Synthesis and characterization of a multicomponent Fe-based bulk amorphous alloy

Journal of Non-Crystalline Solids 354 (2008) 5363–5367

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Synthesis and characterization of a multicomponent Fe-based bulk amorphous alloy M. Iqbal a,b, J.I. Akhter b,*, H.F. Zhang a, Z.Q. Hu a,1 a b

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China Physics Division, PINSTECH, P.O. Nilore, Islamabad, Pakistan

a r t i c l e

i n f o

Article history: Received 29 February 2008 Received in revised form 9 September 2008 Available online 25 October 2008 PACS: 61.05.cp 61.10.Nz 61.43.Dq 62.20.x Keywords: Amorphous metals, metallic glasses Crystallization X-ray diffraction Indentation, microindentation Scanning electron microscopy

a b s t r a c t A new bulk metallic glass (BMG) Fe60B15Zr10Co7Mo5.5Y2Si0.5 was synthesized by Cu mold casting. The alloy was characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) techniques. The thermal stability and glass-forming ability (GFA) of the alloy have been discussed by evaluating a number of thermal parameters. The maximum values of the key thermal parameters like Trg (Tg/Tm, Tg/Tl), c, d and b parameters were found to be (0.66, 0.64), 0.407, 1.84 and 3.83, respectively. The alloy showed double stage crystallization process. The activation energies for crystallization were found to be 606.6 and 623.1 kJ/mol by Kissinger and Ozawa equations, which indicate the high thermal stability. Crystallization behavior of the alloy was explored by XRD. Mechanical properties like Vicker’s hardness, nanohardness and elastic modulus are found to be very promising. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Synthesis and characterization of bulk metallic glasses (BMGs) have attracted increasing attention in the recent years, because of their promising properties, wide range of applications and scientific significance [1–9]. Fe-based bulk amorphous alloys (BAAs) have emerged as a novel class of BMGs having better properties than their crystalline counterparts. Their magnetic behavior combined with good mechanical properties and high corrosion resistance make them very attractive for applications as microgears, welding elements, dental and medical implants, writing goods, solders and brazing elements, power transformers, power supplies, advanced power devices, magnetic sensors, electronic articles and automotive magnets [9]. However, the use of Fe-based alloys as structural materials is still limited due to high cooling rates required for their production to avoid crystallization and high vacuum to overcome oxidation. Efforts are being devoted to enhance the glass-forming ability (GFA) of Fe-based alloys by improving the purity of the starting materials or by addition of metalloids. Hu et al. [10] reported that metalloid addition up to 0.6 at.% in

* Corresponding author. Tel.: +92 51 2207224; fax: +92 51 9290275. E-mail addresses: [email protected] (M. Iqbal), [email protected], [email protected] (J.I. Akhter), [email protected] (Z.Q. Hu). 1 Tel.: +86 24 23992092; fax: +86 24 23971827 (Z.Q. Hu). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.09.027

Fe-based alloys resulted in maximum enhancement of crystallization temperature Tx. The effect was attributed to the production of higher degree of dense random packing, which restricts the mobility of atoms. Minor addition of Y in Fe-based amorphous alloys has also been found very beneficial [11]. Many thermal parameters like supercooled liquid region DTx(Tx  Tg), reduced glass transition temperature Trg(Tg/Tm or Tg/ Tl), c parameter (Tx/Tl + Tg) [12], d parameter (Tx/Tl  Tg) [13] and recently proposed b parameter (Tg * Tx/(Tl  Tx)2) [14] are used to indicate the GFA of alloys. Here Tg, Tx, Tm and Tl represent glass transition, crystallization, melting and liquid temperatures, which can be obtained by thermal analysis of the material. Furthermore, thermal parameters like Weinberg parameter KW(DTx/Tm), Hruby parameter KH(DTx/(Tm  Tx)) [15], K1(Tm  Tg), K2(DTx), K3(Tx/Tm), K4((Tp  Tx)(DTx)/Tm), KSP((Tp  Tx)(DTx)/Tg) [16] and KLL(Tx/ (Tg + Tm)) [17] have also been suggested to evaluate the thermal stability of amorphous materials [18]. Here Tp is the peak temperature. In case of Fe-based amorphous alloys Trg was recommended as the key parameter to determine the maximum homogeneous nucleation rates in the undercooled melts [19]. It was reported that when Trg P 2/3, the amorphous state is easy to obtain because glass transition temperature Tg is closely related to the viscosity (g). At Tg, viscosity of almost all kinds of melts and supercooled liquids is as high as 1013 P, so higher Trg means faster increase of g with the decreasing of temperature from Tm to Tg. High viscosity

