1.Introduction

During the last two decades,bulk metallic glasses(BMGs)have achieved significant progress through the understanding of glass formation,structural features,mechanical properties and deformation mechanisms[1–4].From the remarkable developments,the BMGs have been considered potential Engineering materials for structural applications due to their unique properties,which include its high strength,large elastic limit,and good formability at a super-cooled liquid state[5–7].However,one of the major problems with the BMGs is the unfavorable brittleness at room temperature that originates from an inhomogeneous shear deformation[8–10].The limited plasticity of BMGs is considered as a serious drawback and thus limits their access into engineering applications.Hence,to design high-performing BMGs,the deformation behavior and the fracture mechanisms of the BMGs have been widely studied[10–13].The plastic deformation of monolithic BMGs are localized into shear bands under loading.When shear bands propagate through a sample,the serrated flow by stress drops is observed in the stress-strain curves,which indicates a loading force drop[14].In the shear bands,the materials behave like a thin layer with a low viscosity between the upper and the lower solid plates,and they have a perfect plastic behavior without hardening behavior,which is similar to a viscous fluid[15,16].This viscous flowing during the propagation of a single shear band results in a brittle failure of the monolithic BMGs.

In order to overcome the limited macroscopic plasticity related to the shear localization and the rapid propagation of a few shear bands,researchers have found the solution by developing bulk metallic glass composites(BMGCs)containing the second phases[17–22].It is well known that the Ti-based ultrafine composites containing ductile dendrite phase have exhibited highly improved ductility caused by dislocation movement in the ductile phase and shear banding mechanism in the nanostructured matrix[23–25].Based on the concept of nanostructured composites containing the ductile phase,the BMGCs containing the ductile second phases have exhibited a greatly improved plasticity by combination of dislocation movement in second phase and formation of multiple shear bands in amorphous matrix[18,19,26].Moreover,the interface between the second phases and the metallic glass matrix plays a role in the shear localization(or concentration)site during deformation,which results in the formation of multiple shear bands[18,19,22,27–29].The volume fraction and length scale of the second crystalline phases also influence the ductility of the bulk metallic glass composites with ductile β-Ti phase[27,30,31].Especially,some BMGCs that contain the soft B2 phases have been reported in the CuZr-and the TiCu-based glass forming alloy systems,which represent a highly improved plasticity with obvious work-hardening behavior during the compression as well as the tension[26,30,32–36].The excellent mechanical properties are mainly attributed to the length scale,volume fraction,and relatively large strain accommodation capacity of the B2 particles[34,37].The optimized volume fraction and the effective length scale of the B2 particles are 20 ～40 vol%and over 100µm,which are effective to induce the formation of the multiple shear bands and obstruct the propagation of the shear bands during deformation[30,32].Moreover,stress-induced martensitic transformation of austenite B2 phase to martensitie B19'phase during deformation contributes favorably to the strong work-hardening behavior of the BMGCs[37–39].These strong effects of the B2 particle on the deformation behavior have been verified in many theoretical and experimental studies.However,the fracture behavior of the B2 particlereinforced BMGCs has been investigated in just a few studies.Therefore,to investigate the correlation between the microstructure and the mechanical properties as well as the deformation/fracture behaviors of the B2 particle-reinforced BMGCs,it is very important in order to understand the highly improved mechanical properties and to develop the high-performing BMGCs.

In this study,a Ti45Cu40Ni7Zr5Sn2.5Si0.5 glass forming alloy has been designed to fabricate the monolithic BMG and the BMGCs that have different volume fractions of the B2 particles by controlling the cooling rates of the melt in a single composition.Here,we report the influence of the B2 particles on the phase transformation,the mechanical properties,and the deformation,and the fracture behaviors of the BMGCs with different amount of spherical B2 particles through inclusive investigations.

生态清洁小流域建设要求各项措施应落实到地块、村庄或沟道，并明确反映在措施布局图中。通过将高分辨率遥感影像与治理措施进行空间叠加，可以对治理措施的设计范围与地块实际走向进行对比，从而进一步提高措施规划设计的精确程度。例如：梯田措施设计范围过大，其中包含建筑物、设施用地、道路等其他地物类型；田间生产道路措施未按原有道路走向设计，存在穿越农田、房屋等问题。

