Materials Science and Engineering C 49 (2015) 851–860

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Effects of high-energy electro-pulsing treatment on microstructure, mechanical properties and corrosion behavior of Ti–6Al–4V alloy Xiaoxin Ye a,b, Lingsheng Wang a,b, Zion T.H. Tse c, Guoyi Tang a,b,⁎, Guolin Song a a b c

Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China Key Laboratory for Advanced Materials of Ministry of Education, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China College of Engineering, The University of Georgia, Athens, GA 30602, USA

a r t i c l e

i n f o

Article history: Received 27 June 2014 Received in revised form 29 December 2014 Accepted 23 January 2015 Available online 26 January 2015 Keywords: Titanium alloy Biomaterials Electro-pulsing treatment Microstructure Mechanical properties Corrosion property

a b s t r a c t The effect of electro-pulsing treatment (EPT) on the microstructure, mechanical properties and corrosion behavior of cold-rolled Ti–6Al–4V alloy strips was investigated in this paper. It was found that the elongation to failure of materials obtains a noticeable enhancement with increased EPT processing time while slightly sacrificing strength. Fine recrystallized grains and the relative highest elongation to failure (32.5%) appear in the 11 second-EPT samples. Grain coarsening and decreased ductility were brought in with longer EPT duration time. Fracture surface analysis shows that transition from intergranular brittle facture to transgranular dimple fracture takes place with an increase in processing time of EPT. Meanwhile, corrosion behavior of titanium alloys is greatly improved with increased EPT processing time, which is presented by polarization test and surface observation with the beneficial effect of forming a protective anatase-TiO2 film on the surface of alloys. The rapid recrystallization behavior and oxide formation of the titanium alloy strip under EPTs are attributed to the enhancement of nucleation rate, atomic diffusion and oxygen migration resulting from the coupling of the thermal and athermal effects. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Ti–6Al–4V alloy (Ti–6–4), as an excellent structural material, has been in general use in multi-fields like aeronautic, navigation, biomedical and chemical industries, which rely on its outstanding properties. Among the abstracting performances, high specific strength, good corrosion resistance and fine biocompatibility played a more and more significant role in the research and manufacture. However, realistic applications of the material are strongly dependent on high strength– toughness property and anti-corrosion property at the same time [1]. In order to enhance the anti-corrosion performance of the materials, many scientists utilized various surface modification methods to produce protective films to improve the corrosion behavior [2–5]. Micro-arc oxidation is a well-known technique for improving corrosion resistance by surface modification [6,7]. For example, Yang and Wang [8] explored the relationship between the processing time and surface properties of titanium micro-arc oxidation film. Their results showed that suitable processing time for micro-arc oxidation would produce 10– 20 μm thick oxidation layer to improve the corrosion resistance. Additionally, Baszkiewicz and Krupa [9] studied plasma electrolytic oxidation ⁎ Corresponding author at: Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China. E-mail addresses: [email protected] (X. Ye), [email protected] (G. Tang). 0928-4931/© 2015 Elsevier B.V. All rights reserved.

(PEO) [10,11] and hydrothermal treatment [12,13] on the titanium in an electrolytic solution and the produced oxide layers were porous, highly crystalline and enriched with Ca and P elements, which improves the anti-corrosion behaviors greatly. Others studied comprehensively another surface modification method of laser alloying in improving corrosion behavior [14,15]. In these researchers, Li and Kar [16] utilized a kind of laser to form a laser-alloyed zone (LAZ) to produce commercially pure titanium coating on the aluminum substrate alloy enhancing the antiaffection of the aqueous corrosion. Ultrasonic striking treatment brought in the severe surface plastic deformation and residual compressive stress to restrain the corrosion procedure [17–20]. Selecting the micro-arc oxidation (MAO), plasma electrolytic oxidation (PEO), hydrothermal treatment and laser-alloyed method in the introduction part resulted from their wide range of research and application in the surface modification. These techniques all aim to improve the material corrosion properties as our goal in this paper. On the other hand, it's representative of these methods widely distributed in the physical and chemical fields, which could be an overall introduction of surface modification methods to the potential readers of various subjects. However, although these kinds of methods possessed many advantages in the processing and service, a number of potential weaknesses exist in that it's hard to control the thickness and structure of the prepared oxide film, dangerous working environment or chemical medium and complex procedure inducing highly cost, which all limit titanium


