1.Introduction

The properties of the initial alloy melt show great influence on the subsequent solidification state in the fields of materials science and metallurgy because of the hereditary character of alloy melts[1–3].Unfortunately,the evolution of liquid metal at high temperature is known much less than their solid states.As a pearl quickly frozen from alloy melt,metallic glass materials have demonstrated their excellent soft-magnetic,mechanical and chemical properties[4–6],and have applications in the many fields of aerospace Engineering,Electricity transformation,electronic information,etc.,and have become one of the key functional materials to meet the demand of energy saving,carbon emission reduction,intelligent manufacturing and enhancement of industrial base[7–9].The influence of initial alloy melt is especially relevant to the preparation and property control of amorphous alloys.As amorphous alloys are known to be frozen from the liquid melt in a very short time of about 1 ms,the liquid structure of the alloy is maintained in the solid state to a maximal degree,and thus they are ideal candidates for exploring the merits and controlling the properties of solidified alloy by adjusting the conditions of the initial melt.

To provide a better understanding of initial melt on the their quenched glasses,the structural evolution of glass-forming melts has been investigated by nuclear magnetic resonance for La50Al35Ni15 melt[10],neutron diffraction for Fe[11]and Fe-B melt[12],Ge-Te melt[13], and synchrotron X-ray, including liquid metal X-ray diffractometer for Al-Si melt[14]and Fe-B melt[15],in situ X-ray diffraction for Pd-Ni-P and Zr-Nb-Cu-Ni-Al(VIT106)melt,extended X-ray absorption fine structure(EXAFS)for Co-Pd melt[16-18],in situ X-ray absorption near-edge structure[XANES]for Al,Mg K-edge[19]and Fe silicate melt[20].Through these methods,some interesting phenomena including the liquid-liquid transition were revealed[10,16].

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However,the structure information of Fe-based glass-forming alloy melt obtained from these direct detection instruments is still obscure,and there is a long way to go to explore the mysterious liquid.Reported here are three challenges reached by consensus during workshop discussions:1)The message concerning clusters,metastable phase or heterogeneity in liquid is usually too slight to be traced.Some argues that the clusters in liquid state appear and disappear instantly,which is due to the energy and composition fluctuations.2)For Fe-based glassforming alloy,the required high temperature range for the thermal treatment of the alloy melt is 1400–1900 K,which has led to great difficulties in obtaining experimental results,and thus,only limited data are available for study.3)The total content of metalloid atoms,such as B,Si,P and C atoms,is usually less than 10 wt%.Especially,the size of B atom,which is almost a permanent-contained element in Febased metallic glasses,is so small that its signal at high temperature is difficult to be traced.These difficulties leaded to the experimental conclusions that there is no clear difference between the structures at different temperatures[12,15,21,22].

The emergence of structural models for metallic glasses(MGs),including efficient cluster packing(ECP)of fcc/hcp lattices[23,24],icosahedral short-range order[25],quasi-equivalent clusters packing[26],spherical-periodic order[27],and the Bernal polyhedron[28],have shed the light on the secrets of local atomic structure of amorphous alloy and alloy melt.MGs and its melt can be considered as densely packed atomic clusters with short-range atomic order(SRO),which are constructed the medium-range orders(MROs)through the sharing of vertices,edges,faces and tetrahedral.The cluster packing scheme preferring fivefold symmetry is in fact intrinsically associated with the dense packing in amorphous materials,and is not uncommon in the literature for the packing of small clusters or particles[26,29].Both the short and medium-range atomic order of MGs have been confirmed by Hirata et al.by direct observation techniques [30].However,these models focused on effective spatial packing are not truly predictive because they do not directly account for the chemical contributions to the stability[23,31].

It is acceptable to treat the glass-forming alloy melt as cluster solute dispersed in ideal solution.On the one hand,these clusters in melt prevents the appearance of structurally different ordered phase[24],tends to stabilize the undercooled liquid[32,33],and lowers the driving forces for nucleation of the crystalline phase[34].On the other hand,cluster favors the crystallization,for it has the chance to promote the crystallization even in the undercooled melt[35,36]by participating in nucleation without breaking chemical bonds[37,38].So,further investigations on the effects of clusters are needed.

