简介概要

Plasticity and microstructure evolution of W-CeO₂ rods with different short-duration pulse currents

来源期刊：Rare Metals2017年第12期

论文作者：Wen-Guang Zhu Jian-Can Yang Jie Cao Lin Huang Yu-Chen Xi Zuo-Ren Nie

文章页码：981 - 986

摘要：To improve the formability of W-rare earth electrode, the influence of high-energy pulse on the plasticity property of W-CeO₂ rods was investigated. The effects of current density（J₀）, pulse width（tw）, frequency（f）, and strain rate on the plasticity of W-CeO₂ rods were discussed in detail. Results of tensile tests show that the W-CeO₂ rods applied with the electrical pulses obtain a maximum percentage total elongation at fracture（9.65 %）, increased by118.7 % compared to that without pulses. This is owing to both the heat effect and the interaction of current between dislocations and rare earth additions. Electron back scattered diffraction（EBSD）-generated grain boundary（GB） maps suggest that the length of low-angle grain boundaries composed of high-density dislocations decreases after deformation while applying the pulse current. This demonstrates that the short-duration pulsed current enhances the mobility of dislocations. Scanning electron microscopy（SEM） images of the rods after deformation with the pulse current show that the long fiber-shaped additions become discontinuous,which could reduce the stress concentration and hinder the crack propagation.

稀有金属(英文版) 2017,36(12),981-986

Plasticity and microstructure evolution of W-CeO₂ rods with different short-duration pulse currents

Wen-Guang Zhu Jian-Can Yang Jie Cao Lin Huang Yu-Chen Xi Zuo-Ren Nie

School of Materials Science and Engineering, Beijing University of Technology

收稿日期：13 September 2014

基金：financially supported by the National Key Technology R&D Program of China (No. 2012BAE06 B02);the Beijing Municipal Science and Technology Project (No. Z141100003814008);

Plasticity and microstructure evolution of W-CeO₂ rods with different short-duration pulse currents

Wen-Guang Zhu Jian-Can Yang Jie Cao Lin Huang Yu-Chen Xi Zuo-Ren Nie

School of Materials Science and Engineering, Beijing University of Technology

Abstract：

To improve the formability of W-rare earth electrode, the influence of high-energy pulse on the plasticity property of W-CeO₂ rods was investigated. The effects of current density(J₀), pulse width(tw), frequency(f), and strain rate on the plasticity of W-CeO₂ rods were discussed in detail. Results of tensile tests show that the W-CeO₂ rods applied with the electrical pulses obtain a maximum percentage total elongation at fracture(9.65 %), increased by118.7 % compared to that without pulses. This is owing to both the heat effect and the interaction of current between dislocations and rare earth additions. Electron back scattered diffraction(EBSD)-generated grain boundary(GB) maps suggest that the length of low-angle grain boundaries composed of high-density dislocations decreases after deformation while applying the pulse current. This demonstrates that the short-duration pulsed current enhances the mobility of dislocations. Scanning electron microscopy(SEM) images of the rods after deformation with the pulse current show that the long fiber-shaped additions become discontinuous,which could reduce the stress concentration and hinder the crack propagation.

Keyword：

Pulse current; Plasticity; W-CeO₂ rod; Lowangle grain boundary;

Author： Jian-Can Yang e-mail:yjcan@bjut.edu.cn;

Received： 13 September 2014

1 Introduction

Tungsten,due to its high melting point,low work function,and outstanding high temperature strength,is widely used in the thermal electron emission field.The development of W electrode experienced from pure W,W-ThO₂ to ternary rare earth additions.The additions mainly contain La₂O₃,Y₂O₃,CeO₂,and ZrO₂ to replace ThO₂,in order to eliminate the radiation damage.In the 1950s,the Soviet Union scholar first reported some new kinds of tungsten electrode materials [ 1] .Sadek et al. [ 2, 3] systematically studied the welding performance,working temperature,work function values,and oxidation behavior of W-La₂O₃ and W-CeO₂electrodes.Shanghai Lighting Factory manufactured the W-CeO₂ electrodes which were awarded patent [ 4] .In recent years,composites of La₂O₃,Y₂O₃,and CeO₂ were added to tungsten matrix to improve the electron emission properties.Beijing University of Technology devoted several years to exploring multiple addition (La₂O₃-CeO₂-Y₂O₃)-W electrode [ 5, 6, 7] .