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of the supercooled liquid restricts the diffusion and rearrangement of atoms, thus suppressing crystallization, nucleation and growth which results in large GFA. Xing et al. [20] also emphasized that high value of Trg is more relevant to high GFA of amorphous alloys instead of large DTx. However, Chen et al. [13] listed thermal parameters of a number of Fe-based alloys and demonstrated that the parameter d is more representative of the GFA. The Inoue’s empirical rules [1] and the Greer’s confusion principle [21] have been considered useful for the design of BMGs. In addition, Dong et al. [22] provided excellent guidelines and composition rules for BMGs. In the present study, Fe60B15Zr10Co7Mo5.5Y2Si0.5 alloy was synthesized by Cu mold casting and the thermal stability and GFA of the alloy is discussed systematically by evaluating many parameters described above. The mechanical properties of amorphous and heat-treated samples have also been presented. 2. Experimental The buttons of Fe60B15Zr10Co7Mo5.5Y2Si0.5 alloy were prepared by arc melting a mixture of 2–3 N pure metals and Fe-B master alloy under vacuum (1  104 Pa) in a Ti gettered atmosphere. Melting of the alloy buttons was carried out at least four times to

achieve the extended chemical homogeneity. Bulk metallic glassy cylinders having thicknesses of 2 mm and lengths of 60 mm were synthesized by Cu mold casting technique. X-ray diffraction (XRD) was conducted for structural characterization by a Rigaku X-ray diffractometer D/Max-2500 using Cu Ka1 (k = 1.54056 Å) radiation. In order to measure the thermal stability and thermal parameters, high temperature differential scanning calorimetry (DSC) was conducted at heating rate ‘r’ of 10, 20 and 40 K/min using NETZSCH DSC 404 apparatus under high purity Ar atmosphere. Samples were heat-treated in quartz capsules under inert atmosphere at 953 K and 1123 K for 20 min and their morphology and composition were examined in the scanning electron microscope (SEM) LEO 440i with EDS attachment. Vicker’s hardness ‘HV’ of the as-cast and annealed samples was measured by Mitutoya hardness tester MVK-H3 under an appropriate load. MTS Nanoindenter XP (Berkovich diamond indenter with three sided pyramidal tip) was used under a constant load (P) of 10 mN to measure the nanohardness ‘H’, elastic modulus ‘E’ and other nanohardness parameters at room temperature using Oliver and Pharr method [23]. Density was measured using the Archimedes principle. 3. Results

α -Fe -1

34 12

Intensity (a.u.)

FeCo - 2

4

4 4

34 4

Fe2Si - 3

44 4

Fe14 BY2 - 4

4 3 2 4 1 4

4

4 3 2 4 1

1123 K/20 min

953 K/20 min

As-cast 20

30

40

50

60

70

80

2 theta (deg) Fig. 1. XRD patterns of the as-cast and the annealed samples.

XRD pattern of the as-cast bulk sample is shown in Fig. 1, and it consists of a broad band indicating the amorphous structure of the alloy. Physical appearance of the bulk samples also indicates good metallic luster. The density of induction cast fully amorphous ingots was found to be 8.171 g/cm3. XRD patterns of the samples heat-treated at 953 K and 1123 K for 20 min are also shown in Fig. 1. The sample annealed at 953 K shows a broad band along with a few small Bragg diffraction peaks, which indicate nucleation of some crystalline phases in the amorphous matrix. Analysis indicated presence of phases like a-Fe, FeCo, Fe2Si and Fe14BY2 in the samples heat-treated at 1123 K. SEM examination of the as-cast bulk sample taken from the center of the ingots, revealed featureless surface with out any second phase particles or any segregation. It reconfirms the amorphous structure of the alloy. The microstructure and surface morphologies of the annealed samples are shown in Fig. 2(a–d) that reveal formation of various crystalline phases.

