Effects of alloying elements on glass formation, mechanical and soft-magnetic properties of Fe-based metallic glasses

Intermetallics 19 (2011) 1502e1508

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Effects of alloying elements on glass formation, mechanical and soft-magnetic properties of Fe-based metallic glasses Z.B. Jiao, H.X. Li, J.E. Gao, Y. Wu, Z.P. Lu* State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2011 Accepted 24 May 2011 Available online 14 June 2011

Effects of alloying additions on glass formation, mechanical and soft-magnetic properties of Fe-(Si,P,C,B)based bulk metallic glasses (BMGs) were systemically studied in detail. It was found that the glassforming ability (GFA) and the optimum doping content strongly depend on the electronegativity of the alloying elements, which are discussed in terms of liquid phase stability and crystallization resistance of the competing crystalline phases. These BMGs exhibit high fracture strength ranging from 2800 to 3800 MPa, which closely relates to the atomic size distribution in the alloys. Furthermore, appropriate additions of Co, Ga and Cu could improve not only the GFA but also the saturation magnetization due to different coupling mechanisms. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: B. glasses, metallic B. microalloying B. mechanical properties at ambient temperature B. magnetic properties

1. Introduction Fe-based bulk metallic glasses (BMGs), also known as bulk amorphous steels, have attracted considerable attention due to not only their unique combination of excellent magnetic properties, high fracture strength, high hardness and good corrosion resistance, but also the low cost resulting from plentiful nature resources of iron on the earth [1e6]. However, engineering commercialization of Febased BMGs as industrial soft-magnetic materials is hindered by their low glass-forming ability (GFA), limited plasticity and unsatisfactory saturation magnetization. Recently, it is found that minor addition of large rare-earth elements (RE) is an effective approach to improving the GFA and thermal stability in Fe-based BMGs [7e9]. As such, scientific emphases were subsequently placed on understanding roles of RE elements in glass formation of these Fe-based BMGs with extraordinary GFA. Unfortunately, these RE-doped Febased BMGs are non-magnetic and extremely brittle at room temperature. Meanwhile, available experimental data already revealed that alloying of metallic elements could greatly influence glass formation, mechanical and magnetic properties of the Fe-based BMGs. For example, the maximum size for glass formation in the FeCr-Mo-Y-(B,C) system is increased from 12 to 16 mm via substitute Fe with 7% Co in the alloy [10]. In addition, the strength of Fe-B-based alloy with additions of Nb reaches 4.85 GPa while that with Zr * Corresponding author. Tel.: þ86 10 82375387; fax: þ86 10 62333447. E-mail address: [email protected] (Z.P. Lu). 0966-9795/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2011.05.020

additions is 4.15 GPa [11]. However, alloying effects of metallic elements in the Fe-based BMGs are far from being fully understood. In order to enable bulk glass formation, multiple metallic elements have to add simultaneously. Consequently, currently available systems are not feasible for investigating alloying effects of single metallic element because it is hard to differentiate individual contributions from each dopant. Recently, a relatively simple pseudobinary Fe-(Si,P,C,B) system with a maximum diameter of 1 mm for glass formation (i.e., Fe76Si3.3P8.7C7.0B5.0, in atomic percentage) has been developed [12], which provides a prototype for unraveling the alloying effects of individual metallic elements on GFA, mechanical and magnetic properties in the Fe-based BMGs. Moreover, the selected base glass contains a high percentage of Fe contents (>75 at.%) without expensive RE constituents, and can be prepared using industrial-grade raw materials, which further reduces its production cost. More importantly, the base alloy exhibited promising soft-magnetic properties and had a great potential to be commercialized as engineering materials [12]. In this paper, we will report our experimental results about effects of alloying additions including Al, Ti, V, Cr, Mn, Co, Ni, Cu, Ga, Zr, Nb, Mo, and Ta on glass formation, mechanical and soft-magnetic properties in the simple Fe-(Si,P,C,B) system. The current findings could better our understanding on the roles of metallic elements in glass formation, mechanical and soft-magnetic properties, and more broadly, advance the alloy design technology of the soft-magnetic BMGs with high Fe concentrations.