As we all know,W is hard to deform due to its low plasticity and high strength.Although brittle additions could enhance the emission properties and decrease the working temperature,the formability deteriorates ultimately due to the incoherent boundary between tungsten matrix and hard additions.Traditionally,the formability of metal is increased by plastically deforming the blanks at an elevated temperature [ 8] .But along with the temperature rising,some new problems arise,such as fastening the oxidation,increasing the thermal defect,enlarging the adhesion between blanks and dies,and reducing the service life of the equipment.In order to improve the formability at relatively low temperature,some works were investigated on the electroplastic effect,which was first reported by Troiskii [ 9, 10] in the 1960s.Conrad et al. [ 11, 12, 13] also did a great deal of theoretical studies and experiments on electroplastic.Recent research of Tang et al. [ 14, 15] is mainly about exerting highenergy pulses during wire drawing of 304L stainless steel.All of these studies showed that electroplastic reduced the deformation resistance,improved the ductility,and ameliorated the surface quality during drawing,and it especially has potential applications on refractory metal forming.In this paper,tensile test was carried out on W-CeO₂ electrode by imposing high-energy short-duration pulses to investigate the mechanical properties as a function of current density (J₀),pulse width (t_w),frequency (f),and strain rate.

2 Experimental

The experimental installation consists of the following parts.(1) HT-2402S25 tension tester which is made by HUNGTA.The stress-strain curve was generated on this tester at the speed of 0.5 mm·min^-1.To verify the repeatability of the results,three specimens were tested for each parameter and the average value was calculated.(2) High-frequency inverter pulse current device with its maximum output voltage of 20 V,peak current of10000 A,range of pulse width of 0.1-99.0 ms,and the pulse interval of 1-99 ms.(3) Insulation clamps.During tension tests,there is a high current loading to the rods,so the insulation modification of wedge gripping is needed.Moreover,the output pulses were closely monitored by the DS1000E digital oscilloscope,and the peak current was measured by the inverter pulse current device.In addition,the sample (Φ2.4 mm W-CeO₂ rod) was supplied by BGRIMM Advanced Materials Co.Ltd with2 wt%CeO₂.The influence of different current parameters on the mechanical property of the sample was studied by changing current density (J₀),pulse width (t_w),and frequency (f).FEI Quanta 640 scanning electron microscopy (SEM) and electron back-scattered diffraction(EBSD) techniques were employed to study the morphology changes due to CeO₂ additions and low-angle grain boundary (GB).

3 Results and discussion

3.1 Room-temperature tensile test applied high-energy pulses

The dependence of mechanical properties on current density (J₀),pulse width (t_w),frequency (f),and strain rate were considered in this experiment.J₀ was selected from800 to 1600 A·mm^-2 based on many tentative tests and reports from Conrad [ 11, 16] .Higher current density will lead to obvious oxidation and soften the rods,and if the current density is too low,brittle fracture will appear.

3.1.1 Influence of current density (J0)

Tensile tests were carried out by applying different current density pulses on W-CeO₂ rods,and the stress-strain curves are shown in Fig.1.As shown in Fig.1a(f=285 Hz),the strength of the bar decreases gradually as J₀ increases.When J₀ increases to 1000 A·mm^-2,the strength drop (Δσ/σ₀) is 21.8%compared to that of nonelectric tension test,whereσ₀ represents the initial strength andΔσis the strength drop after tests.The total elongation at fracture of the rod without current applied is 4.62%.However,the elongation reaches 9.47%while the J₀ is800 A·mm^-2,and slightly reduces to 8.43%asJ₀increases to 1000 A·mm^-2.It is obvious that appropriate J₀could reduce the strength and meanwhile improve the plasticity dramatically.In addition,elastic modulus(E) decreases a little as J₀ increases.In addition,if the current pulse is imposed during drawing,the fracture mechanism would change from brittle intercrystalline fracture without any obvious yielding to typical ductile fracture.And it can be drawn from Fig.1 that only if J₀ is>700 A·mm^-2,the remarkable yield platform would emerge,that is to say,the occurrence of electroplastic needs a critical value of current density.

