Crystallization behaviour of the Fe60Co10Ni10Zr7B13 metallic glass

Materials Science and Engineering A 449–451 (2007) 552–556

Crystallization behaviour of the Fe60Co10Ni10Zr7B13 metallic glass A. Grabias a,∗ , D. Oleszak b , M. Kopcewicz a , J. Latuch b a

b

Institute of Electronic Materials Technology, W´olczynska Street 133, 01-919 Warsaw, Poland Department of Materials Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland Received 22 August 2005; received in revised form 10 February 2006; accepted 26 February 2006

Abstract The aim of this work was to investigate the influence of annealing on the structure of the ferromagnetic Fe60 Co10 Ni10 Zr7 B13 metallic glass. Crystallization behaviour of the alloy was studied by differential scanning calorimetry, X-ray diffraction and M¨ossbauer spectroscopy measurements. A combination of transmission and conversion electron M¨ossbauer techniques allowed us to compare the crystallization processes occurring in the bulk and in the surface layers of the ribbon samples. The first stage of crystallization corresponding to the partial crystallization of the bcc Fe–Co solid solution was followed by the formation of iron borides and the paramagnetic iron-poor fcc Fe–Ni phase. The surface crystallization started at lower annealing temperature than the bulk one. The bcc Fe–Co and tetragonal Fe2 B phases were observed at the surfaces of the annealed ribbons. © 2006 Elsevier B.V. All rights reserved. Keywords: Amorphous materials; Crystallization; Calorimetry; X-ray diffraction; M¨ossbauer spectroscopy

1. Introduction Multicomponent (Fe,Co,Ni)–(Zr,Hf,Nb)–B amorphous alloys have attracted a lot of attention since they exhibit good soft magnetic properties combined with high glass-forming ability [1–4]. These two features enable researchers to produce soft ferromagnetic materials in a bulk amorphous form. Thermal stability and crystallization behaviour of the iron-based bulk amorphous alloys have been studied widely [5–8]. In the present paper, we investigate the influence of annealing on the structure of the ferromagnetic Fe60 Co10 Ni10 Zr7 B13 metallic glass that was prepared in a ribbon form by the meltspinning technique. This material has been used to obtain the bulk amorphous alloy by mechanical alloying and hot pressing techniques [4]. Thermal stability of the amorphous alloy was studied by differential scanning calorimetry (DSC) that allowed us to determine the glass transition and crystallization temperatures. Thermally induced structural transformations were investigated by X-ray diffraction (XRD) and M¨ossbauer spectroscopy techniques. The latter method is very useful in studying the local environment around Fe atoms allowing the qualitative and quantitative analysis of iron-containing phases. ∗

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Transmission M¨ossbauer measurements allowed us to follow the structural changes in the bulk of the alloy, whereas conversion electron M¨ossbauer spectroscopy (CEMS) was applied to study the surface layers of the samples about 100 nm in depth [9,10]. 2. Experimental details The amorphous Fe60 Co10 Ni10 Zr7 B13 alloy was prepared in the ribbon form by melt-spinning. The samples were 5 mm wide and about 40 ␮m thick. The DSC measurement was performed with the scanning rate of 20 K min−1 . In order to investigate the crystallization behaviour the samples were collected after heating up to 833, 903 and 993 K. Isothermal annealing was performed in vacuum for 1 or 5 h at temperatures ranging from 773 to 1023 K. Structure of the annealed samples was investigated by X-ray diffraction and M¨ossbauer spectroscopy. A standard powder X-ray diffractometer with Cu K␣ radiation was employed. All M¨ossbauer spectra were measured at room temperature using a 57 Co-in-Rh source. A proportional counter for the detection of M¨ossbauer ␥ radiation was used in the measurements performed in transmission geometry. The CEMS measurements were performed by using a gas flow electron counter with He + 6%CH4 gas mixture, in which the samples were placed. Fitting of the M¨ossbauer spectra resulted in the determination of the hyper-