Although different microstructures have very close relations with the various properties in amorphous alloy systems,structural or/and chemical heterogeneity is always a mysterious phenomenon in multicomponent amorphous alloys.Viscosity investigations have suggested that the melt properties of some amorphous alloys cannot be maintained with changing temperatures[39,40],and this phenomenon is considered to be able to affect the properties of soft magnetism[41].Recent work has revealed the presence of liquid-liquid transitions in Albased amorphous alloys,including the structural polymorphism characteristics in Ce55Al45 metallic glass[42]and the liquid-liquid transition in the La50Al35Ni15 and VIT106 alloy melt[10,17].The existence of heterogeneous metastable phase in glass-forming alloy melt has been recognized and considered to influence the rapidly quenched alloys[43,44].

However,the detailed information,such as the size,microstructure and make-up of these metastable phases are still obscure.Although a proper thermal treatment of alloy melt above melting point is found to be effective for improving the material properties[14],the treatment temperature now is mostly depend on the experience rather than the accurate thermodynamic data.So more information about the temperatures at which the liquid-liquid phase transition or the structure transition occurred is urgently needed.

The classical methods for exploring the merits of glass-forming alloy melt are mainly relied on the large undercooling of several hundreds of kelvins[45].The phenomenon of an abnormal change in viscosity that occurs during the heating process is reported in viscosity-temperature plots[39,43,46].However,the nature of this transition remains unknown at present.Although the amorphous alloy and melt are considered to be composed of clusters,it remains difficult to study the structure and its regulation evolution in Fe-based glass-forming alloy melts[21].

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For the homogeneous Fe-based alloy melt,as shown in the second run of thermal cycling of Figs.1–2,the viscosity increases with the increasing cluster size during cooling processes and decreases with the decreasing cluster size during heating processes.

As viscous flow not only happened in alloy melt,but also appeared in amorphous solid state[47],investigation the viscosity of glassforming alloy melts may offer some opportunities.The viscosity of glass-forming alloy melts is an important factor in the casting process due to its effect on the crystallization kinetics in under-cooled liquids,as well as on the morphology and the properties of the amorphous products.Moreover,viscosity is one of the most sensitive parameters affecting the structure at the cluster level,so the viscosity evolution can provide indirect indications about the cluster heterogeneity of these alloys in the liquid state.

First principles theoretical calculations of structures present in deeply supercooled melts should further shed light on such important aspects as network connectivity and viscosity-temperature profiles[48].As a standalone route,Ab initio molecular dynamics(AIMD)simulation is capable of providing reliable simulation for investigating the microstructure at the atomic scale.AIMD has been used to model the local atomic structure of MGs and alloy melts,and has been shown to reliably reproduce experimental observations,e.g.,the extended Xray absorption fine structure and neutron scattering spectra,and the results from other simulation methods, e.g., reverse Monte Carlo[23,24,26,30,49,50].

The Fe-Si-B system is the key amorphous core material widely applied in industrial field[44].Their excellent soft magnetic properties depend on the homogeneous structure of the alloy melts.The goal of present study is to investigate the hidden disintegration of cluster heterogeneity in order to obtain stable,ideal and uniform Fe-based amorphous ribbons.Here,we revealed the microstructural evolution of Fe78Si9B13 alloy melt as a function of temperature and the nature of their related phy-chemical performance via measurement of the viscosity of Fe78Si9B13 alloy melt during the thermal treatment and differential scanning calorimeter(DSC)combined with simulation of AIMD.

