金属及合金表面功函数研究

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Changes of hardness and electronic work function of

Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 bulk metallic glass on annealing K. Luo a ; W. Li a ; H. Y. Zhang a ; H. L. Su a

a Faculty of Material and Photoelectronic Physics, Key Laboratory of Low Dimensional Materials &

Application Technology (Ministry of Education), Xiangtan University, Hunan, Xiangtan 411105, PR

China First published on: 02 February 2011

To cite this Article Luo, K. , Li, W. , Zhang, H. Y. and Su, H. L.(2011) 'Changes of hardness and electronic work function of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5 bulk metallic glass on annealing', Philosophical Magazine Letters, 91: 4, 237 — 245, First published on: 02 February 2011 (iFirst)

To link to this Article: DOI: 10.1080/09500839.2010.539989

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Philosophical Magazine Letters

Vol.91,No.4,April 2011,237–245

Changes of hardness and electronic work function of

Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5bulk metallic glass on annealing

K.Luo,W.Li *,H.Y.Zhang and H.L.Su

Faculty of Material and Photoelectronic Physics,Key Laboratory of Low Dimensional Materials &Application Technology (Ministry of Education),Xiangtan University,

Hunan,Xiangtan 411105,PR China

(Received 5April 2010;final version received 9November 2010)

The hardness and electronic work function (EWF)of a bulk metallic glass,

namely Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5,have been studied experimentally,with

an emphasis on the effect of heat treatments.The glass was annealed at

different time and temperatures,and its hardness and EWF measured using

the Rockwell indentation technique and a scanning Kelvin probe system,

respectively.It is found that the EWF decreases with annealing time and

temperature,whereas the hardness increases.This study shows a close

relationship between hardness and EWF,indicating that the EWF could be

a sensitive parameter for characterising and investigating the mechanical

behaviour of BMG at the electronic level.

Keywords:bulk metallic glass;annealing;electronic work function;

hardness

1.Introduction There has been much interest in bulk metallic glasses (BMGs)because of their potential engineering applications [1,2].Compared with crystalline alloys,metallic glasses exhibit excellent mechanical properties including high compressive strengths and hardness values [1–4].Besides,they show high corrosion and wear resistance,as

well as good magnetic properties [5].Alloy systems,such as Zr–Ti–Cu–Ni–Be and Pd–Cu–Ni–P,have good glass-forming ability (cooling rates below 100K/s)and can be prepared using recent developments [6,7].However,these glasses are extremely brittle [8,9],which compromises their potential engineering applications.Despite all the virtues of BMGs,the disadvantage of low plasticity,arising from their disordered atomic structure [10],needs further investigation.

To investigate the disordered atomic structure,the glass transition of BMGs has attracted much attention,because it impinges on the development of new systems of BMGs as well as being of intrinsic interest.The glass transition,i.e.the transition from a state of internal equilibrium (supercooled liquid)into a non-equilibrium state (glass)and back,is associated with a change in enthalpy;see for example [11,12].In the study of BMGs,the glass transition temperature,T g ,is one of the most *Corresponding author.Email:wenl@ualberta.ca

ISSN 0950–0839print/ISSN 1362–3036online

?2011Taylor &Francis

DOI:10.1080/09500839.2010.539989

https://www.360docs.net/doc/485067765.html,

D o w n l o a d e d B y : [S u , H . L .] A t : 01:20 14 M a r c h 2011

important characteristic parameters.When a glass is annealed at a temperature 5T g ,structural relaxation (so-called physical ageing)takes place [11,12].In this process,the molecular mobility changes and there is a decrease in enthalpy and free volumes [13].The decrease in free volume during the annealing process makes plastic deformation more difficult and thus embrittlement occurs [14,15].Characterisation of the mechanical properties of BMGs is very important for structural applications.Hardness,as a measure of resistance to permanent deformation and thus to wear,is an important parameter for applications.The Rockwell hardness (HR)test is the most widely used mechanical method for determining hardness.The HR test,introduced in the 1920s,was developed based on force and displacement calibrations [16,17].Its measurement uncertainties have been largely reduced in recent years by new methods,such as the employment of stylus and laser interferometry techniques [17].In order to achieve a fundamental understanding of the mechanical properties of BMGs,it is necessary to take investigations to the electron level.As a fundamental characteristic of solid surfaces,the electronic work function (EWF)is a promising parameter suitable for such studies.The EWF of a metal is defined as the difference between the electrochemical potential inside the metal and the electrostatic potential just outside it [18].It can be easily determined using the scanning Kelvin probe (SKP)technique [19].The measurement system consists of a digital oscillator,a data acquisition unit and a sample translation device,controlled by a host PC.On account of its inherent high surface sensitivity and lateral resolution,it can be employed more powerfully for the analysis of a wider range of materials,at different temperatures and pressures,than any other surface analysis techniques [18,19].In this letter,we report the measurements of hardness and EWF of BMG samples,which were heat treated for different annealing times and temperatures.The aim is to establish a relationship between the hardness and the EWF,which can be useful when investigating the microstructure of BMGs.2.Experimental details