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2. Experimental Multi-component Fe-based alloy ingots with compositions of Fe76-xSi3.3P8.7C7.0B5.0Mx (M ¼ Al, Ti, V, Cr, Mn, Co, Ni, Cu, Ga, Zr, Nb, Mo, and Ta, x ¼ 0e10 at. %) were prepared by arc-melting a mixture of pure Fe, M, Si, C, B (>99%) and Fe-P master alloy (P: 26.75, Si: 1.42, C: 0.37, S: 0.035, Mn: 0.32, wt. %) in a Ti-gettered argon atmosphere. Cylindrical rods with diameters ranging from 0.5 to 6 mm were fabricated by copper mold casting. Amorphous nature of the as-cast samples was examined by X-ray diffraction (XRD) with Cu Ka radiation and transmission electronic microscopy (TEM) with a JEOL-2100 microscopy. The TEM specimens were prepared by low energy ion milling. Thermal properties associated with glass transition, supercooled liquid region, crystallization and melting behavior were investigated by a differential scanning calorimeter (DSC) at a heating rate of 0.333 K/s. The total scattering patterns of three representative glasses were obtained using high energy x-ray diffraction (HEXRD) at Advanced Photon Source of Argonne National Laboratory. Room-temperature compression tests were conducted at a strain rate of 104 s1 for the glassy rod specimens with an aspect ratio of 2. Density of the as-cast amorphous rods was measured by the Archimedean principle. Magnetic properties of saturation magnetization (Ms) and coercive force (Hc) were measured with a vibrating sample magnetometer (VSM) under an applied field of 800 kA/m and a DC B-H loop tracer, respectively. 3. Results 3.1. Glass formation and thermal properties To investigate effects of alloying additions on glass formation, the maximum attainable diameter (Dmax) for fully amorphous rods and the optimum doping content were identified by both XRD and SEM. As an example, Fig. 1a shows XRD patterns of the as-cast Fe76-xSi3.3P8.7C7.0B5.0Mox (x ¼ 1, 3 and 5) alloys with different diameters. For the alloy with 1% Mo, the specimen in a 4 mm diameter shows several sharp crystalline peaks superimposed on a main halo, suggesting that this sample is partially amorphous with considerable fraction of crystalline phases. For the alloy containing 3% Mo, XRD trace of the 5 mm sample reveals only a broad diffuse peak without any evidence of crystalline phases, and its glassy structure was further confirmed by TEM observations in the central region, as shown in Fig. 1b. The bright-field image reveals a modulated contrast, and the selected-area electron diffraction (SAED) pattern also consists of only halo rings, indicating the formation of a single glassy structure. Nevertheless, crystalline diffraction peaks appeared again as the casting size is increased to 6 mm, indicating that GFA of this alloy is enhanced and the critical diameter for glass formation is 5 mm. However, with the further increase of Mo, some sharp crystalline peaks superimposed on the main halo were seen for the 4 mm sample, implying that this sample has partially amorphous structure. Based on the above observation, one can conclude that the optimum doping content of Mo in this system is around 3 at.% and the maximum size for glass formation in the Mo-containing alloys is 5 mm. Similarly, critical diameters of alloys with different amounts of Al, Ti, V, Cr, Mn, Co, Ni, Cu, Ga, Zr, Nb, and Ta were studied, and Fig. 2 shows the critical diameter for glass formation as a function of doping content for each alloying element. Then, the maximum diameter for glass formation and the optimum doping content for alloys added each element could be obtained, as tabulated in Table 1. Fig. 3a shows DSC curves of the optimally doped alloys cast with the critical diameters. For comparison, DSC trace of the base alloy