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2.Methods

The crystallization order of the Fe-B binary alloy is as follows:

The viscosity was measured using a rotational vibration-type hightemperature melt viscometer (NZ-B, Shandong University). The working principle was to determine the vibration attenuation rate of the melt and calculate the viscosity according to the relationship between vibration attenuation rate and viscosity.An Al2O3 crucible was used as the melt container.During the measuring process,the vacuum pressure was 2×10-3 Pa and the heating rate was 10°C/min.A viscosity value was measured at the set temperature every half hour.The cooling processes started once the melt reached its highest temperature and this viscosity measurement was finished.The temperature interval in the heating and cooling processes was 25/50°С.The viscosity data were averaged from 5 individual measurements for each temperature.The repeatability error was less than 3%.

For melts containing different amounts of Fe2B-and Fe3B-type cluster heterogeneity,the initial disintegration temperature may be different.For relatively pure Fe3B-type cluster heterogeneity,the disintegration temperature should disappear because of the almost identical structure between the melt and ingot.

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Fig.1.Relationship of the viscosity of Fe78Si9B13 alloy with temperature during the heating and cooling processes(1-the first run,2-the second run).

3.Results and discussion

3.1.Abnormal viscosity curves

The viscosity as a function of temperature for the Fe78Si9B13 alloy ingot during the heating and cooling processes is shown in Fig.1,where the numbers,1 and 2,refer to the first and second runs of thermal treatment cycling.The homogeneous structure of ingot was checked by SEM/EDS.Based on this viscosity-temperature plot,it can be seen that the viscosity increased slowly and continuously with decreasing temperature for the two cooling processes.Compared to the viscosity in the cooling process,an abnormal change of the viscosity in the temperature ranging from 1300°C to 1500°C can be recognized during the heating process in the first cycle.This abnormal change in the viscosity shows two different behaviors.At first,the viscosity decreased slowly in the temperature range of 1300–1400°C,and then,the viscosity decreased rapidly in the temperature range of 1400–1500°C.It should be pointed out that this abnormal change of the viscosity only presented in the heating process of first cycle and never appeared in the cooling process.Also,the abnormal behavior of the viscosity during the heating process disappeared in the successive thermal cycles.Another feature shown in Fig.1 is that the values of the viscosity in the cooling process are always lower than those in the heating process at the same temperatures.In addition,the measured viscosity in the second cycle less than the viscosity in the first cycle reveals that there is a hysteresis occurred between the first and successive thermal cycles.

Since the viscosity is one of the most sensitive parameters related to the microstructural evolution of melts,the abnormal behaviors of the viscosity present in Fig.1 most strongly suggests a structural transition in the alloy melt happened at the specific temperature range.The transition characteristic around 1400 ℃in the gray transition zone,is very similar to the melting platform of liquid phase.

To avoid the possible influence of initial ingot on this structural transition,the viscosity and structure of amorphous Fe78Si9B13 ribbons were also tested and results are exhibited in Fig.2.The viscosity-temperature relation demonstrated in Fig.2(a)is almost consistent with that of Fe78Si9B13 ingots in Fig.1.As shown in Figs.2(b)and 2(c),the solid structure was perfect amorphous state,supported by the random or disorder atomic arrangement and the diffuse halo ring pattern.

The data from Figs.1 and 2 can suggest that,the structural transition phenomenon occurs both in the amorphous ribbons and alloy ingot,probably being a universal phenomenon for Fe-based glassforming alloys.

To explore the nature of the abnormal behavior of the viscosity in the Fe-based glass-forming alloy melt,the temperature-viscosity relation for Fe78Si9B13(Fe78)and Fe73.5Si13.5B9Cu1.2Nb2.8(Fe73.5)ingots below 1400°C was also investigated,as shown in Fig.3.The heating and cooling viscosity curves were nearly coincident with each other within experimental error below 1350°С.This fact suggests that the viscosity is reversible at below 1350°C and indirectly verifies that a structural transition indeed occurs near 1400°C.

3.2.Structural transition arises from Fe-B atom pairs

To qualitatively demonstrate that the theory of cluster heterogeneity disintegration leads to the structural transition seen in the viscosity curves,the initial heterogeneous Fe78Si9B13 ingots were tested by SEM/EDS and viscometry.Fig.11 shows the SEM/EDS spectra of the artificially selected heterogeneous Fe78Si9B13 ingot and its corresponding viscosity-temperature plots.