Fig. 2. (a–d) SEM micrographs of the alloy samples annealed at 953 K for 20 min (a, b) and at 1123 K for 20 min (c, d).

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20 K/min

Tg Tx1

-10.0

Tx2

Kissenger Ozawa 3.5

-11.0

3.0

-11.5

2.5

Tp2

Tp1 900

Tl

ln(r)

Tm

750

-10.5 2

40 K/min

600

4.0

Tm Tl

ln(r/Tp )

Heat flow (a.u.)exothermic

M. Iqbal et al. / Journal of Non-Crystalline Solids 354 (2008) 5363–5367

1050

1200

1350

-12.0

1500

1.005

1.010

Temperature (K)

1.015 -1

2.0 1.025

1.020

1000/Tp(K )

Fig. 3. High temperatures DSC curves for the alloy. Fig. 4. Kissinger and Ozawa plots for the alloy.

EDS analysis confirmed the presence of a-Fe, FeCo, Fe2Si and Fe14BY2 phases. High temperature DSC experiments were conducted and the results are plotted in Fig. 3 for heating rates 20 and 40 K/min, which revealed an endothermic reaction, characteristic of the glass transition followed by a multiple exothermic event corresponding to the multistage crystallization processes in the alloy. At high temperature two endothermic peaks showing melting and liquid transitions are also evident. The parameters Tg, Tx1, Tp1, Tm and Tl were determined from the DSC plots and are given in Table 1(a). Using these values a number of thermal parameters like DTx, Trg, c, d and b were evaluated and are summarized in Table 1(a). The maximum values of the thermal parameters Tg/Tm, Tg/Tl, c and d for the present alloy were found to be 0.658, 0.639, 0.407 and 1.84, respectively, which are superior to those calculated for more than 20 Febased BMGs reported by Gu et al. [24]. In addition many other thermal parameters like KH [15], KSP [16], KW, KLL, K1, K2, K3 and K4 [17] were also calculated and are given in Table 1(b). All the calculated thermal parameters are much better than those reported for FeCoBSiNbCr and FeCSiBPCrMoAl BMGs alloys [25,26]. The thermal stability of BMGs can be estimated from the activation energy (Eac) of crystallization. The Eac was evaluated using Kissinger equation lnðr=T 2p Þ ¼ Eac =RT p + constant and Ozawa equation ln(r) = Eac/Tp + constant [27], where Tp is the peak temperature at heating rate ‘r’ and R is the real gas constant 8.3145 J/mol K. Kissinger and Ozawa plots drawn in Fig. 4, are straight lines with slope Eac/R = B, where B is a constant. Putting the values of R and B (determined from Kissinger and Ozawa plots), the

activation energies were calculated and the values are found to be 606.6 and 623.1 kJ/mol for first stage crystallization by Kissinger and Ozawa equations, respectively. The corresponding activation energies for the second stage crystallization are found to be 462.3 and 480.3 kJ/mol. The high values of activation energy indicate high thermal stability against crystallization. Vicker’s hardness (HV) of the as-cast and heat-treated samples is given in Table 2. The average value of HV of the as-cast alloy was found to be 1048 while maximum hardness value of 1294 was obtained for the sample heat-treated at 1123 K for 20 min. Vicker’s hardness in the present case is much better than the values reported for Fe48Cr15C15Mo14B6Er2 [28] and Fe41Cr15C15Mo14Co7B6Y2 bulk amorphous alloys [29]. The results on nanohardness (H), elastic modulus (E), ratio H/E, maximum displacement of the nanoindenter (hmax), final indentation depth (hf), contact depth (hc), displacement of the surface at the perimeter of the contact (hs) elastic recovery hf/hmax and percentage elastic recovery of displacement on unloading %R = [(hmax  hf)/hmax) * 100%] for the ascast BMG alloy and heat-treated samples are summarized in Table 2. Hardness and elastic moduli of the annealed samples are found to be higher than the as-cast samples. The enhancement in hardness and elastic moduli in the annealed samples is due to the nucleation of crystallization phases, which act as obstacles to the dislocation movements [30,31]. Loading and unloading (P  h) curves for the as-cast alloy and annealed samples are shown in Fig. 5, which reveals pop-in marks (displacement discontinuities) in the loading curves of indentation and pop-out marks in unload-

