Rare Metals2017年第11期

Isothermal nanocrystallization behavior of bulk metallic glass composites in supercooled liquid region

Xiang-Yun Zhang Zi-Zhou Yuan Xue-Lei Feng Li-Zhi Cui Duan-Xian Li

State Key Laboratory of Advanced Nonferrous Materials,Lanzhou University of Technology

收稿日期：23 September 2013

基金：financially supported by the National Natural Science Foundation of China (Nos.51061008 and 50961008);

Isothermal nanocrystallization behavior of bulk metallic glass composites in supercooled liquid region

Xiang-Yun Zhang Zi-Zhou Yuan Xue-Lei Feng Li-Zhi Cui Duan-Xian Li

State Key Laboratory of Advanced Nonferrous Materials,Lanzhou University of Technology

Abstract：

Bulk metallic glass composites(BMGCs)reinforced by micro-(spherical size of about 10 μm) and nano-sized(20-50 nm) Al₃Zr crystals were synthesized by copper mold suction casting method. The isothermal crystallization of Cu₄₀Zr₄₄Ag₈Al₈ BMGCs in the supercooled liquid region was studied by differential scanning calorimeter(DSC), X-ray diffractometer(XRD), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). The mechanisms for nucleation and growth of the crystalline phases precipitated in two-stage crystallization process and the effect of the isothermal treatment on the in situ crystalline phases were discussed.Results show that the BMGCs experience a primary crystallization process. Nano-sized Al₃Zr and Cu₁₀Zr₇ phases precipitate from the amorphous matrix successively. Furthermore, isothermal annealing has no effect on the preexisting particles in the as-cast sample, which creates a new method to synthesize BMGCs strengthened by duplex crystalline phases with a large size difference. And the reinforced phase could also be dominated with intention if the as-cast BMGCs preparation and isothermal treatment process can be controlled more accurately.

Keyword：

Composite; Bulk metallic glasses; Thermal stability; Heat treatment; Crystallization;

Author： Xiang-Yun Zhang e-mail:zhangxiangyun86@163.com; Zi-Zhou Yuan e-mail:yuanzz@lut.cn;

Received： 23 September 2013

1 Introduction

Despite the noticeable advantages of monolithic bulk metallic glasses (BMGs) [ 1, 2] ,almost all the BMGs fracture catastrophically with nearly no macroscopic plasticity at room temperature due to the formation of highly localized shear band,which significantly restricts its applications in engineering [ 3, 4] .For this reason,considerable efforts were devoted to the development of BMGCs enhanced by secondary particles both by in situ and ex situ methods,which gained a series of gratifying achievements [ 5, 6, 7, 8, 9] ,especially for the BMGCs strengthened by duplex crystalline phases.For example,Mondal et al. [ 10] found that the post-annealed BMGCs with certain volume fraction of free volume,and nanocrystallization showed significant plasticity up to 16%.More recently,Li et al. [ 11] found that the Zr_47.3Cu₃₂Al₈Ag₈Si_0.7Ta₄ BMGCs enhanced by combining in situ precipitate (20-30μm) and ex situ micro-sized Ta particles (about 20μm) exhibited similar thermal properties compared with their base alloy counterpart,and the compressive plastic strain of more than 25%and the fracture strength of 1800 MPa were obtained at room temperature.Furthermore,a replication of microscaled hologram pattern by hot pressing was successfully synthesized [ 12] ,owing to the viscous flow behavior within its supercooled liquid (SCL) temperature region.But,all the previous researches focused on the mechanical properties of BMGCs but ignored the fact that they are also metastable materials.With their pure amorphous counterparts,the mechanical properties of BMGCs are very susceptible to partial or full crystallization.

The thermal stability and microstructure evolution in the crystallization process were widely studied at the very beginning of researches on BMGs [ 13, 14] .As for BMGs,during thermal treatment above the glass transition temperature (T_g),crystallization occurs.According to Xing et al. [ 15] ,annealing between T_g and the first crystallization peak of the BMGs usually produces nanocrystals embedded in a residual amorphous matrix.Annealing at temperatures above the first crystallization peak leads to interconnecting nanocrystallites.But,the study on the thermal stability and microstructure evolution of these BMGCs was rarely reported.

In this paper,the microstructure evolution of an in situ particle strengthened BMGCs was explored by isothermal heat treatment.To our surprise,its crystallization process revealed an opportunity to fabricate new BMGCs strengthened by combination of micro-size and nano-size crystalline phases.Furthermore,the volume fraction of particles with different sizes can be adjusted randomly by controlling the process of multi-stage crystallization.

