A. Dependence of GFA on casting atmosphere species

Figure 1 shows the XRD patterns of as-cast Fe₇₆Si₉B₁₀P₅ rods prepared in argon, air, and helium atmospheres under chamber pressure of 9 × 10⁴ Pa, respectively. The pattern of the rod specimen with a diameter of 1.5 mm fabricated in argon atmosphere shows crystalline diffraction peaks identified as Fe₃(B,P) and α-Fe phases, suggesting that this sample is a partially amorphous structure. On the contrary, for the sample with a diameter of 2.5 mm prepared in air, it can be seen that the pattern consists of only a broad amorphous peak and has no evidence of the existence of crystalline phase. Apparently, these results show that the GFA of Fe₇₆Si₉B₁₀P₅ alloy increases obviously in air condition compared with that in Ar atmosphere. Here, it is noteworthy that the unusual enhancement of GFA has not been reported for any alloy so far.

FIG. 1.

XRD patterns of as-cast Fe₇₆Si₉B₁₀P₅ rods prepared in argon, air, and helium atmospheres, respectively.

Kato et al. have reported that helium as the casting atmosphere can lead to an improvement of cooling effect, especially in the low temperature range. ^{Reference Setyawan, Kato, Saida and Inoue25,Reference Setyawan, Kato, Saida and Inoue26} The knowledge has been applied to improve the GFA of Zr₆₅Al_7.5Ni₁₀Pd_17.5 glassy alloy, and the D _cr was increased from 5 mm in argon atmosphere to 7 mm in helium. The reason is attributed to the higher thermal conductivity of helium compared with that of argon during the low temperature range. ^{Reference Ito, Ueki and Nakamura27} However, the XRD pattern of the as-cast Fe₇₆Si₉B₁₀P₅ rod with a diameter of 1.5 mm prepared in helium atmosphere shows strong diffraction peaks characterized as Fe₃(B,P) and α-Fe phases compared with the one fabricated in air that presents only amorphous characteristic even with a larger diameter. This demonstrates that the better thermal conductivity of chamber atmosphere does not attribute to the unusual enhancement in GFA of Fe–Si–B–P alloys. Therefore, we can fairly conclude that oxygen or nitrogen in air may lead to the significant change in GFA since the main compositions of air are oxygen (about 21% by volume) and nitrogen (about 78% by volume).

To elucidate the effect of oxygen and nitrogen on the GFA, the Fe₇₆Si₉B₁₀P₅ alloy rods synthesized in the two atmospheres were prepared subsequently. Figures 2(a) and 2(b) show the XRD patterns of as-cast Fe₇₆Si₉B₁₀P₅ rods prepared in nitrogen and oxygen atmospheres with various diameters under different chamber pressures, respectively. As shown in Fig. 2(a), the specimen with a diameter of 1.5 mm cast in nitrogen atmosphere under a pressure of 3 × 10⁴ Pa exhibits sharp diffraction peaks superimposed on a broad halo, implying the existence of crystalline phases. When the pressure of nitrogen increases to 9 × 10⁴ Pa, the diffraction peaks become weaker and disappear gradually. However, we cannot get a fully amorphous rod with a diameter of 2.0 mm in nitrogen atmosphere even under a pressure of 9 × 10⁴ Pa, revealing that the nitrogen casting atmosphere does not give an obvious enhancement in the D _cr of Fe₇₆Si₉B₁₀P₅ glassy alloy. On the contrary, as shown in Fig. 2(b), the structure of the as-cast 3 mm diameter Fe₇₆Si₉B₁₀P₅ rods changes successively from crystalline to fully glassy structure as the pressure of oxygen gradually increases from 3 × 10⁴ to 7 × 10⁴ Pa, indicating that the casting in oxygen atmosphere can significantly improve the GFA of the alloy. Moreover, the 3.5-mm-diameter sample cast under pressure of 9 × 10⁴ Pa, even though partially crystallized, still keeps very high fractions of amorphous phase which can be judged from the broad diffraction background in its XRD pattern. The fully amorphous nature of the 3-mm-diameter rod prepared in oxygen atmosphere is further confirmed by the homogeneous contrast in the high-resolution TEM image and the broad halo ring in the selected area electron diffraction (SAED) pattern, as shown in Fig. 3. Therefore, these results clearly suggest that oxygen is not always playing a negative role in improving the GFA, as previous reports argue that oxygen would increase the heterogeneous nucleation sites of oxide, which will deteriorate the GFA. ^{Reference Wang11,Reference Lu and Liu28} Figure 4 shows the hysteresis M-H loops of Fe₇₆Si₉B₁₀P₅ glassy alloys cast in argon, nitrogen, air, and oxygen. It can be found that these samples display similar M-H loop shape and exhibit similar values of I _s and H _c. The glassy alloy exhibits excellent soft magnetic properties, and its value of I _s is about 1.51 T and H _c is about 1.6 A/m, regardless of the applied casting atmosphere.

FIG. 2.

XRD patterns of as-cast Fe₇₆Si₉B₁₀P₅ rods with various diameters prepared in (a) nitrogen (N₂) and (b) oxygen (O₂) atmospheres.

FIG. 3.

High-resolution TEM image and corresponding SAED pattern taken from the cross section of the as-cast Fe₇₆Si₉B₁₀P₅ rod with a diameter of 3 mm prepared under oxygen atmosphere.

FIG. 4.

Hysteresis M-H loops of Fe₇₆Si₉B₁₀P₅ glassy alloys prepared in argon, nitrogen, air, and oxygen, respectively.