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fine parameters such as: (i) hyperfine field (Hhf ) and isomer shift (δ) for six-line spectral components, (ii) quadrupole splitting (Δ) and isomer shift for quadrupole doublets, and (iii) isomer shift for single-line components, that allowed the identification of the phases. The relative abundance (A) of the iron-containing phases present in the samples was calculated as a ratio of the area of the relevant subspectra to the total spectral area, assuming similar Debye–Waller factors for each phase. Isomer shifts are related to ␣-Fe standard. 3. Results and discussion Thermal stability of the Fe60 Co10 Ni10 Zr7 B13 metallic glass was analysed by the DSC measurement (Fig. 1). The DSC curve shows two main exothermic peaks related to two stages of crystallization. The crystallization onset temperature, Tx1 , determined for the studied alloy is 827 K. The second step of crystallization starts at Tx2 = 968 K. The glass transition temperature evaluated from the DSC curve is 809 K. In order to study the crystallization behaviour of the Fe60 Co10 Ni10 Zr7 B13 alloy the amorphous samples were heated up to the characteristic temperatures determined from the DSC curve. The heating of the ribbon up to 833 K (temperature of the first exothermic peak) caused partial crystallization of the bcc Fe–Co solid solution as revealed by the appearance of the bcc lines in the X-ray diffraction pattern (Fig. 2). The transmission M¨ossbauer spectrum recorded for this sample consists of two sextets: one with Hhf = 35.5 T and δ = 0.00 mm/s, correspondFig. 3. Transmission M¨ossbauer spectra of the samples heated in the calorimeter up to the temperatures indicated.

ing to the bcc Fe–Co phase, and the other one with broadened lines and average hyperfine field of about 20 T related to the remaining amorphous phase (Fig. 3a). The bcc Fe–Co phase can be clearly identified by the hyperfine field parameter that is

Fig. 1. DSC curve measured for the Fe60 Co10 Ni10 Zr7 B13 metallic glass.

Fig. 2. XRD patterns of the samples heated in the calorimeter up to the temperatures indicated and of the sample annealed at 1023 K for 5 h.

Fig. 4. Transmission M¨ossbauer spectra of the samples annealed for 5 h.

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significantly larger than that of pure bcc Fe (Hhf = 32.9 T) [11]. The broadened lines of the Fe–Co sextet indicate the formation of the iron-based solid solution. The relative abundance of the Fe–Co phase is 49%. The heating of the amorphous ribbon up to 903 K induced further crystallization of the bcc Fe–Co phase (Fig. 2). The relative abundance of this phase determined from the M¨ossbauer spectrum is 68%. Beside the dominating Fe–Co sextet two new components appeared in the spectrum (Fig. 3b). The sextet with broad lines and a reduced hyperfine field of about 30 T can be assigned to interfacial regions between

the Fe–Co grains and the remaining amorphous phase. The only paramagnetic component seen in the centre of the spectrum is the quadrupole doublet with Δ = 0.68 mm/s and isomer shift close to 0.00 mm/s. It origins most probably from Fe atoms surrounded by Zr ones, forming Fe–Zr- or Fe–Zr–O-like regions. The XRD pattern obtained after the heating of the sample up to the temperature of 993 K, exceeding Tx2 (Fig. 1), reveals the lines corresponding to the bcc Fe–Co, fcc Fe–Ni and Fe3 B-type phases (Fig. 2). This result is consistent with the M¨ossbauer measurement. The M¨ossbauer spectrum consists of: (i) the

Fig. 5. Transmission (a–e) and CEMS (a –e ) spectra recorded, respectively, for the bulk and shiny side of the Fe60 Co10 Ni10 Zr7 B13 ribbon in the as-quenched state and after annealing for 1 h.