Fig. 1. (a) XRD patterns of the as-cast rods for the Fe76-xSi3.3P8.7C7.0B5.0Mox (x ¼ 1, 3 and 5) alloys and (b) The TEM image and corresponding SAED pattern of the as-cast Fe73Si3.3P8.7C7.0B5.0Mo3 alloy with a diameter of 5 mm.

without any added elements is also included. The glass transition temperature Tg, onset crystallization temperature Tx, supercooled liquid region DTx, liquidus temperature Tl, and GFA parameters Trg [13] and g [14,15] for all investigated alloys are also listed in Table 1. As compared with the base alloy, both Tg and Tx of the alloy doped with 3% Mo are obviously increased, yielding a slight increment of the supercooled liquid region DTx. For the other alloying additions, no significant changes in thermal stability have been observed. In addition, the largest values of Trg and g are also obtained in the alloy with 3% Mo, suggesting that this alloy has the best GFA among all the investigated alloys, which is consistent with the Dmax value determined experimentally. Fig. 3b exhibits melting behavior of the alloys doped with the optimum amount of metallic elements. For comparison, the corresponding result for the base alloy is also included. It can be seen that alloying elements have different influences on the resultant Tl value (Table 1) and the melting behaviors. When elements of Zr, Mn, Ti, V, Nb, Ta, Cr and Al are added, Tl does not change remarkably and is similar to that of the base alloy (w1280 K). With additions of Ga, Co, Ni and Cu, the alloy shows a slightly lower Tl value of 1260 K and a much simpler melting process It is noted that the alloy

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Dmax (mm)

a

3.2. Mechanical properties

Mo

5

Compression tests of the optimally doped alloys were carried out at room temperature, and the corresponding stress-strain curves are shown in Fig. 4. The testing specimens of the Mndoped alloy were 1 mm in diameter due to the GFA limitation, and all the others were 1.5 mm rods. All the present BMGs exhibit similar fracture features, i.e., elastic deformation up to a strain of about 2%, followed by final catastrophic fracture, except for the alloys doped with 3% Mo or 1% Ga which exhibited small plasticity of 0.43 and 0.25%, respectively. The Mo-added alloy shows the highest fracture strength w3800 MPa, followed by the alloys containing Ti (w3600 MPa) and Ga (w3400 MPa); the rest are around 3000 MPa.

4 Cu 3

2 Ni

Co

1 0

2

4 6 Doping Content (%)

8

10

b

Dmax (mm)

3

2

Ga Al,Cr

1

Mn Nb

Ti,V,Ta

Zr 0

0

1

2 3 Doping Content (%)

4

Fig. 2. Critical size for glass formation in the Fe-Si-P-C-B-M alloys as a function of the doping content. (a) M ¼ Co, Ni, Cu and Mo; (b) M ¼ Al, Ti, V, Cr, Mn, Ga, Zr, Nb and Ta.

3.3. Soft-magnetic properties As an example, Fig. 5a illustrates the magnetic hysteresis loops of the Fe76-xSi3.3P8.7C7.0B5.0Cox (x ¼ 0, 1, 3, 5, 7 and 9) alloys. The corresponding Ms of the Fe-Si-P-C-B-M (M ¼ Mo, Co, Ga and Cu) alloys as a function of doping contents is summarized in Fig. 5b. As can be seen, the Ms monotonically decreases from 1.52 to 1.10 T with the substitution of Mo for Fe from 0 to 5 at.%. While for the Cocontaining alloys, Ms increases from 1.52 to 1.56 T when 1% Co addition is added, and gradually decreases to 1.48 T upon further increasing Co content to 9%. Similar to the Co additions, proper additions of Ga and Cu initially increase the Ms value, but excessive additions of these elements decrease Ms. It is worth pointing out that the alloy with 0.7% Cu addition exhibits the highest Ms of 1.61 T in the investigated alloys, which is also much higher than that of the other amorphous alloys reported in literature. The coercive force Hc of all the Fe-Si-P-C-B (M ¼ Mo, Co, Ga and Cu, x ¼ 0e9 at.%) alloys was also measured and found to be smaller than 10 A/m. It can be concluded that the present as-cast Fe-based BMGs simultaneously possess high GFA and excellent magnetic properties, and thereby are viable for practical engineering applications. 4. Discussion

