The glass forming abilities and magnetic properties of Fe–Al–Ga–P–B–Si and Fe–Al–Ga–P–B–C alloys

Materials Science and Engineering A 375–377 (2004) 372–376

The glass forming abilities and magnetic properties of Fe–Al–Ga–P–B–Si and Fe–Al–Ga–P–B–C alloys P. Pawlik a,1 , H.A. Davies a,∗ , M.R.J. Gibbs b a b

Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, UK Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK

Abstract An investigation of glass forming abilities and thermal properties of Fe76−x Al4 P12 Gax B4 Si4 and Fe75−x Al5 P11 Gax B4 C5 (where x = 0, 2, 5) soft magnetic alloys was carried out using melt-spun ribbons of various thicknesses and suction cast rods of various diameters. For the Si-containing alloys, the critical thickness of amorphous ribbon increased from 55 to 110 ␮m between x = 0 and 5. On the other hand, for the carbon containing alloys, increasing x from 0 to 5 resulted in a decrease in critical thickness of amorphous ribbon from 150 to 100 ␮m. These data allowed the critical diameter of glassy cylindrical samples Dc to be estimated for appropriate alloy compositions and these are compared with experimental observations for suction cast rods. The magnetic properties, including the hysteresis loops and coercivity i Hc , were determined by dc inductive magnetometry. The results indicate surprisingly low values of i Hc , even for partially crystalline samples. On the other hand, the hysteresis loop shapes suggest a relatively large value of magnetic anisotropy. The possible reason for this behaviour is discussed. © 2003 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glasses; Melt-spun ribbon; Soft magnetic materials

1. Introduction Until recently, the production of amorphous soft magnetic materials was limited to the thin sections, typically less than 100 ␮m, for unidirectional cooling conditions such as in chill block melt spinning to ribbon. This limitation was determined by high critical cooling rates T˙ c for glass formation of Fe-based alloys in the range 105 –106 K/s. Since the discovery of Pd-based [1] and Zr-based [2] alloys with extremely low T˙ c , which allow the formation of bulk, fully glassy samples, considerable effort has been directed at identifying new ferromagnetic Fe-based compositions which could similarly be formed into bulk glassy samples [3–8]. In order to achieve such high glass formability, multicomponent compositions have to be explored but, even so, the critical diameter of a radially cooled glassy section Dc is reported to be generally limited to ∼3 mm [7] or, in exceptional cases to 6 mm [8]. Such limitations are determined not only by the alloy ∗ Corresponding author. Tel.: +44-114-222-5518; fax: +44-114-222-5943. E-mail address: [email protected] (H.A. Davies). 1 Present address: Institute of Physics, Czestochowa University of Technology, A1. AK19, 42-200 Czetochowa, Poland.

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.165

composition, but also by the process conditions employed. In the present paper, these two aspects are investigated in the context of melt-spun ribbon samples with various thicknesses and suction cast rods of various diameters, for two series of soft magnetic alloys Fe76−x Al4 P12 Gax B4 Si4 and Fe75−x Al5 P11 Gax B4 C5 (where x = 0, 2, 5) and the results of initial studies of the magnetic properties of selected samples are presented and discussed.

2. Experimental Samples of alloys in the two compositional series Fe76−x Al4 P12 Gax B4 Si4 and Fe75−x Al5 P11 Gax B4 C5 (where x = 0, 2, 5) were produced as 20g buttons by induction melting under an Ar atmosphere, using high purity elemental Fe, Al, Ga and B with pre-alloyed Fe–P and Fe–C of known, analysed compositions. Commercial grade Fe–P and Fe–C alloys were used as feedstock for initial ingots and they were purified beforehand by melting and fluxing. Using samples of the prepared ingots, the alloys were melt-spun to ribbons of various thicknesses, >250 ␮m in some cases, by varying the surface speed of the wheel down to 6 m/s. Rod samples of the alloys were produced using a