A. Grabias et al. / Materials Science and Engineering A 449–451 (2007) 552–556

dominating bcc Fe–Co sextet with the relative fraction of 80%, (ii) three sextets with low intensity and hyperfine fields of about 28, 26 and 21 T related to the Fe3 B-like regions [12], (iii) the previously observed quadrupole doublet, and (iv) a new paramagnetic, single-line component with δ = −0.09 mm/s, corresponding to the iron-poor fcc Fe–Ni phase and contributing about 7% to the total spectral area (Fig. 3c). Isothermal annealing for 5 h allowed us to obtain well-formed crystalline structures. The M¨ossbauer spectrum recorded for the sample annealed at temperature of 903 K, at which the first stage of crystallization is completed, consists of the dominating bcc Fe–Co sextet accompanied by the quadrupole doublet related to Fe–Zr-based regions and three sextets that can be attributed to the formation of the Fe3 B-like regions (Fig. 4a). Annealing at temperature of 1023 K, that is higher than the temperature of the second crystallization stage, caused the formation of three phases as revealed by the M¨ossbauer spectrum in Fig. 4b. The magnetic bcc Fe–Co phase (A = 76%) coexists with the paramagnetic fcc Fe–Ni one (A = 13%). The sextet with Hhf = 23.0 T and δ = 0.14 mm/s (A = 11%) can be assigned to the tetragonal Fe2 B phase [12]. This phase transformed from the metastable Fe3 B one observed after annealing at 903 K. The XRD pattern recorded for this sample confirms the phase identification obtained from the M¨ossbauer measurement, however, traces of other borides can be also present (Fig. 2). The qualitative and quantitative comparison of the bulk and surface crystallization of the Fe60 Co10 Ni10 Zr7 B13 metallic glass was performed by transmission and conversion electron M¨ossbauer spectroscopy measurements of the samples annealed for 1 h (Fig. 5). The CEMS spectra shown in Fig. 5 were recorded for the shiny side of the ribbon, that was free of the contact with the quenching copper roller during the melt-spinning process. The transmission and CEMS spectra of the as-quenched alloy are identical (Fig. 5a and a ). They consist of a sextet with broad and overlapped lines characteristic for the ferromagnetic, fully amorphous alloy. The broadening originates from different local atomic neighbourhood around Fe atoms in the amorphous phase, what produces a wide range of hyperfine fields at Fe sites. Therefore, the spectra were fitted using the hyperfine field distribution method. The average hyperfine fields of the Fe60 Co10 Ni10 Zr7 B13 metallic glass are 22.9 and 23.2 T for the bulk and surface layer of the ribbon, respectively.

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Clear differences between the bulk and surface crystallization appear already after annealing at 773 K. Although the transmission spectrum obtained for the bulk of the annealed sample reveals fully amorphous structure (Fig. 5b), the CEMS spectrum shows the sextet with sharp lines characteristic for the bcc Fe–Co phase with relative fraction of 56% (Fig. 5b ), indicating partial crystallization of the amorphous phase in the surface layer of the ribbon. The fact that crystallization starts at the surfaces at lower annealing temperature than in the bulk of the ribbon can be related to the presence of quenched-in crystallites at the surface or to composition fluctuations or surface stresses that could reduce the crystallization temperature locally [9,10]. Annealing at 853 K caused partial crystallization of the amorphous phase in the bulk of the ribbon (Fig. 5c). The transmission spectrum was fitted and analysed in the same way as the spectrum in Fig. 3b. The formation of the bcc Fe–Co solid solution with the relative abundance of 65% was observed. The broadened magnetically split sextet with Hhf = 14.9 T and A = 21% corresponds to the retained amorphous phase. The CEMS spectrum shows completely crystalline structure of the sample annealed at 853 K (Fig. 5c ). It consists of three magnetic components: one with Hhf = 35.4 T, δ = 0.00 mm/s and A = 83% related, as before, to the bcc Fe–Co, and the other two with Hhf1 = 22.4 T, Hhf2 = 23.5 T, δ1,2 = 0.13 mm/s assigned to the tetragonal Fe2 B phase. The good resolution of the CEMS spectra allowed us to fit two sextets corresponding to two non-equivalent Fe sites in the Fe2 B lattice. Smaller hyperfine fields than these of the pure Fe2 B phase [12] suggest the presence of Ni atoms in the lattice. Annealing at higher temperatures caused the domination of the stable Fe2 B phase at the surfaces beside the bcc Fe–Co phase whose abundance was less than 19% (Fig. 5d and e ). The transmission spectra of the samples annealed at 903 and 933 K show mainly the formation of the bcc Fe–Co phase accompanied by the Fe3 B-like regions (Fig. 5d and e). For the sample annealed at 933 K the single line with δ = −0.08 mm/s and A = 3% corresponding to the paramagnetic fcc Fe–Ni phase is also observed (Fig. 5e). The M¨ossbauer results are summarised in Fig. 6 that presents schematically the evolution of iron-containing phases in the bulk and at the shiny surface of the Fe–Co–Ni–Zr–B ribbon as a function of the annealing temperature. The onset of crystallization at the ribbon surface occurs at lower annealing temperature than in the bulk of the alloy. The stable Fe2 B phase forms at the sur-