It is clear that the viscosity of the Fe89.66Si10.34 alloy is almost reversible during the heating and cooling processes,and no evidence of a structural transition presents.On the contrary,decrease of the viscosity of the Fe85.71B14.29 alloy melt becomes clearly slow at around 1400°C,indicating occurrence of a structural transition.These phenomena are consistent with the results obtained from the alloys with similar compositions,i.e.,the Fe85B15 and Fe90Si10 binary alloys[39,55].These data confirm the proposed idea that Fe(B)clusters,rather than Fe(Si)clusters,are the dominant clusters and predominantly affect the Fe-Si-B ternary system.Thus,the structural transition is considered to arise from the contribution of Fe-B atom pairs rather than Fe-Si pairs.

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3.3.Thermal analysis results

The DSC experiment was carried out to explore the nature of structural transition above alloy molten temperature.The heating rate is 10 ℃/min.The results of amorphous Fe78Si9B13 ribbons are offered in Fig.5.There are two endothermic peaks in the temperature range of 1000–1500 ℃.The first one around 1167 ℃is correlated to the melting of this alloy.However,it was interesting to note that,the second endothermic peak is around 1414 ℃in liquid.This peak was so steep that it should be related to the melting of one indecomposable particle(refer to the derivative in Fig.5 inset 2).To our knowledge,the most indecomposable particle of this alloy system in liquid is the B-rich cluster heterogeneity.Although the mass of the DSC samples is in the order of 10 mg,the high sensitivity of this thermal analysis equipment secured the uncover of this structural transition around 1400 ℃.This fact further evidenced the existence of structural transition and supported the conclusions of viscosity data.

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3.4.AIMD simulation results

To find the cause of the structural transition in the Fe78Si9B13 alloy melt, the structural evolution during the cooling process was investigated by AIMD simulation.The content evolution of the dominant atomic clusters and the diffusion coefficient are shown in Fig.6 and Fig.7.The content of dominant clusters increased with decreasing temperature during the quenching process,and the diffusion coefficient decreased.As is known,the viscosity depends on the average size and travelling speed of the clusters,and the enlargement of the average size of the clusters and the reduction of the diffusion coefficient increases the viscosity value.

Fig.2.Relationship of the viscosity of Fe78Si9B13 ribbons with temperature during the heating and cooling processes(1-the first run,2-the second run)(a)and the HRTEM microscopy(b)and EDS(c)of Fe78Si9B13 ribbons.

Fig. 3.Viscosity of Fe78Si9B13 and Fe73.5Si13.5B9Cu1.2Nb2.8 ingots below 1400°C in the heating and cooling processes.

Fig.4.Viscosity of Fe85.71B14.29(FeB)and Fe89.66Si10.34(FeSi)alloys melts,the equivalent binary alloys for Fe78Si9B13 alloy during the heating and cooling processes.

Notably, an abnormity in the content of dominant metalloidcentered clusters is demonstrated in Fig.6 at approximately 1400°C.Another abnormity of the diffusion coefficient for Fe78Si9B13 occurs at 1000°C.Although there is a gap in the quenching speed between the simulation technique and the true viscosity experiment on the order of 108 magnitude,the two abnormities may offer favorable evidence for the existence of structural transitions in the alloy melt at approximately 1400°C.

在使用3种消毒法消毒前开启空调，以第一次采样、经培养后，作为空气中自然界基础微生物样本底的数量，连续4天取其均值。消毒后（静态）采样，桉叶油和甲醛熏蒸消毒12 h后，通风2 h采样、培养微生物，连续4天，取其微生物数量均值。空气洁净器开机60 min后采样、培养微生物，连续4天，取其微生物数量均值。

Fig.5.DSC curves for Fe78Si9B13 amorphous ribbons in the temperature range of 1000–1500 ℃.Inset 1-the enlargement of second endothermic peak;Inset 2–the △DSC as a function of temperature.