Table 1(a) Thermal parameters of the Fe60B15Zr10Co7Mo5.5Y2Si0.5 alloy measured at different heating rates ‘r’ r (K/min)

Tg

Tx1

Tp1

DTx

Tm

Tl

Trg1 = Tg/Tm

Trg2 = Tg/Tl

c

d

b

10 20 40

926 935 946

967 976 987

977 986 995

41 41 41

– 1440 1438

– 1475 1481

– 0.649 0.658

– 0.634 0.639

– 0.405 0.407

– 1.80 1.84

– 3.67 3.83

All temperatures are in K.

Table 1(b) Few more thermal parameters of the alloy r (K/min)

Tx2

Tp2

K1

K2

K3

K4

KH

KW

KLL

KSP

10 20 40

1077 1082 1096

1084 1095 1112

– 505 492

41 41 41

– 0.678 0.686

– 0.285 0.228

– 0.088 0.091

– 0.029 0.029

– 0.411 0.414

0.443 0.443 0.347

All temperatures are in K. Maximum error in temperature measurements is 61%.

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Table 2 Vicker’s hardness HV, elastic modulus E (GPa), nanohardness H (GPa) and nanoindentation parameters of the as-cast and heat-treated samples Sample

HV (±20)

H (±0.2)

E (±5)

H/E

hmax (nm)

hf (nm)

hc (nm)

hs (nm)

hf/hmax

%R

As-cast 953 K 1123 K

1048 1158 1294

14.9 16.4 18.4

240.6 251.3 279.6

0.06193 0.06526 0.06581

201.6 195.8 187.4

142.2 138.4 129.3

164.2 158.5 149.8

37.4 37.3 37.6

0.705 0.707 0.690

29.5 29.3 31.0

12

As-cast 953K/20 min 1123 K/20 min

Load (mN)

10 8

Pop-in mark Pop-out

6

Pmax

Loading curves 4 2 Pop-in mark 0

0

50

Unloading curves

hf hmax 100

150

200

250

Penetration depth (h in nm) Fig. 5. Loading and unloading (P–h) curves of the as-cast and annealed samples.

ing curves. This observation is consistent to that reported previously by many researchers [31–33] in BMGs. The higher slope of the unloading curves indicates a higher stiffness or elastic modulus [32,33]. It is clear that with increasing annealing temperature, penetration depth (h) decreases indicating increase in hardness. 4. Discussion A new bulk amorphous Fe60B15Zr10Co7Mo5.5Y2Si0.5 alloy of 2 mm thickness was synthesized by Cu mold casting technique to investigate the thermal and mechanical properties. High temperature DSC analysis revealed that the alloy has very high glass transition temperature similar to the other Fe-based BMGs. Although the supercooled liquid region for the alloy is small, most of the thermal parameters given in Table 1(a) and (b) indicate very good glass-forming ability of the alloy. For example, the maximum value of Trg(Tg/Tm) was found to be 0.66 that is much higher than those reported for other Fe-based BMGs. The c parameter was found to be better compared with other Fe-based BMGs [34–36]. The values of d parameter was also found to be 1.82 ± 0.02, which is higher compared with other BMGs [13]. The b parameter was calculated too and the value of 3.75 ± 0.08 indicate high GFA of the present alloy. The stability of the alloy was also investigated by evaluating the activation energy of crystallization. It was observed that the alloy undergoes two-stage crystallization with very high activation energies. The activation energy for the first stage crystallization was found to be above 600 kJ/mol, which is much higher than many other Fe-based glassy alloys [32,37]. The high value of activation energy also indicates that the alloy has a high thermal stability. The crystallization behavior of the alloy was also examined. For this purpose two temperatures, one above the glass transition and one above the first crystallization temperature, were chosen. The appearance of very small peaks in the XRD pattern at 953 K (Fig. 1) indicates that the major fraction of the material remains amorphous, which suggests the high thermal stability of the present alloy. The results on mechanical properties presented in Table 2 revealed that the present alloy has much high values of Vicker’s hardness, nanohardness and elastic modulus. The present results

