Scholarly article on topic 'Crystallization of Fe-Based Bulk Amorphous Alloys'

Crystallization of Fe-Based Bulk Amorphous Alloys Academic research paper on "Materials engineering"

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Academic research paper on topic "Crystallization of Fe-Based Bulk Amorphous Alloys"

ARCHIVES OF METALLURGY AND MATERIALS

Volume 60 2015 Issue 1

DOI: 10.1515/amm-2015-0001

K. BLOCH*, M. NABIALEK*, M. DOSPIAL*, S. GARUS*

CRYSTALLIZATION OF Fe-BASED BULK AMORPHOUS ALLOYS

KRYSTALIZACJA MASYWNYCH STOPOW AMORFICZNYCH NA BAZIE ZELAZA

The aim of this paper is to present the results of crystallization studies for the bulk amorphous (Feo.6iCoo.ioZro.o25Hfo.o25 Ti0.o2Wo.o2Bo.2o)98Y2, Fe61Co10Tix Y6B20, Fe61Co10Ti2Y7B20 alloys. The crystallization of the alloys was studied by differential scanning calorimetry (DSC). The amorphicity of the investigated alloys in the as-quenched state was testified using Mossbauer spectroscopy, X-ray diffractometry and transmission electron microscopy. Moreover, X-ray diffractometry was applied to structure investigations of partially crystallized samples. The crystallization process in the investigated alloys occurs in one or two stages. Two peaks in the DSC curves can be overlapped or well separated indicating the complex crystallization processes. From X-ray diffraction we have stated that in both types of devitrification the crystalline phase can be ascribed to the a-FeCo. In the first stage the crystalline grains seem to grow from the nuclei frozen in the samples during the rapid quenching, whereas in the second one both the growth of the existed grains and creation of new ones during annealing may occur.

Keywords: bulk amorphous alloys, differential scanning calorimetry (DSC), X-ray diffractometry, transmission electron microscopy, Mossbauer spectroscopy

W pracy przedstawiono wyniki badan krystalizacji masywnych stopow amorficznych (Fe0,61Co0,10Zr0,025Hf0,025Ti0,02W0,02 B0,20)98Y2, Fe61Co10Ti3Y6B20, Fe61Co10Ti2Y7B20w postaci pr^tow. Krystalizacji tych stopow badano wykorzystuj^c skanin-gowy kalorymetr roznicowy (DSC). Amorficzno^d probek w stanie po zestaleniu zostala zbadana przy uzyciu spektroskopii Mossbauera, dyfrakcji promieni Rontgena oraz transmisyjnej mikroskopii elektronowej. Ponadto, dyfrakj promieni X uzyto do badan probek cz^^ciowo skrystalizowanych. Na podstawie badan z wykorzystaniem DSC stwierdzono, ze krystalizacja moze przebiegad w jednym lub w dwoch etapach. Piki na krzywych DSC odpowiadaj^ce tym dwom etapom mog^ byd polozone blisko siebie lub wyrainie rozdzielone. Pierwsze maksimum odpowiada tworzeniu si§ ziaren a-FeCo z zarodkow powstalych podczas produkcji stopu, drugie natomiast zwi^zane jest z procesem tworzenia si§ ziaren fazy krystalicznej a-FeCo z zarodkow powstalych podczas wygrzewania i rozrostem wcze^niej powstalych ziaren.

1. Introduction

Iron-based amorphous ferromagnetic alloys have attracted much attention due to their great potential application and low material cost [1-3]. Conventional amorphous alloys are usually prepared by rapidly quenching of a molten material on a rotating wheel at cooling rate of the order of 106K s-1 and have a shape of thin ribbons with thickness of about 40 jum. The packing density of the material in magnetic cores is low because of air gaps between layers. Multicomponent systems enable to produce amorphous materials at relatively low cooling rate of 1-102 K s-1 in the form of rods, tubes and thick ribbons [4-7]. One of the important parameters, which determine the application of amorphous material, is the thermal stability of the structure and magnetic properties. Instability of the amorphous alloys, connected with irreversible structure relaxations, leads to their crystallization at high temperature. We may distinguish two types of amorphous alloys [8,9]. Type one devitrifies directly to the crystalline phase. Whereas in the

second type at first the quasicrystalline state is created and then during further heating it transforms to crystalline state [10-11].

