on both air and wheel sides of the amorphous

ribbon. Only the thin oxide layer which is under 3 nm of thickness is visible. With the increased Si composition, the activity of the Fe on the surface which can react with oxygen is predicted to be decreased with respect to the ribbons containing low Si content, which prevents the formation of iron oxides even for the defect containing wheel side. It leads to the conclusion that amorphous ribbons with high Si content is highly oxidation resistant when compared to that of low Si content. This excellent oxidation resistance property of 13.5 at.% Si containing amorphous ribbons could be of immense use upon industrial applications.

3.2. Comparison between the 300 and the 400 °C annealed ribbons In Section 3.1, it is discussed the effect of 300 °C annealing for 24 h on the oxide layer formation of the amorphous ribbons with 4.5 at.% and 13.5 at.% Si. In order to investigate the effect of annealing temperature, the annealing is carried out at 400 °C for

Fig. 11. (a) BF, (b) HR-TEM image of the oxide layer right above the substrate (c) HAADF and (d–f) EDS mapping images of the air side of the ribbon with 4.5 at.% Si annealed at 400 °C for 24 h.

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the 4.5 at.% Si containing ribbon which exhibit evident oxidation and the results are discussed in this section. 3.2.1. Structural analysis Similar to the 300 °C annealed 4.5 at.% Si ribbon, oxidation at 400 °C shows crystalline peaks of a-Fe according to the XRD data in Fig. 10. The ribbon annealed at 400 °C exhibit the distinct crystalline peaks corresponding to Fe2O3 (JCPDS #: 87-1166) for both air and wheel sides in comparison with the 300 °C annealed ribbon. Furthermore, it is noticed that the Fe2O3 peak intensities are relatively small for the air side when compared with that of the wheel side for the ribbon containing 4.5 at.% Si, confirming the high oxidation resistance of the air side compared to the latter. 3.2.2. TEM analysis To further understand the oxidation behavior, the oxidation behavior at 400 °C was investigated through the TEM images as

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shown in Figs. 11 and 12 for the air and wheel sides respectively. According to the HR-TEM image for the air side in Fig. 11(b), the amorphous oxide layer exhibits a thickness around 10 nm. Furthermore, unlike the air side of the 300 °C heat treated ribbons, sparsely distributed Fe oxide is found on the air side. The reason could be the enhanced diffusion rates of Fe, Si and O at this high temperature compared to 300 °C, as well as the crystallization of the amorphous substrate. It should be noted that these crystallites offer effective nucleation sites for the formation of iron oxides as for the case of wheel side of the 300 °C annealed 4.5 at.% Si containing ribbon. The island formation of Fe oxide, instead of a layer when compared to that of the wheel side could be attributed to the decreased number of nucleation sites present on the smooth air side. The interface of the oxide layer and the substrate of the wheel side is shown in Fig. 12. According to the BF-TEM image shown in Fig. 12(a) the thickness of the outer oxide layer seems to vary

Fig. 12. (a) BF, (b) HR-TEM image of the oxide layer right above the substrate (c) HAADF and (d–f) EDS mapping images of the wheel side of the ribbon with 4.5 at.% Si annealed at 400 °C for 24 h.

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Fig. 13. Schematic diagram of the morphology and the oxide layer formation of (a) as-quenched (b) 300 °C annealed and (c) 400 °C annealed ribbons containing 4.5 at.% Si.

in the range of 270–400 nm. This significant increase in thickness confirms the enhanced oxidation of the Fe as the annealing temperature increases. It is very interesting to see the inner oxide layer which exhibits a fully amorphous structure at 300 °C, reveals to contain nanocrystallites when the annealing temperature is increased to 400 °C, as displayed in the HR-TEM image in Fig. 12(b). The thickness of the inner oxide layer is around 30–50 nm which is around 10 times of the amorphous oxide layer when annealed at 300 °C. The formation of the nanocrystallites on the amorphous oxide layer could be attributed to the high annealing temperature. The overall oxidation mechanism for 4.5 at.% Si containing ribbon is summarized in a schematic diagram shown below in Fig. 13. 4. Conclusions The oxidation behavior at 300 and 400 °C of the amorphous Fe based ribbons are investigated according to the Si content. The oxidation of homogeneous air side and the defect containing wheel side shows significant difference and is explained as a result of the effect of nucleation sites on the substrate. In the case of 4.5 at.% Si containing ribbon, air side of the asquenched ribbons exhibit fully amorphous matrix. As a result, air side of the ribbons with 4.5 at.% Si shows excellent oxidation resistance due to the small number of nucleation sites, when subjected to annealing at 300 °C for 24 h. On the other hand the wheel side of the ribbon containing 4.5 at.% Si exhibits deteriorated oxidation resistance and results in two layer oxide formation which could be attributed to the large number of nucleation sites existing in the matrix of the wheel side which are predicted to be formed during the production of the ribbon. Increasing annealing temperature to 400 °C of the 4.5 at.% Si containing ribbon, enhances the diffusion rates of the constituent ions which consequently increases the thickness of the oxide

layers. The amorphous matrix is being partially crystallized when annealed at 400 °C resulting provoked iron oxide formation on the air side as well, but due to the presence of less number of nucleation sites compared to the wheel side, the crystalline oxide layer has only formed in the shape of islands rather than a continuous layer. In the case of the wheel side, as the annealing temperature is increased to 400 °C, the thickness of the amorphous SiO2 layer which forms right above the substrate, increases consequently and reaches a critical thickness. No further amorphous oxides is formed thereafter and nanocrystallites begins to form within the amorphous oxide layer. It is worth noting that the oxidation resistance was substantially enhanced by increasing the Si content to 13.5 at.%. This could be attributed to the decreased activity of the Fe. The presence of surface defects on wheel side could not provoke the oxide formation for this composition when annealed at 300 °C which could be very promising upon utilizing in industrial applications. Acknowledgement This research was financially supported by RIST and POSCO, Pohang, Korea. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jallcom.2014.08. 160. References [1] H.W. Sheng, W.K. Luo, F.M. Almqir, J.M. Bai, E. Ma, Nature 439 (2006) 419–425. [2] A. Inoue, Acta Mater. 48 (2000) 279–306. [3] R.W. Cahn, H.H. Libermann, Rapidly Solidified Alloys, Marcel Dekker, Newyork, 1993.

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