on nanohardness H and elastic modulus E are comparable with Lu’s alloys [35] and better than those reported by Castellero et al. [38] for Fe-based BMGs. The annealing treatment enhanced the Vicker’s hardness and elastic moduli further up to 25% while the nanohardness increased by 16%. Fracture strength of the alloy has been estimated to be 4812 MPa using the empirical relationship between elastic modulus E and fracture strength rf [39] i.e. E/rf  50. This value is close to 4966 MPa determined from the relation rf = H/3. The maximum value of nanohardness to elastic modulus ratio (H/E) for the alloy is 0.066, which is comparable with that for bulk amorphous steels containing Y and Dy and many other BMGs [32,33]. The elastic recovery hf/hmax and %R [40,41] are two important parameters that can be derived from the nanohardness measurements. The limits of hf/hmax are 0 6 hf/hmax 6 1. The lower limit of elastic recovery corresponds to fully elastic deformation, whereas the upper limit is characteristic of rigid plastic materials, for which there is no elastic recovery. For comparison, the value of elastic recovery (hf/hmax) for glass and Al are 0.687 and 0.951, respectively [40]. The value of this ratio and %R are found to be 0.705 and 30, respectively, in the present case, which indicate that the alloy may be deformed elastically. 5. Conclusions The Fe60B15Zr10Co7Mo5.5Y2Si0.5 alloy has high hardness and elastic modulus indicating very attractive mechanical properties. The values of the reduced glass transition temperature Trg(Tg/Tm and Tg/Tl), c, d and b parameters were found to be (0.658, 0.639), 0.407, 1.84, and 3.83, respectively, indicating the good glass-forming ability of the alloy. The activation energies for the first and second stage crystallization processes were calculated to be 606.6 and 462.3 kJ/mol using Kissinger and 623.1 and 480.3 kJ/mol using Ozawa equations, respectively, indicating the high thermal stability of the alloy against crystallization. Phases like a-Fe, FeCo, Fe2Si and Fe14BY2 were identified in heat-treated samples leading to the enhancement of the mechanical properties. Acknowledgements The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (Grant No. 50471077), and the Ministry of Science and Technology of China (Grant No. 2006 CB 60521, 2005 DFA 50860). We are grateful to Professors W.S. Sun, A.M. Wang, H. Li, W. Wei, J.Z. Zhao and members of RDG and Diagnostic Laboratories Physics Division, PINSTECH for cooperation during the experimental work. M. Iqbal is also grateful to PAEC Pakistan for permission to avail the chance to do this work partially at IMR, Shenyang, China. References [1] A. Inoue, N. Nishiyama, MRS Bull. 32 (2007) 651. [2] W.L. Johnson, M.D. Demetriou, J.S. Harmon, M.L. Lind, K. Samwer, MRS Bull. 32 (2007) 644. [3] M.D. Demetriou, G. Duan, C. Veazey, K.D. Blauwe, W.L. Johnson, Scripta Mater. 57 (2007) 9. [4] A.R. Yavari, J.J. Lewandowski, J. Eckert, MRS Bull. 32 (2007) 635. [5] W.H. Wang, C. Dong, C.H. Shek, Prog. Mater. Sci. 44 (2007) 540. [6] A.L. Greer, E. Ma, MRS Bull. 32 (2007) 611.

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