2 Experimental

Ingots with nominal composition of Cu₄₀Zr₄₄Ag₈Al₈ (at%)were prepared by arc melting of mixtures of pure metals(99.99%purity) for 3 times under a high purity argon atmosphere.Then,the ingots were remelted,and suctioncast into a water-cooled Cu molds to get rods with a diameter of 4 mm and length of 70 mm under 7 kW of power and negative pressure of 0.02 MPa by the electromagnetic levitation suction casting equipment.Furthermore,melt-spun ribbons of 30μm in thickness and 1 mm in width were fabricated as a reference by vacuum melt spinning.The ribbons were identified to be pure amorphous structure by transmission electron microscope (TEM).

To understand the microstructure evolution of the BMGCs,heat treatment was employed.The duration of heat treatment was deduced from its isothermal DSC curves at738 K in the SCL region.According to this isothermal DSC curves,the as-cast s amples were isothermally annealed at the same temperature for 15,20,25,30,40,and 60 min in a muffle furnace (TM-04179) under an air atmosphere with the temperature accuracy of±1 K.Then,the samples were dropped to water immediately after reaching the scheduled time.On the one hand,water quenching can avoid structure change.On the other hand,Li et al. [ 16] studied the quenching mode of thermal treatment of BMG and found that water quenching treatments made the Zr₆₄Al₁₀Ni₁₅Cu₁₁BMG have a higher strength.Subsequent mechanical burnish was adopted to remove the oxide layer.Slices with various thicknesses were cut from the rods for different detections by diamond slicing machine.

Micros true tures of samples before and after annealing were characterized by X-ray diffraction spectrometer(XRD,Japan Nikkaku D/max-2400,Cu Kα),scanning electron microscope (SEM,JSM-6700F),and TEM (JEM-2010).The annealed samples used for SEM measurement were first mechanically polished to a mirror finish and then chemically etched in a solution of 1%hydrofluoric acid aqueous solution for 5 s.Samples used for the TEMmeasurement were perforated by chemical jet thinning using a solution of 10%perchlolic acid in ethanol.The nonisothermal properties of the samples were determined by differential scanning calorimetry (DSC,NETZSCH STA 449C Instruments) at a heating rate of 5 K·min^-1.At this heating rate,the errors in determining T_g and the crystallization onset temperature (T_x) were less than 0.5 K using indium as a calibration standard [ 17] .The two parameters of the as-cast sample were determined to be 696and 753 K,respectively.

3 Results and discussion

Figure la shows XRD pattern of the as-cast rod.The sample is basically amorphous as seen from the broadened diffuse hump in the 2θrange of 35^°-45^°.No apparent crystalline peak is detected except a few tiny Bragg peaks from Al₃Zr phase.In parallel,SEM was used to further verify the microstructure of the sample,due to the low resolution of XRD.The metallographic examination by backscattered electron image (BEI) in Fig.lb shows that only a small amount of crystals with a spherical size of about 10μm are homogeneously embedded in the amorphous matrix.The energy dispersive spectrometer (EDS)analysis for the particles shows that they have nearly the same Al/Zr atom ratio as Al₃Zr phase.Both of these SEM examinations are in accordance with the XRD analysis.In addition,TEM observation and the corresponding selected area diffraction (SAD) pattern of the substrate in the BEI image reveal that nano-sized precipitates in the range of20-50 nm are embedded in the amorphous matrix,as shown in Fig.1c.Thus,we can conclude that the as-cast samples are BMGCs with micrometer and nanometer grains embedded in the amorphous matrix.The overall crystallized volume fraction calculated by its DSC curves is 9.555%,which will be shown in the later section.

Fig.1 XRD pattern a and BEI image b of as-cast Cu₄₀Zr₄₄Ag₈Al₈ sample,and c SAD pattern of amorphous matrix

Fig.2 Isothermal DSC curve at 738 K for as-cast Cu₄₀Zr₄₄Ag₈A1₈ BMGCs

Fig.3 Nonisothermal DSC curves of reference film,as-cast sample and samples pre-annealed for various time at 738 K

Figure 2 shows the isothermal DSC scan of the BMGCs annealed at T_a (738 K) for 100 min.Characteristic time was defined with hollow circles,and the corresponding thermal stability and microstructures will be discussed later.The isothermal process of the BMGCs consists of three parts.The first part indicates the incubation time for crystallization,owing to the differences between the annealing temperature and the first crystallization temperature revealed in the continuous-heating DSC scan in Fig.3,which was widely discussed in Refs. [ 15, 18] .