The aim of this paper is to present the results of crystallization studies of the bulk amorphous (Feo.61Coo.1o Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2, Fe61Co1oTi3 Y6B20, Fe61

Co10Ti2Y7B20 alloys.

2. Experimental procedure

Alloy ingots with nominal composition (Fe0.61Co0.10Zr0.025 Hf0.025Ti0.02W0.02B0.20)98Y2, Fe61Co1oTi3Y6B20, Fe61Co1oTi2 Y7B20 were prepared by arc melting of high purity elements in an argon protective atmosphere. Each ingot was remelted four times to obtain the homogenous material. Amorphous rods 1 mm in diameter and 20 mm long (Fig. 1.) were obtained by a suction-casting method of the molten alloy into a copper mould cooled with water [12].

* INSTITUTE OF PHYSICS, CZESTOCHOWA UNIVERSITY OF TECHNOLOGY, 19 ARMII KRAJOWEJ AV., 42-200 CZESTOCHOWA, POLAND

Fig. 1. Sight of amorphous rods obtained by an optical microscope

The structure of the samples was investigated using Mossbauer spectroscopy, X-ray diffractometry and transmission electron microscopy. The Mossbauer spectrum and X-ray diffractions patterns were measured for powdered samples. The crystallization of the alloys was studied by differential scanning calorimetry (DSC) for the samples in the form of rods and after powdering. The heat treatment of the samples was carried out in an argon atmosphere using a differential scanning calorimeter.

X-ray diffraction patterns are characteristic of the amorphous state and consist of broad maxima. No narrow peaks typical of crystalline phases are present.

Transmission Mossbauer spectra and obtained from them hyperfine field induction distributions for the same samples are shown in Fig. 3.

Mossbauer spectra are typical of amorphous alloys and have the form of sextet with broad and overlapped lines. The hyperfine induction distributions do not exhibit the unimodal character and we may distinguished in them at least two components corresponding to the different environment of 57Fe atoms.

v[mm/s] Bhf[T]

Fig. 3. Transmission Mossbauer spectra (a, b, c) and corresponding hyperfine field induction distributions (d, e, f) for amorphous powdered alloys after preparation: (a, d) - (Fe0.61Co0.1o

Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2, (b, e) - Fe6lCol0 Ti3Y6B20> (c, f) -

Fe6iCoioTi2Y7B2o

The amorphous structure of the as-received alloys was also confirmed by transmission electron microscopy working at high resolution mode (HREM). In Fig. 4 the bright-field HREM-image for the sample of

(Fe0.6iCo0.i0Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2 alloy, as an example, is depicted.

3. Results and discussion

In the Fig. 2 X-ray diffractions patterns for as-received

(Fe0.6lCO0.i0Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2, Fe6lCoioTi3 Y6B20, Fe61Co10Ti2Y7B20 alloys are presented.

-1-1-1-'-I-1-1-1-

Fig. 4. Bright-field HREM-image (a) and corresponding electron diffraction pattern (b) obtained for the as-received

(Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2 alloy

20[°] In bright-field image no crystalline phases are observed

Fig. 2. XRD patterns obtained for as-received powdered samples (a) - and corresponding selected area diffraction p^tern has form (Fe0.61Co0.10Zr0.25Hf0.25Ti0.02W0.02B0.2)98Y2, (b) - Fe61Co10Ti3Y6B20, of a halo ring characteristic of an amorphous phase. (c) - Fe61 Co10Ti2Y7B20 Fig. 5. shows DSC tracers obtained for small pieces

(5 mm long) cut out from the as-quenched rods.

850 900 950 975 1000 1025 temperature [K]

Fig. 5. DSC curves of as-received (a) - (Feo.6iCoo.ioZro.25Hfo.25Tio.o2 Wo.o2Bo.2o)98Y2, (b) - Fe^Co^^'Y6B20, (c) - Fe61 Co1oTi2Y7B2o alloys recorded at heating rate of 10 K min-1

DSC curve obtained for the amorphous Fe61 Co10Ti2Y7B20 alloy exhibits one exothermic peak at 969 K corresponding to the crystallization of the sample. As for the amorphous Fe61Co10Ti3Y6B20 sample in the DSC curve we may distinguish near situated two peaks, one at 955 K and the second at 968 K. Well separated two peaks are observed in DSC curve for (Feo.61Coo.1oZro.25Hfo.25Tio.o2Wo.o2Bo.2o)98Y2,at 913 K and 998 K, respectively. DSC results indicate that the crystallization process in the latter two alloys occurs in two stages.