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This XC increases for the 2 at.% Ga alloy to 74–105 ␮m (Fig. 1b), while increasing the Ga further up to 5% does not significantly change XC (Fig. 1c). In contrast, for the Fe75−x Al5 P11 Gax B4 C5 alloys, increasing the Ga content reduces XC from 108–150 ␮m for 0 at.% Ga (Fig. 1d) to 83–108 ␮m for 5 at.% Ga (Fig. 1f). On the assumption of approximately the same interfacial heat transfer coefficient for casting by melt spinning to ribbon and for suction casting to a rod sample, it can be shown [12], that the cooling rate at the centre of the cross section of a rod of diameter D is about equal to the cooling rate at the top surface of a ribbon of the same alloy of thickness D/4. Thus, the critical diameter for a glassy rod should be about a factor 4 larger than XC for melt-spun ribbon. For the Fe76−x Al4 P12 Gax B4 Si4 alloys, Dc initially increases up to ∼0.4 mm on adding 2% Ga and then remains about constant on increasing the Ga content further up to 5 at.%. In contrast, for the Fe75−x Al5 P11 Gax B4 C5 alloys, addition of Ga results in a decrease of the estimated DC of glassy rod from ∼0.6 mm to slightly below 0.5 mm. The X-ray traces for 0.5 mm suction cast rods, corresponding to the highest-estimated glass forming abilities, are presented in Fig. 1c and d for Fe71 Al4 P12 Ga5 B4 Si4 and Fe75 Al5 P11 B4 C5 alloys, respectively. There is clear evidence of much

suction die casting facility integrated into the arc-melting unit. This technique involved suction of the molten alloy from the arc-melting hearth into a cylindrical cavity in a split copper die, using a pressure difference of argon between two chambers within the system [9–11]. The structures of the ribbon and rod samples were monitored, by X-ray diffraction (XRD) using Co K␣ radiation. XRD scans were taken from top surfaces of ribbon samples and cross sections of rod samples. The hysteresis loops for ribbon were obtained by dc magnetometry. The thermal properties, in particular the crystallisation behaviour, were studied by differential scanning calorimetry (DSC) on the ribbon samples of various thicknesses and on the rod samples.

3. Results and discussion 3.1. X-ray diffraction analysis The XRD scans for ribbon samples for Fe76−x Al4 P12 Gax B4 Si4 (x = 0–5 at.%) alloys are shown in Fig. 1. The maximum thickness of fully amorphous ribbon XC for the Ga-free alloy is in the range 37–56 ␮m (Fig. 1a).

(d)

(a) 0.5 mm rod

x=86-137µm x=169-274µm x=84-88µm

x=37-56µm

(b)

(e)

1mm rod Intensities

x=190-221µm x=205-232µm

x=133-173µm

x=108-162µm

x=104-110µm

x=74-105µm Angle, 2Θ[deg]

(c)

(f) 0.5 mm rod

0.5 mm rod x=170-245µm

x=202-313µm

x=105-140µm

x=113-131µm

x=56-83µm 30

40

50 60 Angle, 2Θ[deg]

70

x=64-98µm 80 30

40

50 60 Angle, 2Θ[deg]

70

80

Fig. 1. X-ray diffraction patterns (Co K␣ radiation) for Fe76−x Al4 P12 Gax B4 Si4 (where x = 0, 2, 5) alloys; (a) x = 0; (b) x = 2; (c) x = 5, and the Fe75−x Al5 P11 Gax B4 C5 (where x = 0, 2, 5) alloys (d) x = 0; (e) x = 2; (f) x = 5, for ribbon samples of various thicknesses, and for 0.5 and 1 mm rod samples.

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(a)

(d) 0.5 mm rod x=169-274 µm T c

x=25-32 µm 300

(b)

T T g x

400 500 600 Temperature, T[K]

70 0

800

300

(e)

1mm rod Heat flow [a.u.]