containing 3% Mo, which has the highest GFA, shows the lowest Tl of 1252 K. Compared with those doped by Zr, Mn, Ti, V, Nb, Ta, Cr and Al, the alloys with Ga, Co, Ni and Cu additions are closer to the eutectic. In particular, the alloy containing 3% Mo seems to associate with a deeper eutectic, which might be responsible for its highest GFA [3,16,17].

Table 1 Thermal properties and glass-forming ability of the optimally doped Fe-Si-P-C-B-M alloys. Alloying element

Doping Dmax(mm) Tg(K) Tx(K) DTx(K) Tl(K) Trg content (at.%)

Base alloy e Al 1.0 Ti 1.0 V 1.0 Cr 1.0 Mn 0.5 Co 7.0 Ni 5.0 Cu 0.3 Ga 1.0 Zr 0.5 Nb 0.5 Mo 3.0 Ta 1.0

1.0 2.0 1.5 1.5 2.0 1.0 3.0 3.0 3.0 3.0 0.5 1.5 5.0 1.5

747 761 760 758 759 757 749 746 746 752 763 760 768 756

766 779 778 773 776 772 763 764 763 767 780 778 788 772

18 18 18 15 17 15 14 18 17 15 17 18 20 16

1272 1266 1278 1278 1282 1280 1258 1259 1263 1259 1277 1276 1252 1278

0.587 0.601 0.595 0.593 0.592 0.591 0.595 0.593 0.591 0.597 0.597 0.596 0.613 0.592

g 0.379 0.384 0.382 0.380 0.380 0.379 0.380 0.381 0.380 0.381 0.382 0.382 0.390 0.380

4.1. Alloying effects on glass formation 4.1.1. Correlation between the electronegativity and the glassforming ability As noted above, alloys doped with different metallic elements have apparently different GFA. It is known that these alloying elements locate closely in the period table, and the distinct difference between these alloying elements is their electronegativity which ranges from 1.33 to 2.16 (ref. [18]). Fig. 6a exhibits the critical diameter for glass formation and the electronegativity for all alloying elements investigated. Interestingly, it was found that the variation of Dmax was consistent with that of the electronegativity of alloying elements, which can be seen more directly by their correlation as shown in Fig. 6b. As illustrated, Dmax increases linearly with electronegativity of the alloying element. In other words, the GFA is strongly depended on the electronegativity of the alloying elements in the current Fe-based metallic glasses. To understand the above correlation, it is important to consider chemical bonding between constituent elements in the alloy with the GFA because electronegativity is directly related to the chemical bonding which has been confirmed to be critical for glass formation [19]. Chen et al. [20] have pointed out that electrons could transfer from metalloids such as P, B, C and Si to metals, and then a strong covalent bonding would be formed between them. It can be understood that by adding metals in the base alloy, new covalent

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Fig. 3. DSC heating traces (a) and melting behaviors (b) of the optimally doped Fe-Si-P-C-B-M alloys.

4.1.2. Effect of the electronegativity on the optimum doping contents Fig. 7 exhibits the optimum doping contents as a function of electronegativity differences between Fe and an individual alloying element M, i.e., Dx ¼ (xMexFe), where xM and xFe are the electronegativity of the alloying element and Fe, respectively. Generally, the larger the absolute value of the electronegativity difference Dx, the lower the optimum doping content should be, and vice versa. In other words, alloy elements with a similar electronegativity with Fe (e.g., Co and Ni) can be doped with a high amount. However, the optimum doping content of the alloying elements with a larger