It is well known that it is thermally activated and the location of the peaks in DSC curves depends on the heating rate (Fig. 6).

DSC curves recorded at heating rate of 10 K min 1 for amorphous rods and powdered samples are presented in Fig. 7.

850 900 950 1000 1050 900 950 1000 1050

temperature[K]

Fig. 7. DSC curves measured at the heating rate of 10 K min-1 for the bulk amorphous (A) (Feo.6iCoo.ioZro.o25Hfo.o25Tio.o2Wo.o2Bo.2o)9sY2, (B) Fe61Co10Ti3Y6B20, alloys: (a) - the samples in the form of the rod; (b) - powdered rod

The peaks in the DSC curves obtained for powdered samples are more pronounced and slightly shifted to lower temperature than for rods. It is connected with temperature distribution in the cross-section of rods during measurements of DSC curves.

In order to determine the phase composition of the samples in different stages of crystallization, X-ray diffraction patterns were measured for the samples in the proper stage of crystallization. In Fig. 8 DSC curves for the amorphous powdered (Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2 sample measured in the temperature range from 800 K up to 920 K and from 800 K up to 1010 K and X-ray diffraction patterns recorded after breaking heat treatment at 920 K and 1010 K are shown.

875 900 925 950 975 1000 1025

temperature [K]

Fig. 6. Isochronal DSC curves recorded at heating rate 10 K min-1 (a) and 5 K min-1 (b) for the amorphous Fe61Co10Ti3Y6B20 alloy

It is worth noticing that the first peak in the DSC curves measured at the heating rate of 5 K min-1 shifts from 957 K to 895 K, while the location of the second peak remains practically unchanged in comparison with curves recorded at 10 K min-1.

800 850 900 950 1000 1050800 850 900 950 1000 1050 temperature [K] temperature [K]

30 40 50 60 70 30 40 50 60 70

_29[°]_29[°]_

Fig. 8. DSC curves measured in the temperature range from 800 K up to 920 K (a) and from 800 K up to 1010 K (c) and X-ray diffraction patterns recorded after breaking heat treatment at 920 K (b) and 1010 K (d) for the amorphous powdered

(Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2 alloy

The X-ray diffractograms show the narrow peaks at 20 = 44,8° corresponding to the crystalline phase. The intensity of this peak in X-ray pattern obtained for the sample is higher after the second stage of crystallization. The similar results were obtained for the bulk amorphous Fe61Co10Ti3Y6B20 alloy (Fig. 9).

temperature [K] temperature [K]

30 40 50 60 7030 40 50 60 70

29[°] 29[°]

Fig. 9. DSC curves measured in the temperature range from 800 K up to 955 K (a) and from 800 K up to 980 K (c) and X-ray diffraction patterns recorded after breaking heat treatment at 955 K (b) and 980 K (d) for the amorphous powdered Fe61Co10Ti3Y6B20 sample

The first peak observed in DSC curves for (Fe0 61Co010 Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2 and Fe61Co^Ti3Y6B20 alloys corresponds to creation of crystalline grains from nuclei quenched in during alloys preparation. However, the second peak is connected with the growth of existing grains and nuclei which were created during heating. DSC curve and X-ray diffraction pattern of the amorphous Fe61Co10Ti2Y7B20 alloy are showed in Fig. 10.

900 ' 950 ' 1000 ' 1050

temperature [K]

30 40 50 60 70

20[°]

Fig. 10. DSC curve (a) measured in the temperature range from 800 K up to 990 K at the heating rate of 10 K min-1 and X-ray diffraction pattern (b) recorded after breaking heat treatment at 990 K for the amorphous powdered Fe61Co10Ti2Y7B20 sample

DSC curve obtained for this alloy exhibits only one maximum. From the analysis of X-ray diffraction patterns it has been stated that during crystallization of the investigated alloys a-FeCo phase appears.

4. Conclusions

We found that the crystallization process of bulk amorphous alloys strongly depends on their chemical composition. The amorphous (Fe0.61Co0.10Zr0.025Hf0.025Ti0.02W0.02B0.20)98Y2 alloy crystallizes in two stages with well separated temperature. DSC curve for Fe61Co10Ti3Y6B20 alloy shows near situated peaks, whereas the crystallization process for Fe61Co10Ti2Y7B20 alloy takes place in one stage.

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Received: 20 March 2014.