Fig.6.Content of dominant Fe-,Si-and B-centered clusters during quenching processes.

The functions gBB(r)and gSiSi(r)of the FeSiB melts during the quenching processes are plotted in Fig.8(a)and(b).The nearest peaks of B-B and Si-Si pairs became stronger with the decreasing temperatures.Therefore,it is acceptable to reason that the B-rich and Si-rich clusters need to disintegrate during the heating process to obtain a homogeneous alloy melt.

Fig.7.Diffusion coefficients of Fe,Si and B atoms during quenching processes.

3.5.Structural model of the molten and amorphous alloy

According to the results above,the structure of glass-forming alloys in liquid and solid state can be considered to consist of densely packed atomic clusters with SRO,which can be constructed into MRO through the sharing of vertices,edges,faces and tetrahedrons.The microstructure of the melt and the solid alloy of amorphous alloys can be schematically illustrated in Fig.9.In the schematic,the amorphous alloy contains more fivefold symmetry clusters than the liquid melt.

For the normal liquid melt of a glass-forming alloy,the cluster size can be described as follows:

where DLT represents the average cluster size at T,α represents the augment of cluster size in temperature intervals of(Tnext-T),β represents the derivative of cluster size at T and t represents the relaxation time.

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Fig.9.Schematic microstructure of the melt and amorphous alloy.

3.6.Disintegration of cluster heterogeneity in the supersaturated melt

To understand the nature of the structural transition in the Fe-Si-B system,the distributions of several classical compositions in the FeSiB ternary phase diagram[56]are listed in Fig.10(a).It is clear that all the classic components are localized away from the eutectic line.Thus,according to the solid solution theory,the melt is composed of solute and solution,and there are always superfluous B and Si atoms in the alloy melt.Therefore,these non-eutectic compositions can be treated as supersaturated melts containing much B-rich and Si-rich cluster heterogeneity,which is illustrated in Fig.10(b).

Eliminating the existing B-rich and Si-rich cluster heterogeneity may require more energy.The cluster heterogeneity starts to disintegrate when the temperature is higher than the melting temperature for a homogeneous melt,which leads to the sudden decrease in viscosity during the heating process.The viscosity-temperature curve is completely reversible when the heating temperature is lower than the needed disintegration temperature.Therefore,a homogeneous Fe-based alloy melt can be obtained by a proper heat treatment at a proper temperature.

3.7.Inference of Fe2B type cluster heterogeneity

Fig.8.Partial pair distribution functions gBB(r)(a)and gSiSi(r)(b)of FeSiB melts during quenching processes.

Fig.10.Composition distribution in relation to the eutectic line of the FeSiB ternary phase diagram(a)and a schematic plot of the supersaturated solid solution(b).

Table 1 The structural and thermal characteristics of Fe-B compounds.

Characters Fe3B Fe2B FeB Crystal system Orthorhombic systemtetragonalorthorhombic(similar to Fe3C)systemsystem Melting point 1300°C 1389°C 1550°C State metastable phase stable phase stable phase

To further explain the nature of the structural transition in the Fe-Si-B alloy system,the microstructure and microphases at the nanoscale are reviewed and discussed here.As there is great difficulty to detect the cluster type of Fe-based glass-forming alloy melt even by the synchrotron beam radiation[21,22],indirect techniques are employed usually.Notably,the short-range order of the amorphous alloy is closest to the first metastable phase of the Fe-B binary alloy system[57].The interference function curves for microcrystalline Fe3B clusters were similar to that of liquid Fe-B alloy,as determined using X-ray diffraction[15].These results reveal the existence of Fe3B clusters in the Fe-B melt.This phenomenon has been indirectly confirmed for eutectic Fe-B liquid alloys.At a high temperature(1627 ℃),the X-ray structures of a homogeneous liquid consisting of boron-saturated Fe4B clusters show interactions between constituent atoms as well as the weaker interactions between the clusters[58].