The material used in this study has a composition of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5,which can be prepared by mature processing technology and has shown promising applications.Alloy ingots were prepared by arc melting mixtures of pure metal elements in a titanium-gettered argon atmosphere,followed by suction casting into a copper mould at about 1atm pressure.BMG samples with dimensions of 12?3?2mm 3(the size of the copper mould)were annealed in a resistance furnace for times of 30and 60min,and at four temperatures,320 C,360 C,380 C and 450 C,during both annealing periods.The crystalline structures of the as-cast and annealed samples were characterised by X-ray diffraction.Thermal analysis was performed by differential scanning calorimetry (DSC)under an argon atmosphere at a heating rate of 0.33K/s.

Rockwell indentation experiments were conducted using a Wilson indenter ( ?60 ).Both geometrical and non-geometrical factors affect the hardness performance of the indenters.Geometrical properties include the mean tip radius and the maximum and minimum radii,profile peak and profile valley deviations,the

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mean cone angle as well as the maximum and minimum cone angles,the cone flank straightness,any holder axis alignment error,surface roughness and surface defects.Non-geometrical factors include the mechanical properties of the diamonds and the soldering of the diamond prism into the holder.Here,the maximum loads (F max )were chosen as 10,20,30,40and 50N for the Rockwell test.The loading rate and the holding time at the maximum load were controlled at 0.02N/s and 100s,respectively.Prior to the indentation tests,the samples were polished in successive steps to 1m m finish using diamond pastes.The EWF was measured using an SKP system,which was provided by KP Technology Ltd.(Caithness,UK).The system had high resolution (550m eV)and the probe spacing could be controlled within 40nm.A three-axis microstepper positioner permitted high-resolution sample positioning (0.4m m/step).In this study,a gold tip with a diameter equal to 1mm was used and the oscillation frequency of the Kelvin probe was set as 173Hz.The tested samples were then lightly polished using a slurry containing aluminium oxide powder (0.05m m).After polishing,the samples were ultrasonically cleaned in reagent-grade acetone (10min)and reagent alcohol (5min).All tests were carried out on the polished surfaces without etching in order to reduce the probability of formation of surface films.For the EWF measurement,the surface under study was scanned line by line by the Kelvin probe over an area of 1?1mm which covered 10?10?100points.Each measured value is therefore an average over 100measurements,which is statistically more precise than that of a measurement at a single point.In this study,all presented EWF values were obtained by averaging four measurements.3.Results and discussion Figure 1shows DSC thermograms of the as-cast and the isothermal annealed samples.They exhibit an endothermic feature characteristic of the glass transition.Here,T g is defined as the onset temperature of the glass transition,T x is the onset temperature of the crystallisation event.D T ,defined as T x àT g ,is referred to as the supercooled liquid region.The DSC trace of the as-cast alloy reveals that the glass

transition temperature is 623K and the supercooled liquid region spans 80K before the onset of crystallisation at 703K.A comparison between the DSC scans obtained from the as-cast and the annealed samples shows no obvious difference.However,closer examination of the glass transition regime (indicated by the dotted box in Figure 1a,and shown magnified in Figure 1b)reveals subtle but systematic changes at about 650K.A sharp exothermic peak is observed for the as-cast glass prior to the endothermic reaction caused by the glass transition,and the exothermal enthalpy is about 18J/g.With increasing annealing temperature,the height of exothermic peak reduces gradually.After 523K (12h),573K (12h),593K (1h)and 633K (1h)annealings,the exothermal enthalpy values are 13,7.2,8.4and 2.7J/g,respectively.This means that annealing at 633K for 1h leads to a reduction in the exothermal enthalpy by 85%with respect to that of the as-cast sample.

The exothermic peak prior to T g during the DSC measurements is known to be caused by structural relaxation well below the glass transition temperature [20–24].It has been well-documented that the exothermic enthalpy is the result of annihilation Philosophical Magazine Letters 239

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of excess free volume,D v f .The reduction in the free volume D v f gives rise to a heat release D H ,when the glass sample is heated in DSC,and D H is proportional to D v f

[20–24].Hence,it is possible to estimate the free-volume changes that occur during annealing by monitoring D H [20–24].From Figure 1b,it can be seen that D

H

Figure 1.(a)DSC curves of the as-cast and annealed samples and (b)enlarged view of the glass transition regime.

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gradually decreases on account of the reduction of free volume upon structural relaxation annealing.So,we choose the proper annealing temperatures for low-temperature heat treatment based on the result of DSC to investigate the microstructure of the metallic glass.The result of the latter experiment can be interpreted by the free-volume theory.

The as-cast and annealed samples were studied by scanning electron microscopy (SEM).Shallow wells on the surface of the BMGs were seen (Figure 2)after 60min of annealing.They became more obvious although the amount of them was unchanged as the annealing temperature became higher.These changes were attributed to structural relaxation in the annealing process according to the analysis of Cernoskov et al.[12].