X. Ye et al. / Materials Science and Engineering C 49 (2015) 851–860

materials and surface modification techniques in the widespread application. These limitations stimulated the new idea and technique of applying EPT (electro-pulsing treatment) in the surface modification to improve the surface corrosion due to its high-efficiency, simple procedure, energy saving, environment-friendly and low cost, which implicates that the technique seems to be cheap [21–23]. In addition, it's convenient to realize the on-line treatment in the self-designed equipment with precise control system and temperature monitoring system, which guarantees the controllability and high-efficiency in the wide application in the material industry. Moreover, the microstructure and ductility improvement can be obtained at the same time. However, the realistic application is still demanded of the next step research in the rapid monitoring of the products, enlarging the production scale and final products facing the customers. Therefore, it's a potential technique of EPT in the future application. Recently, high-energy electro-pulse, as a novel style of energy input to process the materials in short procedure aiming to improve the microstructure and material mechanical performance, has emerged and attracted more and more attention [24–31]. In this process, electroplasticity, and electromagnetic field are applied simultaneously within a short time (about 10 s or even shorter) and lower temperature, and the most important is that the electro-pulsing treatment (EPT) can improve the microstructure and ductility of metals and alloys with ultrahigh efficiency. When the electrical flux with the peak value went through the specimen, which is almost 20–30 times of average efficient energy input of current in the certain time duration to transfer the enormous energy to atoms at a ultra-fast procedure. During the EPT process, atomic diffusion acceleration and energy barrier decrease induced by the hybrid effect of the thermal effect and athermal effect can noticeably decrease the requisite temperature and time duration of the procedure. Gromov and Ivanov [32] studied dislocation sub-structure evolution on creep and fatigue property of Al and stainless steel under weak electric potential. Our previous results [25,33,34] also successfully applied the EPT in processing FGM (functionally graded materials) titanium alloys improving the microstructure and mechanical properties of titanium alloy strips. However, the effects of EPT on the microstructure, mechanical properties and corrosion behavior of Ti–6Al–4V alloy have not raised many concerns in fabrication and relevant research. On the one hand, there're seldom researches in the microstructure and mechanical properties of titanium alloys induced by the EPT; on the other hand, surface modification and corrosion behavior of the titanium alloy under EPT are seldom conducted in the previous research. Thus the experiments are developed along these two aspects: microstructure improvement under EPT

induces the mechanical property enhancement and surface oxide layer under EPT induces the corrosion property improvement. To expand further the application and improve the understanding of EPT in the titanium alloy, the microstructure, mechanical property and corrosion resistance performance of the cold rolled titanium alloy (Ti– 6Al–4V) under electro-pulsing treatment (EPT) have been conducted in the research to produce titanium alloys with high strength–ductility property and superior anti-corrosion performance inexpensively. Relative mechanisms have been put forward to try to discuss the phenomena in the paper. 2. Experimental 2.1. Materials and EPT processing The annealed commercial titanium alloy Ti–6Al–4V alloy (6.66% Al, 5.13% V, 0.21% Fe, 0.03% Mo, balance Ti) was utilized. After cold rolling, the processed strip is 1.36 mm thick without obvious side cracks and defects. The entire on-line EPT process is shown schematically in Fig. 1. The titanium alloy strip experiences EPT statically and then the strip moves along the length direction treating the next section. Samples under EPT were subjected to high-energy current pulses with the same frequency and interval pulse duration (see Table. 1). The Ti–6Al– 4V strips move through two conductive electrodes and the pressure between the two electrodes and the strips is enough to keep good electrical contact without any deformation. A self-made electro-pulsing generator supplied the positive pulses within the short time duration (less than 100 μs). EPT was applied to the static strips in the EPT zone with various persisting times. The electrical parameters including frequency, root-mean-square current (RMS), amplitude, charge voltage (CV), effective time and duration of each current pulse were monitored by a Hall Effect sensor connected to an oscilloscope and timing device. 2.2. Measuring and characterizing The surface temperature of strips was measured by a storable K type thermocouple and infrared thermal device (FLIR SC600-series). The micro-hardness was conducted by micro-hardness tester (Struers Duramin 05656242) at ambient condition. The uniaxial tensile tests at room temperature were carried out to measure the mechanical properties by tensile testing machine. Fracture surface was observed by Hitachi S-4800 FEG scanning electron microscope (SEM). X-ray diffraction (XRD) was also utilized to characterize the constitution of material oxide surface by a Rigaku micro-diffraction goniometer

Fig. 1. Schematic view of near net shape rolling–EPT processes.