T

x=15-26 µm

T T g x

c

400 500 600 700 Temperature, T[K]

800

1mm rod x=205-232 µm x=24-39 µm 300

400

(c)

T c

T T g x

x=133-173 µm Tc

T gT x

T c

T T g x

x=22-39 µm

500 600 700 800 900 Temperature, T[K]

300

400

500 600 700 800 Temperature, T[K] 1 mm rod

(f)

0.5 mm rod

0.5 mm rod

x=203-313 µm

x=170-245 µm

x=22-25 µm 300

T

900

T Tx g

c

400 500 600 700 Temperature, T[K]

x=16-18 µm 300

800

400

500 600 700 Temperature, T[K]

800

900

Fig. 2. DSC traces for ribbon samples of various thicknesses and suction cast rods, for Fe76−x Al4 P12 Gax B4 Si4 alloys: (a) x = 0; (b) x = 2; (c) x = 5, and for Fe75−x Al5 P11 Gax B4 C5 alloys (d) x = 0; (e) x = 2; (f) x = 5 (temperature change 40 K/min).

larger content of amorphous phase in a 0.5 mm rod of the Fe75 Al5 P11 B4 C5 alloy than in the case of the 0.5 mm rod of the Fe71 Al4 P12 Ga5 B4 Si4 alloy. This suggests better glass forming ability for the Fe75 Al5 P11 B4 C5 alloy, which agrees with the results for ribbon samples. For both compositions, the coexistence of amorphous and crystalline phases was revealed. This suggests that the heat transfer coefficient in the case of suction casting conditions is slightly lower than for melt spinning. It is believed that the quenching efficiency during rod casting is impaired by the preheating of the die by conduction from the arc-melting hearth and attempts are currently under way to overcome this limitation. Suction casting is considered to have a number of potential

advantages over injection casting from a nozzle for casting amorphous rods, including reduction of melt stirring. 3.2. Thermal characterisation DSC scans for ribbon samples of various thickness for the Fe76−x Al4 P12 Gax B4 Si4 and Fe75−x Al5 P11 Gax B4 C5 alloys are shown in Fig. 2. The thermal stability parameters Tg and Tx determined for all ribbon samples are presented in Table 1 and are similar to values previously reported by Inoue in [4,6]. For the Fe76−x Al4 P12 Gax B4 Si4 alloys Tg , decreases from 745 to 728 K while Tx increases from 757 to 781 K with increase of x from 0 to 5. For the Fe71 Al4 P12 Ga5 B4 Si4 alloy,

Table 1 Thermal stability and magnetic properties of Fe76−x Al4 P12 Gax B4 Si4 and Fe75−x Al5 P11 Gax B4 C5 (where x = 0, 2, 5) alloy ribbons Tc (K)

Fe76 Al4 P12 B4 Si4 Fe74 Al4 P12 Ga2 B4 Si4 Fe71 Al4 P12 Ga5 B4 Si4 Fe75 Al5 P11 B4 C5 Fe73 Al5 P11 Ga2 B4 C5 Fe70 Al5 P11 Ga5 B4 C5

623 628 610 608 612 600

Tg (K)

745 737 728 705 709 721

Tx (K)

757 778 781 758 767 778

Tx (K)

12 41 53 53 58 57

µ0 Ms (T)

1.38 1.14 1.09 1.29 1.15 1.06

χ As-quenched

J Hc

Thin <25 ␮m

Thick >100 ␮m

Thin <25 ␮m

Thick >100 ␮m

Thin <25 ␮m

Thick >100 ␮m

155 160 139 75 111 145

33 32 36 29 40 43

3 8 1.7 11 12 6

6.5 10 1.4 2 2 3

0.16 0.07 0.005 0.18 0.45 0.25

0.02 0.02 0.005 0.005 0.005 0.02

(A/m)

Mr /Ms

Tc : Curie temperature; Tg : glass transition temperature; Tx : crystallisation temperature; Tl : liquidus temperature; Tx : Tx −Tg ; Ms : saturation magnetisation, χ: susceptibility.