2.Material and methods

Master ingots, with a nominal composition of Ti45Cu40Ni7Zr5Sn2.5Si0.5(at%),were prepared using arc melting with high purity elements of 99.9 at%in an argon atmosphere.The ingots were re-melted at least 5 times in order to achieve chemical homogeneity.From the alloy ingots,a rapidly quenched ribbon sample,with width of 4 mm and 40±5µm in thickness,was fabricated using a single-roller melt spinning wheel with a speed of 40 m/s.The as-cast rod samples were produced using suction casting into cylindrical rodshape Cu molds with diameters of 1,2,and 3 mm and 50 mm in length.In order to analyze the phase and the structure of the as-spun ribbon and the as-cast rod samples,X-ray diffraction(XRD,Rigaku-D/MAX-2500/PC)with CuKα was used.The thermal characteristics associated with glass transition and crystallization was measured by differential scanning calorimeter(DSC,Perkin-Elmer DSC8500)under a flowing argon atmosphere with a constant heating rate of 40 K/s.The cross sectional microstructures of the rod samples were examined using a scanning electron microscopy (SEM: JEOL JSM-6390). Cylindrical samples with a 2:1 aspect ratio for the compression tests were prepared.Mechanical properties of the rod samples were tested under a compressive mode at a strain rate of 1×10-3 s-1.To examine the phase transformation after the fracture,an electron backscattered diffraction(EBSD)was conducted using an FE-SEM(TESCAN CZ/MIRA I KMH)device that was equipped with an EBSD detector in conjunction with a transmission electron microscopy(TEM,Technai F20).The thin film specimens for TEM observation were prepared using ion milling with liquid nitrogen cooling.The lateral and the fracture surface morphologies of the failed samples were investigated using SEM.

3.Results and discussion

Fig.1.(a)XRD patterns and(b)the DSC traces of the as-spun ribbon and the ascast rod samples with diameters of 1,2 and 3 mm.

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Fig.4 displays the SEM micrograph obtained from the lateral surface of the failed 2 mm sample in conjunction with the inset EBSD phase map of the inside area of the B2 particle.The tiny and the strong multiple-shear bands were observed around the B2 particles and the amorphous matrix.This result indicates that the B2 particles are effective to obstruct and scatter the propagation of the shear bands.Moreover,the severe plastic deformation features,such as the slip bands,is detected inside the B2 particle.It is well known that the B2 phase formed in the TiCu-and CuZr-based BMGCs usually undergoes a plastic deformation,and the stress-induced martensitic transformation during deformation contributes to the improvement of the plasticity and empowers work-hardenability [28,30,32,34,37–39]. The EBSD analysis is a useful method to measure phase evolution and dispersion.The inset EBSD phase in Fig.4 obtained from the inside area of a deformed B2 particle reveals the 3 types of microstructural features,which consists of the B2(red),the B19'(yellowish green)and the slip band(black).The B19'martensites are mainly distributed in the inside and around the slip bands,which implies that the stress-induced martensitic transformation from the B2 austenite to the B19'martensite phase occurred around the vicinity of the slip bands.However,the EBSD phase map does not clearly reveal the phase transformation behavior because the stress-induced B19'martensites are too small to identify in FE-SEM level.Therefore,the TEM investigation was conducted in order to obtain a deeper insight into the stress-induced martensitic transformation behavior of the B2 particles.

Fig.2.(a)-(c)SEM BSE micrographs obtained from the cross sectional areas of the as-cast rod samples with diameters of 1,2 and 3 mm,respectively.