Fig.1 Stress-strain curves with different J₀ at a pulse frequency of 285 Hz and b pulse frequency of 50 Hz

Fig.2 Stress-strain curves with different t_w at a pulse frequency of 50 Hz and b pulse frequency of 100 Hz

Fig.3 Effect of frequency on W-CeO₂ rods:a stress-strain curve at different frequencies and b frequency versus elongation curve

3.1.2 Pulse width (tw)

Figure 2 shows the effect of pulse width on the mechanical property of W-CeO₂ rod under the condition of J₀=1000 A·mm^-2.When the frequency (f) is 50 Hz,the rod does not appear yielding and is fractured directly as t_w=2 ms.While t_w=3 ms,the yield platform is obviously observed and the total elongation at fracture increases to 8.53%.The elongation drops to 8.04%,and the strength declines when t_w=4 ms.As shown in Fig.2b,f is 100 Hz,while t_w is 2 ms,and the elongation can reach9.56%.The strength of the rod decreases along with the pulse width increasing.As t_w is 4 ms,both strength and plasticity dramatically deteriorate.Hence,it is clear that there is an optimum point of t_w which has an utmost enhancement on plasticity when f and J₀ are fixed.

3.1.3 Frequency (f) and strain rate

A great deal of present research of electroplastic was focused on the effect of current density.There seems to be rare investigations conducted to characterize the effect of frequency.Figure 3 reveals the dependence of the stressstrain curve of W-CeO₂ rods on frequency.The total elongation at fracture does not monotonically increase with the frequency.When J₀=800 A·mm^-2,as the frequency increases,the elongation of the rod initially decreases and then increases.The maximum of total elongation at fracture is 9.38%with f=285 Hz.The mechanism of this effect needs further investigation.Figure 4 shows the relationship between strain rate and stress-strain curve when J₀=1000 A·mm^-2 and f=50 Hz.At low strain rate (<0.08 min^-1),the elongation of the rods does not significantly change and the strength enhances slightly as strain rate increases.However,the elongation deteriorates obviously as strain rate increases to 0.16 min^-1.This indicates that at a higher strain rate,the current parameter in static tensile test is no longer suitable.So the current parameters need to be adjusted during the drawing process.

3.2 High-temperature tensile test and thermo effect

As a matter of fact,the rods were produced under a specific temperature at drawing mill.In order to substantiate the effect of current on mechanical properties at high temperature,high-temperature static tensile test was introduced.The mechanical properties were characterized by force-displacement curves instead of stress-strain curve because the deformation becomes more heterogeneous at high temperature.In this part,the diameter of the Ce-W rods is 1.0 mm.

Fig.4 Stress-strain curves with different strain rates

Figure 5 indicates that with the temperature increasing,the tensile strength becomes lower gradually,while the elongation increases first and then decreases.The maximum total elongation at fracture is obtained at the temperature of 600℃.By comparing Fig.5a with Fig.5b,it can be seen that the elongation of the rods with pulse current applied is higher than that without the current.However,it is notable that the enhancement of elongation at high temperature is lower than that at room temperature.

Both room-temperature and high-temperature tensile tests substantiate that high-energy pulse current lowers the strength,but dramatically improves the plasticity of tungsten rods.However,whether this improvement of plasticity is mainly due to the Joule heat generated by pulse current or the facilitation of dislocation glide generated by electron wind is still intensely disputed.Gallo et al. [ 17, 18] used viscoelastic theory model to explain the effect of pulse current on the mechanical properties of materials that only considered the Joule heat.And Troitskii [ 10] expounded the electroplastic by electron wind force which improved the movability of dislocation.Conrad [ 16] also calculated the value of electron wind force and clarified the electroplastic in terms of thermally activated plastic deformation processes.He suggested that the influence of electron wind was limited and the main effect was on the pre-exponential.In regard to discussing the influence of temperature on tungsten,firstly,the temperature of the rod with pulse current applied was calculated.Based on the theory of steady-state heat transfer,the heat input is equal to the heat consumption on tungsten rods.Temperature rise calculated as the environment temperature is 600℃.The heat consumption mainly includes convective heat transferring to the surroundings and heat conduction in contact with the fixtures.The heat input per second input power (P) on tungsten rods is given by:

where I is the current through the rod,R is the resistance of tungsten at 600℃,D₀ is the duty cycle,S is the cross section area,J is the current density,l is the length of the rods,μis the pulse width,and p₀ is the resistivity.P was calculated as 1.4797 W.