B. The influence of casting atmospheres on the component and thermal stabilities of Fe₇₆Si₉B₁₀P₅ glassy alloy

It has been reported that a proper doping of oxygen (0.1 at.%) can improve the GFA in some Fe- and Zr-based alloys. ^{Reference Wang, Yang, Lim and Li29,Reference Li, Gao, Jiao, Wu and Lu30} However, the mechanism for the effectiveness of casting atmosphere species on GFA of Fe₇₆Si₉B₁₀P₅ glassy alloy seems to be different from that of doping oxygen in the BMG systems. Table I shows the contents of oxygen and nitrogen in the master alloy and the BMG rods prepared in various casting atmospheres. The order of oxygen content is consistent with the previous reports. ^{Reference Meng, Li, Pang, Wang, An, Yin, Song and Wang31,Reference Wang, Zhang, Gebert, Roth and Schultz32} The difference in oxygen or nitrogen contents among the BMG samples prepared in various atmospheres is small enough to be ignored, suggesting that oxygen or nitrogen is not introduced into the alloy even casting in oxygen or nitrogen atmosphere. Thermal stabilities of the fully glassy rods with a diameter of 2 mm prepared in air and oxygen atmosphere were studied by DSC, together with the data for the melt-spun glassy alloy ribbon prepared in argon atmosphere (as shown in Fig. 5). No appreciable difference in T _g, T _x, or crystallization process is observed between the melt-spun ribbon and the rod samples, which provides supportive evidence for the results of oxygen and nitrogen content analyzed using the oxygen/nitrogen analyzer. From the above-described results and analyses, it is expected that oxygen in casting atmosphere appears to act as a kind of catalyst toward the enhancement of GFA for Fe₇₆Si₉B₁₀P₅ glassy alloy.

TABLE I.

Oxygen and nitrogen contents of the master alloy and as-cast Fe₇₆Si₉B₁₀P₅ glassy rods with a diameter of 2 mm prepared in various atmospheres.

FIG. 5.

DSC curves of as-cast Fe₇₆Si₉B₁₀P₅ glassy rods with a diameter of 2 mm prepared in air and oxygen atmospheres, together with the data for the melt-spun glassy ribbon prepared in argon atmosphere.

C. The underlying mechanism concerning the influence of casting atmospheres on GFA of Fe₇₆Si₉B₁₀P₅ glassy alloy

We try to understand the origin of the enhancement of GFA in oxygen atmosphere from thermodynamic and kinetic points. The heterogeneous nucleation rate ^{Reference Tumbull and Fisher33,Reference Turnbull34} can be defined as:

(1) $$I\; \propto\; D\exp \left( { - {{16{\rm{\pi \sigma }}_{{\rm{L}}-{\rm{C}}}^3 } \over {3kT({\rm{\Delta }}G_{{\rm{L}}-{\rm{C}}} )^2 }}\;f({\rm{\theta }})} \right)\quad ,$$

where D is the atomic diffusion coefficient, k the Boltzmann's constant, T the absolute temperature, ΔG _L–C the free energy difference between the liquid and crystal, f(θ) = (2 − 3cos θ + cos³θ)/4 the catalytic factor with θ as wetting angle, σ_L–C the liquid-crystalline interfacial tension. During the casting of BMGs in gas, the σ_L–C can be defined as:

(2) $${{\rm{\sigma }}_{{\rm{L}}-{\rm{C}}}} = {{{{\rm{\sigma }}_{{\rm{W}}-{\rm{G}}}} - {{\rm{\sigma }}_{{\rm{W}}-{\rm{C}}}} - {{\rm{\sigma }}_{{\rm{L}}-{\rm{G}}}}\cos \phi } \over {\cos {\rm{\theta }}}}\quad ,$$

where φ is the contact angle of liquid and σ_L–G is called the surface tension of liquid. The corresponding schematic diagram is shown in Fig. 6. It is known that surface tension is an important thermal parameter of a metal or alloy, and the value of the surface tension is a reflection of the nature of the surface of the specimen. Certain elements prefer to surface sites which are known as surface active components. And the presence of active components (even at concentrations of ppm) at the surface results in a dramatic decrease in surface tension. According to Eqs. (2) and (1), the change of σ_L–G induced by the species of the atmosphere can significantly vary, then the I (may in many orders of magnitude) ^{Reference Nastar and Clouet35} and the GFA. The thermal enthalpies between O and the elements Fe, Si, B, and P are −824, −910, −1272, and −2984 kJ/mol, respectively. These data show that the P is the most reactive among the four elements. It was reported that the σ_L–G of Fe–P alloy liquid decreases with an increase of phosphorus content, and contamination with oxygen or nitrogen will cause the lack of phosphorus on the Fe–P alloy surface, which can significantly increase the σ_L–G of Fe–P alloy. ^{Reference Xue, Jiang, Sui, Ding and Hu36} During the casting procedure in our study, the phosphorus on the free surface of the alloy melt is inevitably contaminated by the oxygen or nitrogen atmosphere, resulting in an increase of the σ_L–G of the alloy melt. The liquid-crystalline interfacial tension σ_L–C will increase as the φ > 90° in practice. Then the I will decrease based on Eq. (1). During the preparation of Fe₇₆Si₉B₁₀P₅ BMG, it was usually observed that if crystallization occurred, crystallites would be formed on the surface of the rod, which is similar to the previous report in FeSiBPC alloy system. ^{Reference Lavorato, Fiore, Tiberto, Baricco, Sirkin and Moya37} Hence, it can be deduced reasonably that if the nucleation on the surface was suppressed, the GFA of the whole bulk sample would be improved. The high phosphorus-scavenged ability of oxygen can explain reasonably why the composition of the alloy remains unchanged and consequently the magnetic properties are maintained.

FIG. 6.

Schematic diagram of heterogeneous nucleation. (W = wall, G = gas, L = liquid, C = crystal, θ = wetting angle, φ = contact angle of liquid.)