The second part includes two rapid heat release peaks,indicating that crystallization of different phases may occur in the amorphous matrix when the heating time enters into this region.

The third part with very slow heat output indicates that the crystallization process slows down [ 15] .A considerable amount of grains nucleation and growth in part two may make the grains impinge to each other,and the rate of crystallization becomes very sluggish during continued isothermal annealing.Basu et al. [ 19] also found that when the crystallized volume fraction of a La-based BMG reached a critical point,longer annealing time induced no significant change in crystallized volume fraction.

Samples are scattered to pieces due to the large stress brought about by water quenching,and the signals detected in the DSC scans are obscure when the heat treatment time reaches 40 min.So,the samples annealed for not more than30 min were detected by DSC.Figure 3 shows the DSC scans of the reference amorphous film and BMGCs annealed for various time (0,15,20,25,and 30 min) at a constant heating rate of 5 K·min^-1.Compared with DSC scans of the BMGCs,the exothermal peak intensities of the pure amorphous film are stronger and narrower,which may be caused by the different preparation processes.

As for the BMGCs,the overall areas of exothermal peak gradually diminish with the isothermal annealing time increasing,indicating the crystals precipitate from the amorphous matrix during annealing.In addition,T_g,T_x,and the exothermal peak (T_p) all shift toward lower temperature with the isothermal time increasing,which indicates that the precipitation of crystalline decreases the thermal stability of the residual amorphous matrix [ 19] .The crystallized volume fractions (V_f) calculated by the decrease of the exothermic heat of crystallization on the continuous DSC curves are shown in Table 1.The values agree with the isothermal DSC curve.On the one hand,the increased volume fractions (Vf) of crystalline for the isothermal time corresponding to the two exothermal peaks in the isothermal DSC scan reach 38.87%and 27.58%,respectively.On the other hand,for the smooth rise in isothermal DSC curve between 20-25 min,there is only3.74%increase in V_f.In addition,the critical V_f corresponding to the third part with the crystallization process slowing down is 79.746%,which is comparable to the results of the La-based BMGs [ 15, 19] .

The BMGCs synthesized at high quenching rates are in a nonequilibrium structural state at room temperature and will automatically change to metastable state or stable state upon heating.Figure 4 shows XRD patterns of the as-cast samples annealed at T_a for different annealing time.The as-cast sample shows tiny crystalline peaks of Al₃Zr phase and a broad diffraction peak centered at 2θof 39^°(named as Peakα) corresponding to the short region orders (SRO)of Zr-Cu,owing to the large negative heat of mixing of atomic pairs of Zr-Cu (Table 2) [ 20] .But,it does not manifest distinct broad diffraction peak of SRO of Zr-Al with the largest negative heat of mixing in this composition,owing to the very small amount of aluminum content.

Fig.4 XRD patterns of as-cast and thermal-treated Cu₄₀Zr₄₄Ag₈Al₈BMGCs

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Table 2 Heat of mixing (△H_mix) between atomic pairs (kJ·mol^-1)

With the structural relaxation taking into effect during the long incubation time of 13 min as shown in the isothermal DSC curve in Fig.2,more Zr-Cu and Zr-Al atomic clusters form,which leads to a higher volume fraction of SRO.Therefore,the broad diffraction peak centered at 2θof 21°(named as Peakβ) corresponding to the SRO of Al-Zr appears,and the intensity of Peakαincreases with the isothermal annealing time increasing.Furthermore,continued isothermal annealing leads to a considerable amount of grain nucleation and growth and eventually results in the appearance of distinct Bragg peaks of Al₃Zr phase (crystallized in body-centered tetragonal structure with a=0.4009 nm and c=1.7281 nm) when the annealing time increases to 15 min.