In order to obtain a deeper insight into the fracture behavior of the Ti45Cu40Ni7Zr5Sn2.5Si0.5 BMGCs with the spherical B2 particles in the compression,we investigated the fracture morphologies of the failed rod samples with diameters of 1 and 2 mm.Fig.6(a)shows the fracture surface morphology obtained from the failed 1 mm sample that consisted of mostly the amorphous phase with very few B2 particles,which displayed the typical vein patterns of the failed monolithic BMG with a uniform arrangement of the veins,which indicates a mode II fracture behavior[43].This result implied that the 1 mm sample failed due to the rapid propagation of the main shear bands on the maximum shear stress plane.On the other hand,the SEM images in Fig.3(b)-(d)obtained from the fracture surface of the failed 2 mm sample with 22 vol%B2 exhibits complex characteristic features of the fracture morphologies.The low magnified fracture morphology in Fig.6(b)reveals three types of regions:i)the severely deformed islands(marked by yellow dotted circles),ii)the intermittent smooth region around the deformed islands(indicated by white arrows),and iii)the highly branched shear bands on the whole fracture surface.A severely deformed island and surrounding the intermittent smooth region can be seen in Fig.6(b).A distinct interface can be found between the spherical island and the intermittent smooth region.In addition,the lengths of the major islands marked by yellow dotted circles in Fig.6(b)are about 5 ～30µm,which coincide with the lengths of the B2 particles in the as-cast rod sample with a diameter of 2 mm as shown in the SEM BSE micrograph in Fig.2(b).Based on this observation,it can be inferred that the spherical islands in the fracture surface are the B2 particles that suffered a severe plastic deformation during the fracture.The severe plastic deformation features of the B2 particles were already mentioned in the previous study[30,34,37–39].Moreover,a very interesting smooth region was found around the failed B2 particles,which is shown in Fig.6(c).The smooth region is considered as one of characteristic features of the tensile fracture morphologies of the BMGs[44,45],which is often observed on compressive fracture surfaces of the BMGs and the BMGCs[46,47].The B2 particles and the amorphous matrix usually exhibit different elastic modulus in the TiCu-and the CuZr-based BMGCs[38,39,42].Due to the elastic mismatch between the B2 particles and the amorphous matrix,the stress is concentrated on the vicinity of the interface between the B2 particles and the amorphous matrix during the deformation.This stress concentration zone at the interface leads to the formation of a complicated microscopical internal stress state mixed with the tensile and compressive stresses[48].Based on previous results,it can be inferred that the smooth region observed around the B2 particles is considered as an evidence that the local tensile fracture occurred by local tensile stress at the interface region.In addition,the nano-scale dimple patterns in the B2 particles and the cone-type fracture morphologies of the B2 particles also can be evidence of the local tensile fracture.Fig.6(d)illustrates the highly magnified SEM micrograph that was obtained from the highly branched shear bands area.It was observed that there are a lot of tiny branching shear bands with lengths of a few micrometers,and the inter-shear bands spacing is a few micrometers[marked by double-headed arrow in the inset in Fig.6(d)].These shear bands strongly intersect with each other,and they interact with the B2 particles to accommodate the excessive shear strain.Moreover,the nano-scaled honeycomb-like structures are observed on the micro-scale multiple-shear planes,as shown in the inset in Fig.6(d),which represents a mode I fracture behavior[43].These observations reveal that the uniformly distributed micro-scale multiple shear banding regions on the metallic glass matrix have been formed throughout the whole sample upon the deformation of the BMGC in order to accommodate the applied shear stress rather than the accumulation of the shear stress for generation of the main shear bands.Moreover,the fracture occurred due to the complex stress state instead of the maximum shear stress normally encountered in the monolithic BMGs.Based on these observations,it is inferred that the stress concentration and the complex stress states zone around the interfaces between the amorphous matrix and the B2 particles can accelerate to operate the shear transformation zone and the nucleation of the shear bands.Moreover,the severe interaction of the tiny branching shear bands effectively redistributes the local shear strain and helps to impede the crack generation and the propagation.

Fig.3 displays the compressive stress-strain curves of the as-cast rod samples with diameters of 1,2 and 3 mm.The values of the yield strength and the plastic strain are 1935±25 MPa and ～0.3%for 1 mm sample,1795 MPa±25 MPa and 5.6±1.5%for 2 mm sample,and 1656±25 MPa and 2.9±1%for 3 mm sample,respectively.The yield strength of the BMGCs is not greatly affected from the different diameter of the samples[41].Instead,the volume fraction of the B2 particles greatly influences the yield strength of the BMGCs.The yield strengths of the present BMGCs,including the different amounts of the B2 particles,exhibit a tendency to decrease with an increase in the volume fraction of the B2 particles,which is caused by the early deformation of the softer B2 particles than the amorphous matrix[30,34,37].However,the 3 mm sample with 40 vol%B2 reveals a lower plasticity compared with the 2 mm sample with 22 vol%B2,which can be demonstrated from the previous studies.The fracture strain model of the BMGCs was described using the three-microstructure-element body that was developed by Fan and Miodownik[32,34,42].From this theoretical model and the experimental results from previous reports,the correlation between the volume fraction of the B2 particles and the fracture strain have demonstrated[32,34,41].The fracture strain of the BMGCs strongly depends on the volume fraction of the B2 particles and greatly increases until the volume fraction of the B2 phase reaches to 30～35 vol%.However,when the volume fraction of the B2 particles is over the 35 vol%,the fracture strain of the BMGC exhibits a tendency to decrease slightly.In previous report on the Ti-Cu-Ni-Zr BMGCs with the B2 phase[34],the optimized fracture stain(23.7%)was achieved on the BMGC with 28 vol%B2,whereas the fracture strain also decreased when the volume fraction of B2 phase increased up to 38%.In the present Ti45Cu40Ni7Zr5Sn2.5Si0.5 BMGs,the maximum fracture strain is achieved with B2 volume fraction of 22%,which is reasonable results in agreement with previous report.Therefore,it is believed that the Tibased bulk metallic glass composites with the improved fracture strain can be achieved by formation of the B2 phase with critical volume fraction(30 ～35%),which can be tailored by controlling cooling rate.