The raw materials of Fe-B(20 B mass%),Si(99.999 mass%),Fe(99.98 mass%)were melted into ingots with nominal compositions of Fe78Si9B13 by vacuum induction melting.The fracture morphology and element distribution of the sample ingots were measured by scanning electron microscopy(SEM,FEI Nova nanoSEM450)and energy dispersive spectroscopy(EDS,X-Max50),respectively.The amorphous structure of Fe78Si9B13 ribbons was determined by transmission electron microscopy(TEM,Fei Tecnai F30).The thermal properties including the endothermic peak of phase transition was obtained by differential scanning calorimeter (DSC, TGA/DSC 1600, Mettler Toledo).

Yamasaki held that Fe-rich-type and Fe2B-type clusters exist in hypoeutectic and hypereutectic Fe-Si-B alloy melts,respectively[59].Popel and Sidorov systematically studied the anomalies in the density and resistivity curves as functions of temperature for many systems(Fe-Sc,Fe-Ge,Fe-B,and Co-B melts)[43].The curves indicate that the abnormal point corresponds to the melting point of the compound,which suggests that the abovementioned abnormal phase transition results from the dissolution of the particles.Calvo-Dahlborg[39]also presented a similar view,suggesting that the anomalous point corresponds to the onset of dissolution in the anisotropic particles.The typical structural characteristics of Fe-B compounds are shown in Table 1.

At the same time,for the eutectic composition,the structural transition phenomenon should be very slight or disappear entirely.The disappearance of this transition from the second thermal cycling viscosity curves of the FeSiB alloy indicates that prolonged heat treatment at high temperatures can effectively eliminate cluster heterogeneity,facilitating obtaining a homogeneous alloy melt.

The melting temperatures of the Fe2B and Fe3B phases are 1389°C and 1300°C,respectively.The former is around 1400 ℃,and this was the temperature at which the viscosity suddenly dropped in Figs.1–3,the second endothermic peak occurred in DSC Fig.5 and the abnormity results happened in Fig.6 in FeSiB alloys.According to the viscosity,DSC proof,simulation results and phase information discussed above,combined with the presence of Fe2B and Fe3B,we speculate that the nature of the structural transition in the FeSiB system is the disintegration of analogous Fe2B clusters into analogous Fe3B clusters.Through this disintegration,a more stable and homogeneous melt is obtained.

Notably,this disintegration is irreversible.There are two reasons for the irreversibility.1)The characteristics of the Fe-based glass-forming alloy melt are strongly inherited from its nearest precursor.The size of the cluster heterogeneity will decrease at the nanoscale,and the sign of this transition will become slighter after prolonged heat treatment at high temperatures.2)The Fe3B phase with a melting temperature of 1300°C is itself a metastable phase during the crystalline process,so the Fe3B-type clusters at the nanoscale are probably relatively stable and can exist for a long time in the FeSiB alloy melt.This result is verified by data from the same Fe3B-type cluster that exists in FeSiB amorphous alloy[54].

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As a standalone route,AIMD is often used to obtain the static liquid and amorphous structure of materials and deduce a general trend by systematically comparing a number of amorphous alloy systems with different chemical compositions.AIMD has been used to model the local atomic structure of MGs and alloy melts and shown to reliably reproduce experimental observations.In the present simulations,a cubic cell containing 100 atoms with periodic boundary conditions and 3000 configurations was used for structural analysis after the Fe78Si9B13 melt was rapidly quenched at a rate of 1014 K/s from 1600 ℃and equilibrated at every targeted temperature for 9 ps.

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Fig.11.SEM/EDS spectra of heterogeneous FeSiB ingot and its viscosity-temperature plot.

Fig.12.SEM/EDS spectra of the homogeneous FeSiB ingot and its viscosity-temperature plot.

3.8.Confirmation by reproducing the structural transition in melt

To check out the internal cause of structural transition,we compared the element distributions for the samples before and after the viscosity measurement.