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Table 1 Crystallized volume fractions of thermal-treated samples

Fig.5 SEM images of Cu₄₀Zr₄₄Ag₈Al₈ BMGCs annealed for20 min:a amorphous matrix and b pre-existing grains

For the sample annealed for 20 min corresponding to the end of the first exothermal peak in the isothermal DSC scans,almost the entire Peakβis replaced by Bragg peaks of Al₃Zr phase and minor peaks of Cu₁₀Zr₇ phase.Nevertheless,the intensity of Peakαcontinues to grow.For the sample annealed for 30 min corresponding to the end of the second exothermal peak in the isothermal DSC scans,a majority of Peaksαare replaced by sharp peaks of Cu₁₀Zr₇phases (base-centered orthorhombic system,with the unit cell dimensions of a=1.2675 nm,b=0.9313 nm,c=0.9347 nm,and Z=4),and eventually the intensity of Cu₁₀Zr₇ peaks obviously exceeds that of Al₃Zr peaks.The ultimate crystallization products are A1₃Zr,Cu₁₀Zr₇,and some unknown phases (cannot be identified for lacking enough Bragg peaks).

In general,the BMGCs naturally crystallize according to time sequence and go through a series of intermediate links:formation of SRO of Zr-Cu and Zr-Al atomic clusters→precipitation of Al₃Zr phase→precipitation of Cu₁₀Zr₇ phase,which is in good accordance with the isothermal DSC scans.Therefore,BMGCs strengthened by specific particles can be synthesized with intention by carefully controlling the volume fraction of crystalline.However,there is an exception with the crystalline phase that a few tiny peaks of an unknown phase emerge in the XRD pattern during the crystallization process,which may be resulted from the already existing crystal nucleus in the as-cast BMGCs.Moreover,the solidification sequence and the identified microstructure are somewhat in contradiction with the other observations in Refs. [ 21, 22] for the same composition,which may be caused by the different treatment methods.

Fig.6 BEI image a and SEM image b of Cu₄₀Zr₄₄Ag₈Al₈ BMGCs annealed for 40 min

In addition,it is rather interesting to note that the crystal peaks move to low temperature with annealing time increasing.And the peak positions do not agree with the standard PDF peaks until the annealing time reaches30 min,reflecting the chaotic arranged atoms in nonequilibrium state keep getting close to equilibrium state through atom moving during isothermal annealing.

To further survey the evolution of the size of the particles in the BMGCs,the samples annealed for 20 and40 min were detected by SEM.Figures 5 and 6 show the SEM observations of the samples annealed for 20 and40 min,respectively.

In general,the amorphous structure is known to improve the corrosion resistance of an alloy,which is attributed to the lack of defects like dislocations or grain boundaries,the chemical homogeneity and a rapid formation of thin barrier-type films composed of the oxides of the valve-metal components [ 23, 24, 25] .It is for this basic reason that the crystalline phases are prior to be corroded to a certain depth as soon as the polished samples were put into the hydrofluoric acid aqueous solution.In addition,the amorphous matrix with high corrosion resistance still keeps its smooth surface even though the etching time reaches 5 s.Figure 5a shows SEM image of the amorphous matrix of the BMGCs annealed for 20 min.Particles with a spherical size of about 200 nm precipitate from the amorphous matrix.But Fig.5b shows that the original grains with a spherical size of about 10μm in the as-cast samples have hardly any growth after annealing for 20 min.

Figure 6a shows BEI images of the samples annealed for 40 min.The pre-existing grains still keep its original size of about 10μm,indicating that heat treatment has no effect on the size of the pre-existing particles in the as-cast BMGCs,which creates a new method to synthesize BMGCs strengthened by in situ particles with a large size difference.In other words,we can get BMGCs strengthened by duplex crystalline phases with different volume fractions of nano-size and micro-size particles by carefully controlling pre-BMGCs preparation and heat treatment process,respectively.

Figure 6b shows SEM image of the amorphous matrix of the samples annealed for 40 min.The particles with size reaching～500 nm impinge on each other,but there is still residual amorphous matrix left with annealing time further increasing up to 40 min.The microstructure observation is consistent with the isothermal DSC scan,as discussed in the previous section.

4 Conclusion

The microstructure of the Cu₄₀Zr₄₄Ag₈Al₈ BMGCs consists of micro-and nano-sized Al₃Zr crystals embedded in the amorphous matrix.Nano-sized Al₃Zr and Cu₁₀Zr₇phases precipitate from the amorphous matrix successively during annealing.But,thermal treatment has no effect on the pre-existing micro-sized particles in the amorphous matrix,which creates a new method to synthesize BMGCs strengthened by duplex crystalline phases with intention.The phases of strengthened particles and the sizes of different phases can be controlled by carefully processing the pre-BMGCs preparation and the following heat treatment.

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Nos.51061008 and50961008).

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