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Fig. 3. Examples of magnetic hysteresis loops measured for Fe71 Al4 P12 Ga5 B4 Si4 alloy ribbons with different thickness (x); (a) x = 22–25 ␮m (amorphous); (b) x = 63–110 ␮m (amorphous); (c) x = 105–140 ␮m (amorphous + crystalline), and for Fe75 Al5 P11 B4 C5 alloy ribbons; (d) x = 15–26 ␮m (amorphous); (e) x = 108–150 ␮m (amorphous); (f) x = 169–274 ␮m (amorphous + crystalline).

the supercooled liquid region Tx has the largest value in the series, 53 K. In the case of the Fe75−x Al5 P11 Gax B4 C5 series, the increase of Ga content causes an increase in Tg from 705 to 721 K and an increase in Tx from 758 to 778 K, between x = 0 and x = 5. Surprisingly, the Fe75 Al5 P11 B4 C5 alloy has the lowest Tx = 53 K in this series, in spite of having the highest glass forming ability. DSC traces for 0.5 mm rod samples, corresponding to the Fe71 Al4 P12 Ga5 B4 Si4 and Fe75 Al5 P11 B4 C5 alloys, are included in Fig. 2c and d, respectively. The crystallisation peaks at the expected temperatures, indicate the presence of a fraction of glassy phase in both samples. However, the glass forming abilities of examined alloys are significantly lower than for alloys reported in [4,6], which may be due to the different cooling conditions in two cases. 3.3. Hysteresis loop measurements Selected dc hysteresis loops are shown in Fig. 3, for ribbon samples of the Fe71 Al4 P12 Ga5 B4 Si4 and Fe75 Al5 P11 B4 C5 alloys (in both cases, the composition with the highest glass forming ability). The coercivity i Hc is as expected, very low for all fully amorphous ribbon samples. In this case, magnetic properties are similar to those previously reported by Inoue in [4,6]. Of particular note are the data for samples with coexisting amorphous and crystalline phases, which have unusually low coercivities but very high anisotropy fields. This phenomenon may be attributed to the stress field associated with the nucleation of a thin layer of ferromagnetic crystalline phases on the surface of the amorphous ribbon during casting. The effects of surface crystallisation on magnetic properties in Fe-rich metallic glasses were widely discussed in [13–15]. TEM investigation carried out on the thick (>100 ␮m) ribbon samples of Fe76−x Al4 P12 Gax B4 Si4

alloys (for x = 0 and 5) proved the existence of relatively large crystals up to 800 nm diameter on the top surface of the ribbon as well as nanocrystallites, with diameters down to 1 nm, embedded in the amorphous matrix phase. The magnetic parameters for ribbon samples are also given in Table 1. In the case of the Fe75−x Al5 P11 Gax B4 C5 series, all thick ribbons (>100 ␮m) with coexisting amorphous and crystalline phases have much lower coercivity, in the range 2–3 A/m, than for thin, fully amorphous ribbon samples (<25 ␮m), for which i Hc is in the range from 11 A/m for x = 0 to 6 A/m for x = 5. The geometrical dispersion of a small number of large surface crystallites has little effect on coercivity. If fully amorphous ribbon is subsequently devitrified, the coercivity rises rapidly, in line with earlier work [14,15], where there are many small grains at the surface. The Mr /Ms ratio and the susceptibility χ are always lower in the case of thick samples for all alloys examined. The magnetic characteristics and structures of these ribbons are currently being investigated further.

4. Conclusions For Fe76−x Al4 P12 Gax B4 Si4 alloys, an increase in XC with increasing Ga is observed. Whereas, for Fe75−x Al5 P11 Gax B4 C5 alloys, XC decreases with increasing Ga. DC is estimated to be roughly ∼0.6 mm for Fe75 Al5 P11 B4 C5 alloys while, experimentally, an 0.5 mm diameter cast rod was found to be mainly crystalline with only a small fraction of glassy phase. This suggests that for the suction casting of rod the interfacial heat transfer coefficient is lower than for the melt spinning process. The magnetic measurements revealed that J Hc for fully amorphous ribbons is, small for all samples examined. However, the ribbon samples with co-

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existing amorphous and crystalline phases have even smaller J Hc but, on the other hand, high anisotropy fields. This phenomenon can be attributed to the presence of stress fields associated with surface precipitation of a few large surface crystallites in thick ribbons. Acknowledgements Research sponsored by the EU under a Research Training Network on “Bulk Metallic Glasses” (contract no. HPRN-C7-200-00033). References [1] A. Inoue, Acta Mater. 48 (2000) 279–306. [2] H. Kato, Y. Kawamura, A. Inoue, Mater. Trans. JIM 37 (1996) 70–77.

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