Fig.3.compressive stress-strain curves of the as-cast rod sample with diameters of 1,2 and 3 mm.

Fig.2 shows the SEM backscattered electron(BSE)micrographs obtained from the cross sectional areas of the rod samples with diameters of 1,2 and 3 mm,respectively.The 1 mm rod sample in Fig.2(a)reveals very few particles with a dark contrast and lengths of ～10µm embedded in a bright matrix[marked by black dotted circles in the inset in Fig.2(a)].With a decrease in the cooling rates,the volume fraction and the length of the homogeneously dispersed dark particles increase substantially as shown in Fig.2(b)and(c),which demonstrates a typical microstructure of the particle-reinforced BMGC.The spherical particles in the rod samples can be identified as the B2 phase with a length of 1 ～50µm,and the bright matrix is considered the amorphous phase based on the XRD analysis and the previous research[30].The volume fraction of the B2 particles was estimated from the SEM images using a pixel analysis,and the measured values are as ～1±0.5 vol%in 1 mm sample,21±2 vol%in 2 mm sample and 40±2 vol%in 3 mm sample,which coincides well with the measured volume fraction using the crystallization enthalpy values from the DSC analysis in Fig.1(b).Based on the above phase and the microstructural analyses,it is confirmed that controlling the cooling rates on the Ti45Cu40Ni7Zr5Sn2.5Si0.5 glass forming liquids is very effective to form the B2 particles-reinforced BMGCs and to vary the volume fraction of the homogeneously distributed B2 particles in the amorphous matrix.

Fig.1 shows the XRD patterns and the DSC traces that were obtained from the as-spun ribbon and the as-cast rod samples with diameters of 1,2 and 3 mm.In Fig.1(a),the XRD pattern of the as-spun ribbon sample exhibits a broad diffraction characteristic of the amorphous phase and didn’t have any evidence of crystalline peaks,which indicates the formation of monolithic metallic glass.The XRD pattern of the rod sample with 1 mm diameter shows a very weak crystalline peak in the range of 40 ～42°superimposed broad diffraction.With the decreasing in cooling rates,or the increasing in diameters,the intensity of the crystalline peaks gradually increases,which implies an increase in the volume fraction of the crystalline phase.The detectable crystalline phase in the rod samples with diameters of 1,2 and 3 mm was identified as the B2 phase,which has been systemically investigated in previous reports on the Ti-Cu-Ni-Zr-Sn-Si glass forming alloy system[30].Fig.1(b)shows the DSC curves that were obtained from the asspun ribbon and the as-cast rod samples.The as-spun ribbon sample displays an endothermic event that corresponded to the glass transition of the amorphous phase and three distinct exothermic peaks,which corresponded to the crystallization of the super-cooled liquid phase.For the rod samples with diameters of 1,2 and 3 mm,the glass transition and the three-step crystallizations are also observed.The rod samples did not show any significant temperature difference for the thermal events compared to the ribbon sample.Subsequently,it was observed that the exothermic heat values that correspond to the crystallizations in each sample gradually reduced with the decreasing cooling rates,which indicates a decrease in the volume fraction of the amorphous phase.In a previous report on Ti-Cu-Ni-Zr-Sn-Si BMGCs,it is well known that the B2 phase and the amorphous matrix consists of similar chemical compositions[30].In the present study,all the samples exhibited similar glass transition and crystallization temperature,which indicates that the amorphous phases in the all rod samples are chemically identical to the monolithic metallic glass ribbon sample.Therefore,the volume fractions of the amorphous matrix and the B2 phase in the rod samples can be estimated from the crystallization enthalpy values comparing with the crystallization enthalpy value of the monolithic metallic glass ribbon sample[40]:

取铁皮石斛100 g，剪碎后用电动匀浆器匀浆，装入离心管离心后取上清液，通过细菌滤器后备用。用打孔器将滤纸加工成直径为6 mm的原形滤纸片，灭菌后分别在不同的铁皮石斛匀浆液（质量分数分别为10%、20%、40%、80%）和无菌水中浸泡2 h。用移液枪吸取200 uL菌液于培养基表面，涂布均匀，然后将纸圆片从浸泡的匀浆液或无菌水中取出、沥干，等距离放置于培养基表面相应位置，每个浓度取3次重复。将培养基放入培养箱内倒置培养（37℃，24 h）。待菌落长出后，测定抑菌圈直径并比较抑菌效果。