2.0 1.5 1.0 0.5 0.0

Ti Al

Co0 (1.52T) Co1 (1.56T) Co3 (1.54T) Co5 (1.52T) Co7 (1.50T) Co9 (1.48T)

-0.5 -1.0 -1.5 -2.0

-800

-400

b

Cr Mn Co

Ni

Cu

Nb

400

800

Cu

1.6

Co

1.5 Ga

Ga V

0

Magnetic field (kA/m)

Ta

1.4 Ms (T)

Stress (MPa)

a

Mo

4000

3000

electronegativity than Fe, i.e., a positive Dx value, is much higher than that of the alloying elements with Dx < 0. The above correlation could be understood in terms of liquid phase stability and crystallization resistance of competing crystalline phases [23]. Firstly, for the alloy elements having a marginal

Magnetization (T)

bonding between alloying metals and metalloids could be generated. During solidification, however, it is necessary to break these metal-metalloid bonds so that competing primary phase can be formed. With the increase of the electronegativity, the metalmetalloids covalent bonding nature will become stronger because the ability of metals to attract electrons from metalloids is enforced according to the Pauling’s theory [21]. Thus, the liquid phase stability is enhanced and formation of primary phases is slowed down, leading to large GFA. Similar phenomenon was also observed in Al-based conventional metallic glasses where the liquid phase stability increases almost linearly with the electronegativity of rare-earth elements [22].

1.3 1.2

2000

Mo 1.1

1000

0

Strain Fig. 4. Engineering stress-strain curves of the optimally doped Fe-Si-P-C-B-M alloys.

4

6

8

10

Doping content (%)

2% 0

2

Fig. 5. (a) Typical magnetic hysteresis loops of 1-mm amorphous rods of the Fe76(x ¼ 1, 3, 5, 7 and 9) alloys, and (b) the saturation magnetization Ms as a function of doping contents for the Fe-Si-P-C-B-M (M ¼ Mo, Co, Ga and Cu) alloys.

xSi3.3P8.7C7.0B5.0Cox

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a Dmax (mm)

5 4 3 2 1

Electronegativity

0

2.2 2.0 1.8 1.6 1.4 1.2 Al Ti V Cr Mn Co Ni Cu Ga Zr Nb Mo Ta Alloying element

b

4.2. Alloying effects on mechanical properties

5

Yang et al. [25] have pointed out that there exist remarkable similarities between the physical processes of plastic deformation and glass transition of glassy materials, and derived a simple equation between fracture strength and glass transition temperature normalized with molar volume, as described below:

Dmax (mm)

4 3 2

sf ¼ 55

1 0 1.2

Secondly, from a chemical bonding point of view, for elements with an electronegativity value larger than that of Fe, their proper additions could tighten the atomic structure of supercooled liquid and increase the atomic packing density, which would lower the ground-state energy of the supercooled liquid and thus stabilize the supercooled liquid [24]. While for elements with an electronegativity value smaller than that of Fe, i.e., Dx < 0, their additions would tend to destabilize the liquid phase because of the formation of weak bonding atomic pairs. Consequently, the optimum doping content of these elements is usually small (below 1.5% in the current base alloy) and relatively lower than that of the elements with an electronegativity difference Dx > 0. In addition, it is worth pointing out that copper has a slightly larger electronegativity than Fe but its optimum doping content is very low (0.3%). The main reason is that copper has a positive heat of mixing with Fe and thus is hardly miscible with Fe, which limits its addition content.

1.4

1.6 1.8 Electronegativity

2.0

2.2

Fig. 6. (a) Dmax of the Fe-Si-P-C-B-M alloys and the electronegativity of alloying elements M; (b) Relationship between Dmax and the electronegativity of alloying elements. The lines are used to guide the eye.

electronegativity difference with Fe, their small additions would have limited impact on the GFA because of the high tendency for forming solid-solutions [24]. For atoms having a large positive or negative electronegativity difference Dx with Fe, however, a large amount of their additions would destabilize the liquid phase and facilitate the compound formation (i.e., new competing crystalline phases). Thus, their optimum doping content is normally low.