The amount of convective heat transfer (Φ_t) can be written as:

where h_c is natural convection coefficient of heat transfer,A is the cross section of specimen,andΔt=(t_w=t_f)represents the time of temperature rising.

Fig.5 Force-displacement curves of W-CeO₂ rod under different temperatures:a without current applied and b with 1000 A·mm^-2 pulse current

And the heat flow direction was considered as from center to both ends of the rods,so the length of this distance (δ) is 0.01 m.The heat conductivity (Φ_c) calculation of thermal conduction is given as:

where k₀ and A_S represent thermal conductivity and cross section,respectively.

AsΦ_t+2Φ_c=P,the temperature rise was calculated to beΔt=68.64 K using MATLAB,andΔt at different furnace temperatures was calculated in the same way as shown in Table 1.

Figure 6 illustrates the elongation at different temperatures.The curves represent elongation applied with highenergy pulse current at the calculating temperature and that without the current.It indicates that the elongation of the rod with the pulse current during the tensile test is higher than that without the current in the range of 300-600℃.This suggests that the improvement of plasticity of tungsten rods is not only due to the Joule heat effect but also as a result of electroplastic effect.The interaction between current pulses and dislocations improves the movability of dislocation and enhances the plasticity of tungsten rods.

3.3 Microstructure evolution after pulse current applied

Tensile test indicates that the plasticity is obviously improved by the pulse current.These remarkable changes on the mechanical property are caused by the microstructural evolution.In order to reveal the morphology changes of dislocations and CeO₂ additions,SEM and EBSD were carried out.As shown in Fig.7,CeO₂ additions show a long fibrous morphology which is parallel to the tungsten matrix in normal tensile test;with current pulse imposed,as seen in Fig.7b,d,the additions are fractured and even become spheroidizing.These changes of CeO₂ additions increase the plasticity of the rods,as the fracture releases the internal stress which permits the additions to be consistent with the tungsten matrix in the subsequent deformation.Moreover,as the fibrous additions are mainly distributed along grain boundaries,during the further deformation,the dislocations are multiplied at the grain boundary,generating crack source.This crack spreads quickly along the hard-brittle additions and leads to a split easily.However,when the addition is fractured,the spread of crack is hindered,thus enhancing the plasticity.

下载原图

Table 1 Temperature correction in Fig.5b (℃)

Fig.6 Temperature versus elongation with/without current

Fig.7 SEM images of W-CeO₂ rod at room temperature:a,c without current applied;b,d with current pulse of 1000 A·mm^-2 and 50 Hz applied

Fig.8 EBSD-generated GB and band contrast map of W-CeO₂ rods:a without current applied and b current pulse of 1000 A·mm^-2 and50 Hz applied

The differences of grain boundary of W-CeO₂ rods with and without the pulse current applied are shown in Fig.8.The low-angle grain boundary (misorientation<15°) is composed of high-density screw dislocations,and the highangle grain boundaries show misorientation of>15° [ 19, 20] .It is interesting that the specimen from the rod with current pulse applied which shows higher elongation has low dislocation density,while that without current applied which shows lower elongation has much more low-angle grain boundaries instead.This unusual phenomenon obviously suggests that pulse current enhances the slip of dislocations remarkably,thus enhancing the plasticity of the rods.

4 Conclusion

It is shown that high-density pulse current remarkably improves the plasticity ofΦ2.4 mm W-CeO₂ rod and reduces the strength.The improvement on plasticity has a current density threshold (800-1000 A·mm^-2).Moreover,there is a more significant effect with the frequency ranging from 50 to 300 Hz and the duty cycle from 0.20 to 0.35.

High-temperature tensile tests suggest that the significant increase in elongation is not only due to the Joule heat effect but also as a result of the electroplastic effect.The interaction between the current pulse and dislocations improves the movability of dislocations and enhances the plasticity of tungsten rods.

SEM images show that fibrous CeO₂ additions are fractured into several discontinuous particles.EBSD-generated GB and band contrast maps show that the rod showing a higher elongation which imposes high-energy pulse current during tensile test has low dislocation density instead.This suggests that pulse current enhances the mobility of dislocations and thus improves the plasticity and decreases the flow stress.

Acknowledgments This study was financially supported by the National Key Technology R&D Program of China (No.2012BAE06B02) and the Beijing Municipal Science and Technology Project (No.Z141100003814008).This study is also in collaboration with the BGRIMM Advanced Materials Co.Ltd at Beijing.