Fig.5 shows the TEM bright field(BF)images and the selected area electron diffraction(SAED)patterns of the as-cast and the failed 2 mm samples.The BF image in Fig.5(a)displays the inside area of a single B2 particle formed in the as-cast 2 mm sample.The SAED pattern obtained from the single B2 particle represents[113]zone axis of the CsCl-type austenite B2 phase without any other diffraction spots.On the other hand,the BF image in Fig.5(c)obtained the failed 2 mm sample represents the needle type phases in the whole area of the B2 particle.Moreover,the SAED pattern in Fig.5(d)obtained from the deformed B2 particles reveals the additional weak spots with the spotty diffractions that correspond to[113]zone axis of the B2 phase.The additional weak spots were identified as[331]zone axis of the twined B19'martensite phase from the inset fast Fourier transform(FFT)pattern.From the microstructural analysis in Figs.4 and 5,it is identified that the stressinduced martensitic transformation from the austenite B2 phase to the martensite B19 phase occurred at the highly stress concentrated and severely deformed regions,such as the slip bands in the deformed B2 particle during the deformation.

Fig.4.SEM BSE micrograph obtained from the lateral surfaces of the failed rod sample with a diameter of 2 mm,an inset EBSD phase map obtained from the inside area of the deformed B2 particle.

Fig.5.[(a)and(c)]the TEM BF images and[(b)and(d)]the SAED patterns of the as-cast and the deformed B2 particles.

各观察指标数据均采用SPSS22.0统计学软件处理分析，且组间对比存在明显差异P＜0.05时，具有统计学意义。

在航空制造业领域本体中，包含设计方法类知识、材料类知识、工艺制造类知识、智能品类知识等顶层概念。以材料类知识为例，通过集成其属性特征，用本体描述语言OWL对其进行描述，本体片段如下：

where,ΔHribbon and ΔHrod are the crystallization enthalpies of the ribbon and the rod,respectively.The volume fractions of B2 phase with small variation estimated from the DSC curves of the as-cast rod samples are about 3±3 vol%in the 1 mm sample,20±3 vol%in the 2 mm sample and 42±3 vol%.%in the 3 mm sample,respectively.Based on combining the results of the XRD and the DSC trances,it can be concluded that only the ribbon sample forms a monolithic metallic glass upon solidification,whereas the rod samples with diameters of 1,2 and 3 mm form BMGCs that contain different volume fractions of the B2 phase due to the lower cooling rates to the critical cooling rate to form the monolithic BMG during solidification.

4.Conclusions

Fig.6.SEM micrograph obtained from the fracture surfaces of the failed(a)rod sample with a diameter of 1 mm,(b)-(d)rod samples with diameters of 2 mm.

The microstructural features and the influence of the B2 particles on the mechanical properties and the deformation/fracture behaviors of the Ti45Cu40Ni7Zr5Sn2.5Si0.5 BMGCs have been investigated using BMGCs with different volume fraction of the B2 particles.The monolithic BMG without macroscopic plasticity exhibits typical vein patterns formed by the main shear band propagation on the maximum shear stress plane.On the other hand,the BMGCs containing the B2 particles reveal an improved plastic strain to 5.6%and work-hardening behavior that resulted from a strong interaction between the B2 particles and the shear bands as well as the stress-induced martensitic transformation of the B2 particles.Moreover,the BMGCs that contained the B2 particles displayed distinguishable fracture characteristics. The BMGCs containing 22 vol%B2 represent significant complicated fracture characteristics that consisted of the severely deformed B2 particles,the intermittent smooth region around the B2 particles,and the highlybranched tiny shear bands with the nano-scale honeycomb-like patterns in the micro-scale shear planes.These complicated fracture features mainly relate to the complex stress state around the B2 particles originated from the elastic mismatch between the B2 particles and the amorphous matrix.The complex stress state around the B2 particles leads to the formation of the multiple branching shear bands before the accumulation of the shear stress that is required to activate the main shear bands and the cracks.

Acknowledgements

This work was supported by the National Research Foundation(NRF)of Korea grant funded by the Korea government(Ministry of Science and ICT) (No. 2017R1C1B5017092 and No.2018R1A2B3007167), Industrial Infrastructure Program for fundamental technologies(Project No.N0000846)funded by Ministry of Trade,Industry and Energy(MOTIE,Korea).

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