DTg V

(1)

where sf is the fracture strength, DTg is the temperature difference between glass transition temperature Tg and ambient temperature, and V is the molar volume. Fig. 8a shows sf of the current Fe-based BMGs as a function of DTg/V. Although there is a general tendency for sf to increase with the increase of DTg/V, large scattering in the data is also observed. The relationship shown in Eq. (1) is based on an assumption that plastic deformation and glass formation are activated by similar basic units (i.e., the shear transformation zones) [25]. In the brittle Fe-based BMGs, however, the final fracture actually occurs before the occurrence of macroscopic yielding since their failure behavior is virtually controlled by the critical distensile stress of the materials [26], which is different from that in the ductile BMGs fractured via the shearing mode. Therefore, there must exist the other factors controlling the fracture behavior of the current Fe-based BMGs. Essentially, the fracture strength of BMGs is believed to be directly associated with the atomic bonding due to the lack of crystallographic defects. As such, both of the bonding length and bonding strength are crucial factors determining the fracture strength. In the present BMGs, all the alloying metallic elements are larger than the base elements. In other words, additions of these alloying elements could cause more sequential change in the atomic size on the order of M > Fe >> Metalloids and then enlarge the atomic size distribution. The atomic size distribution can be described by [24]

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n  uX r 2 d ¼ t Ci , 1  i r i¼1

(2)

where ri is the atomic radius of element i, and r is the mean radius for a compound and can be calculated as follows:

r ¼

n X

Ci ,ri

(3)

i¼1

Fig. 7. The optimum doping content versus electronegativity difference Dx between the alloying metallic element and Fe.

Fig. 8b shows the correlation between the fracture strength sf and the atomic size distribution d for the current Fe-based BMGs. It

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a

1507

moments; hence, the resultant Ms would be significantly influenced by the doped elements and their concentrations. Below, effects of Mo, Ga, Co and Cu metals on Ms will be discussed in terms of their contribution on the total magnetic moment. 4.3.1. Diluting effect Cobalt is a ferromagnetic element while copper, molybdenum and gallium are non-magnetic. Cobalt has an atomic magnetic moment of 1.7 mB, which is lower than that of pure Fe. Therefore, additions of these metals into the base alloy would dilute the total Fe concentration and thus decrease the total atomic magnetic moment.

b

Fig. 8. Correlation of the compressive fracture strength sf with DTg/V (a), and atomic size distribution d (b) for the studied samples.

is found that the fracture strength sf increases linearly with the increase of the atomic size distribution d. Effects of atomic size distributions on the fracture strength can be associated with the following two aspects: (i) a wider atomic size distribution can enhance the topological ordering, leading to the formation of a highly dense random packed structure [27]. As a result, the alloy has good ability to resist from shearing deformation and fracture under uniaxial compression and thereby possesses higher fracture strength, and (ii) a denser packing structure also indicates that bonding lengths between adjacent atoms are shorter, giving stronger bonding forces among constitute elements [28]; consequently, the sample can withstand higher compressive load before fracture. Based on the above analyses, it can be concluded that in present BMGs the wider of the atomic size distribution, the higher of the fracture strength, and vice versa.

4.3. Alloying effects on the saturation magnetization The magnetic moment is an important factor characterizing the magnetism of materials. In this study, the ferromagnetism of the base alloy is determined mainly by the magnetic moment of Fe atoms and thus the total Fe content in the alloy. By doping metallic elements into the base alloy, the valence electronic structure and magnetic moment alignment of the added elements as well as their electronic interacton with Fe will markedly affect the magnetic