One can see from Figure 2that the graininess is in existence both before and after annealing.This means that crystallisation has not taken place,thus allowing the study of microstructural changes during low-temperature annealing in this study.Figure 3shows the variations in the EWF of annealed Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5samples with respect to annealing temperature and annealing time.From Figure 3a,one can see that the EWF decreased as the annealing temperature increased.When the annealing temperature increased below the onset of crystallisation at 430 C (703K),both curves decreased gradually following an approximate linear relation-ship,and then decreased further when the annealing temperature reached 445 C.Moreover,by comparing the two curves in Figure 3a,one can see that the EWF of the 60min annealed samples was lower that of the 30min annealed ones at each annealing temperature,with an approximately constant decrement,except for the point at 450 C.As can also be seen in Figure 3b from another perspective,the EWF decreased as the annealing time increased while the as-cast sample and a 10min annealed sample were added for comparison,for an annealing temperature of 380 C.The decrease in the EWF becomes more gradual as the annealing time is continuously increased.

These results indicate that the annealing treatment lowers the minimum energy required to extract an electron from the inside of a bulk solid to the outside.This may arise from a falling free volume in the annealed BMG.Wigner and Bardeen [25]proposed that the work function can be expressed as

?à tD ?à àep "0:e1TFigure 2.SEM micrographs of the (a)as-cast,(b)360 C annealed (t ?60min)and (c)450 C annealed (t ?60min)alloys.

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The first term in Equation (1)is the bulk chemical potential of the electrons relative to the mean electrostatic potential in the metal interior,and the second term corresponds to the energy necessary to penetrate the dipole barrier D at the surface.The surface dipole barrier is formed by the redistribution of electron density on the surface and the chemical potential is a parameter that is inversely proportional to the free volume [26].Since is inversely proportional to the free volume of the bulk

[27],

Figure 3.Variations of the EWF of the annealed samples treated during (a)isochronal annealing and (b)isothermal annealing.

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the EWF decrease as the free volume is decreased by annealing is consistent with the EWF results.Therefore,when the free volume decreases gradually by low-temperature isochronal annealing,the EWF decreases approximately linearly as shown in Figure 3a.The decline became more obvious when the annealing

temperature lies beyond the onset of crystallisation temperature,which results in the annihilation of excess free volume.Similarly,the EWF decreases as the annealing time increases,which also has a positive effect on the decline of free volume.Figure 4shows the variation of hardness of 60min annealed Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5BMG with respect to various annealing temperatures,and compared with the work function curve.It can be seen that the hardness

increases with the annealing temperature during low-temperature annealing because of structural relaxation,which is consistent with previous studies [26,28–30].The hardness displays an obvious rise in the first period of increasing annealing temperature,after which the curve becomes smoother when the annealing temper-ature exceeds the onset of crystallisation temperature.An increase in hardness with annealing has been reported for a wide variety of BMGs [31].In most cases,the hardness increase is linear with the crystalline volume fraction and this is attributed to micromechanisms in the nanocrystalline phase [32].It has also been suggested that solute enrichment in the amorphous phase arising from primary crystallisation could be responsible for the continuous increase in the hardness even when the crystalline phase is softer [32].Structural relaxation occurs through the annealing treatment and the free volume decreases.Thus,the two curves in Figure 4show an inverse relationship.A higher EWF value corresponds to a lower hardness.In summary,structural relaxation of the BMG,caused by an increase in the annealing temperature or the time,decreases the free volume,resulting in an increase in the Figure 4.Variations of the EWF and hardness of 60min annealed samples with respect to annealing temperatures.Philosophical Magazine Letters 243

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EWF and a decrease in the hardness.The relationship between EWF and hardness indicates that the EWF is a very promising parameter for fundamental understand-ing of the mechanical properties of BMG.

4.Conclusions

In this study,we have investigated the effects of isochronal and isothermal annealing treatments on the hardness and the EWF of a BMG with a composition of Zr 41.2Ti 13.8Cu 12.5Ni 10Be 22.5.The experimental results show that the EWF of the metallic glass decreases with an increase in the annealing time and temperature while the hardness increases with an increase in the annealing temperature.Moreover,there is a relation between the variation of EWF and hardness,namely if the annealing temperature is below the glass transition temperature (430 C),the EWF decreases and the hardness increases gradually,whereas if the annealing temperature is above 445 C,the decrease in EWF and the increase in hardness both become significantly greater.Such results can be interpreted using the concept of structural relaxation and free-volume theory.The relationship between hardness and EWF indicates that the latter is closely related to mechanical properties and could thus be used a sensitive parameter for characterising and investigating the mechanical behaviour of BMGs.

Acknowledgements The authors acknowledge the financial support of the Natural Science Foundation of China (Reference Nos.10972190,10872177and 20973146245),and the State Key Laboratory of Advanced Metals Materials 246(Reference No.EG512677781CN).The project is also sponsored by the Scientific Research Foundation for Returned Overseas Chinese Scholars,State Education Ministry.References

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