Fig. 6. Relative abundance of the iron-containing phases determined from transmission and CEMS spectra of the samples annealed for 1 h.

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face and dominates at higher annealing temperatures applied, whereas in the bulk of the ribbon the dominating bcc Fe–Co and the metastable Fe3 B-like phases are observed. The paramagnetic fcc Fe–Ni phase detected for the bulk of the alloy is not observed at the ribbon surface. The differences between the bulk and the surface crystallization observed by M¨ossbauer measurements may origin from different quenching rates experienced by the surface and the inside parts of the ribbon during the meltspinning process, that could cause compositional inhomogeneity across the thickness of the ribbon. Furthermore, the shiny surface of the ribbon (free of the contact with the spinning wheel) may reveal more potential nucleation sites than the inside parts of the ribbon. 4. Conclusions Crystallization behaviour of the Fe60 Co10 Ni10 Zr7 B13 metallic glass was studied as a function of annealing temperature. The first stage of crystallization is related to the partial crystallization of the amorphous phase causing the formation of the bcc Fe–Co solid solution. Between the first and the second stages the formation of small amounts of Fe3 B-like regions were observed in the M¨ossbauer spectra. The next step of crystallization induced the formation of the paramagnetic iron-poor fcc Fe–Ni phase, in addition to the previously observed bcc Fe–Co and Fe3 B phases. For longer annealing time or at higher annealing temperature the

metastable Fe3 B phase transformed to the stable tetragonal Fe2 B one. The surface crystallization started at lower annealing temperature than the bulk one that was attributed to composition fluctuations or surface stresses that could reduce the crystallization temperature locally. Two crystalline phases (bcc Fe–Co and Fe2 B) were observed in the surface layers of the annealed ribbons. References [1] A. Inoue, T. Zhang, T. Itoi, A. Takeuchi, Mater. Trans. JIM 38 (1997) 359. [2] T. Itoi, H. Onodera, S. Miura, A. Inoue, Mater. Trans. JIM 41 (2000) 1392. [3] A. Inoue, A. Makino, T. Mizushima, J. Magn. Magn. Mater. 215–216 (2000) 246. [4] A. Grabias, D. Oleszak, M. Kopcewicz, J. Latuch, T. Kulik, F. Stobiecki, J. Non-Cryst. Solids 330 (2003) 75. [5] A. Inoue, H. Koshiba, T. Zhang, A. Makino, J. Appl. Phys. 83 (1998) 1967. [6] A. Castellero, M. Motyka, A.L. Greer, M. Baricco, Mater. Sci. Eng. A 375–377 (2004) 250. [7] M. Shapaan, J. Gubicza, J. Lendvai, L.K. Varga, Mater. Sci. Eng. A 375–377 (2004) 785. [8] D.Y. Liu, W.S. Sun, A.M. Wang, H.F. Zhang, Z.Q. Hu, J. Alloys Compd. 370 (2004) 249. [9] M. Kopcewicz, A. Grabias, J. Appl. Phys. 80 (1996) 3422. [10] N. Saegusa, A.H. Morrish, Phys. Rev. B 26 (1982) 6547. [11] R. Br¨uning, K. Samwer, C. Kuhrt, L. Schultz, J. Appl. Phys. 72 (1992) 2978. [12] M.B. Fernandez van Raap, F.H. Sanchez, Y.D. Zhang, J. Mater. Res. 10 (1995) 1917.