For the FeSiB alloy melt,the liquid structure is composed of Fe(B)and Fe(Si)clusters and an ideal solution component[51–53].However,the Si atoms is embedded relatively freely in the Fe-Si solid solution,and the Fe-B binary alloy demonstrates much stronger chemical interaction than the Fe-Si pairs,according to their different glass forming abilities[54].Therefore,we conclude that Fe(B)clusters,rather than Fe(Si)clusters,should be the dominant cluster and should predominantly affect the Fe-Si-B ternary system.Thus,the structural transition of the Fe-Si-B ternary system should be induced mainly by the transition of the Fe-B binary system. To verify this speculation, we divided the Fe78Si9B13 composition into equivalent Fe85.71B14.29 and Fe89.66Si10.34 alloys according to their invariable nominal compositions.Then,the viscosity of the two alloys as a function of temperature was measured in both the heating and cooling processes and the results are shown in Fig.4.

It is clear from the SEM/EDS micrograph that the structure of the Fe78Si9B13 ingot is heterogeneous,especially in the segregation of Si and B elements(a)-(c).The viscosity-temperature plot of this heterogeneous ingot is almost consistent with Figs.1–2,which can be considered a reproduction.The viscosity-temperature result from the artificially selected heterogeneous Fe-B ingot also shows a similar structural transition trend.

However,for the initial heterogeneous ingot after one run of viscosity measurement and cooling to room temperature,the SEM/EDS spectra and viscosity-temperature curve demonstrate a homogeneous trait,shown in Fig.12.The viscosity curves are continuous and almost reversible during the heating and cooling processes,showing a homogeneous melt.

This characteristic is similar to the second run of Figs.1–2.Thus,the structural transition in liquid can be eliminated and a homogeneous Febased alloy melt can be obtained by a proper heat treatment at a temperature higher than the needed disintegration energy.This result confirms that the structural transition in the melt at high temperatures can be reproduced in artificially selected heterogeneous Fe78Si9B13 ingots.

These findings enrich our knowledge about the nature of Fe-based alloy melts at high temperatures and provide theoretical references for preparing amorphous ribbons by melt-spinning techniques.

4.Conclusions

Based on the experimental results,following conclusions can be drawn:

(1)A structural transition around 1400 ℃occurs in three types of FeSiB alloy melts from initial homogeneous ingot,heterogeneous ingot and amorphous ribbons.It has been found that the viscosity of FeSiB alloy melts decreases suddenly during the heating process between 1350 and 1400°C,and the viscosity curve does not coincide with that observed during the cooling process.This irreversible feature indicates that a structural transition exists in the Fe-based glass-forming alloy melt.If the alloy melts are cooled below this transition temperature,the heating and cooling curves will coincide completely.

人们在用数学的方式解决每一个实际问题时，其本质就是简化所表达的问题，然后将其中的抽象知识用一个数学模型的方式简单的展现出来，利用一些先进的技术，如计算机技术来求解模型，最后再重复的推理以及完善结果的准确度，直至满足事情原本所需的要求。

(2)The DSC result reveals an endothermic peak at 1414 ℃,supporting the existence of structural transition in FeSiB alloy melt.

(3)First-principles calculations show that an anomalous dominant clusters of Si and B atoms is observed at approximately 1400°C,which further demonstrates the possibility of a structural transition.

(4)Fe(B)clusters,rather than the Fe(Si)clusters,are the dominant clusters and predominantly affect the structural transition in the Fe-Si-B ternary system.

(5)Combined with the characteristics of Fe-B phases,the nature of the structural transition is that Fe2B clusters transform into Fe3B clusters.

Acknowledgement

This work was supported by the National Natural Science Foundation of China(Grant No.51501043,51571132),the New Star Plan of Science and Technology of Beijing (Grant No.Z161100004916048),the National Key R&D Program of China(Grant No.2016YFB0300500)and the Key Science and Technology Project of Jiangsu province,China(Grant No.BE2015216).

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