4.3.2. Coupling effect In addition to the above diluting effect, elements Mo, Co, Cu and Ga also have strong interaction with Fe and thus change the atomic magnetic moment of the Fe atoms. It has been found that the exchange interaction between Mo and Fe atoms is an ‘antiferromagnetic’ type in metallic glasses [29]. In the current alloy system, additions of Mo introduce antiferromagnetic moments into the alloys and cancel out some ferromagnetic moments of Fe, leading to the reduction in the ferromagnetic moments. Therefore, the Ms value monotonously decreases from 1.52 to 1.10 T with additions of Mo, as shown in Fig. 5b. Previous studies also show that cobalt has a very strong ferromagnetic exchange coupling with Fe and additions of Co could lead to alignment of the Fe moments and thereby increase the atomic magnetic moment of individual Fe atoms [30,31]. According to the Slater’s theory, the 3d bands of Fe can be divided into 3dþ and 3d sub-bands due to the interaction of electrons. It was found that by doping Ga, the valence electrons of Ga first filled into the 3dþ sub-bands of Fe atoms, leading to a larger difference between 3dþ and 3d sub-bands [32]. As a result, the atomic magnetic moment of Fe increased as the Ga content rises, giving rise to the increase of the Ms. However, with more electrons filling into the 3dþ sub-bands, the number of positive spin electrons tends to reach the maximum value. Subsequently, the valence electrons of Ga starts to fill into the 3d sub-bands and shorten the difference between 3dþ and 3d sub-bands of Fe. Consequently, the magnetic moment of Fe would decrease due to the excessive Ga doping and so did the Ms. The heat of mixing between Cu and Fe is positive [33], suggesting that Fe and Cu atoms would impulse each other in the Cudoped alloys. Proper additions of Cu could promote the bonding of FeeFe pairs, leading to the increasing of the nearest-neighbor Fe atoms [34]. According to Heisenberg [35], the atomic magnetic moment is assumed to depend on the number of Fe atoms in the nearest-neighbor shell, i.e., a higher number of the nearestneighbor Fe atoms can lead to a higher atomic magnetic moment. As discussed above, the Ms value is simultaneously determined by the two opposite effects on the atomic magnetic moment in all the Co-, Ga- and Cu-doped metallic glasses. The competition between the diluting and coupling effects gives rise to the maximum Ms. For example, when 1% Co is added, effect of Co on promoting the alignment of Fe moments is overriding, thereby raising the total atomic magnetic moment and the Ms. With the further increase of Co, the Fe content continues to decline and the diluting effect becomes dominant, thus decreasing the Ms.

5. Conclusions Effects of metallic elements including Al, Ti, V, Cr, Mn, Co, Ni, Cu, Ga, Zr, Nb, Mo and Ta on the glass formation, mechanical and softmagnetic properties in a Fe-based soft-magnetic BMG system were

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systematically investigated. The main results are summarized as follows: (1) The GFA of the Fe-Si-P-C-B-M alloys was found to strongly depend on the electronegativity of the alloying element. Elements with a large electronegativity value could effectively enhance the GFA due to the enhanced liquid phase stability and crystallization resistance. In addition, it was found that the optimum doping content of alloying elements is mainly determined by the electronegativity difference between the alloying element and Fe. (2) The fracture strength linearly correlated with the atomic size distribution. A wide atomic size distribution is effective in increasing the fracture strength due possibly to the strong atomic bonds resulted from the dense atomic packing structures. (3) Influences of Mo, Co, Ga and Cu on the maximum saturation Ms are twofold: diluting and coupling effects. Molybdenum antiferromagnetically couples with Fe and its additions monotonously decrease the atomic magnetic moment and reduce Ms. Proper additions of Co, Ga, and Cu could increase the atomic magnetic moment of Fe atoms, thus raising the Ms; however, excessive additions decrease the Ms due to the diluting effect resulting from the concentration decrement of Fe atoms. Acknowledgments This research was supported by National Natural Science Foundation of China (Grant Nos. 50725104, 50841023), the 973 program under the contract number of 2007CB613903 and China Postdoctoral Science Foundation (20080430019).

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