X. Ye et al. / Materials Science and Engineering C 49 (2015) 851–860 Table 1 Electropulsing parameters for treatment of the cold-worked Ti–6Al–4 V alloy. Sample number

Frequency (Hz)

Time (s)

Je (A/mm2)

Jm (A/mm2)

Measured temperature (°C)


– 160 160 160 160 160 160 160 160 160 160 160 160

– 5 6 7 8 9 10 11 12 13 14 15 16

– 13.6 14.2 11.5 12.8 13.7 12.9 14.5 13.4 13.0 13.5 12.8 14.1

– 169 167 172 171 169 170 166 173 165 153 168 188

– 403 420 476 490 515 531 560 593 606 653 650 648

Notes: Je represents the root-mean-square value of current density in EPT processing and Jm is the amplitude of current density.

equipped with a one-dimensional detector using Cu Kα radiation operated with 40 kV/200 mA. For the evaluation of the electrochemical corrosion behavior, sample electrodes were prepared by epoxy cold-resin mounting and the immersion solution is 6% volume fraction dilute HF electrolyte solution at a pH of 3.0. The corrosion potential was measured as a function of time for 30 min with a stability of 0.016 mV/s. Subsequent polarization tests were conducted after a 30 min immersion. Potentiodynamic polarization scans were carried out with a scan rate of 1 mV/s. All tests were conducted in naturally aerated conditions at 37 °C. Optical microscopes (OMs) were used to study the microstructure, vertical fracture plane, oxidation layer and corrosion surface with an Olympus GX51 microscope and a HiROX KH-7700 microscope. 3. Results 3.1. Effect of EPT on the mechanical property and anti-corrosion property of the Ti–6–4 strip The effect of EPT on the mechanical property and corrosion property of cold-rolled titanium alloy has been studied. The dependency of the ultimate tensile strength (UTS) and the elongation to failure (EL) of the EPT samples on the processing time has been shown in Fig. 2. The EL of the cold-rolled sample was enhanced with lengthening the processing time of EPT. The whole process of enhanced ductility could be divided into three parts: (a) preliminary part (persisting time is shorter than 6 s), in this period the EL and UTS changed slightly, (b) intermediate part (the persisting time is from 6 s to 12 s), EL greatly improved and (c) decline part, the strength and plasticity both lowered at some extent.

Fig. 2. Dependency of the ultimate tensile strength (UTS) and the elongation to failure (EL) of the EPT samples on the EPT processing time.


According to the changed law of the mechanical properties with the persisting time evolution, at EPT7 (exceeding 10 second-processing time of EPT) elongation reached the maximum value (32.5%). While at this increasing EL process the UTS of the specimen has just a slight decrease from the cold-rolled state. It's convenient to determine the relative characteristics Δσ and Δδ as measuring the effects of the EPT on the mechanical property with the changes of the persisting time: Δσ ¼

σ cr −σ t  100%; σ cr


δt −δcr  100% δcr


and Δδ ¼

where σt means the UTS of the samples under EPT and σcr means the UTS of the cold-rolled samples, δt is the EL of the samples under EPT and δcr means the UTS of the cold-rolled samples. For the EPT8sample, the Δδ can be 550% and Δσ is only 15%, so it's obvious that EPT enhanced the ductility greatly at the expense of a little strength. In the additional increasing persisting time of EPT to 14 s, the UTS and EL both obtained a bit decline due to grain coarsening. Fig. 3 shows the potentiodynamic polarization curves for cold-rolled (CR) sample and the ones under the EPT tested in dilute HF solution at 37 °C. The corrosion potentials (Ecorr) ranged from −16.56 mV/SCE to − 738.74 mV/SCE for all the samples. The corrosion potentials (Ecorr), corrosion current densities (Icorr), and passive current densities (Ipass) of the samples from their polarization curves are summarized in Table 2. The corrosion current densities (Icorr) are obtained from the polarization curves using the Tafel extrapolation method fit. With the help of EPT the anti-corrosion performance enhanced by the effect of EPT, Ecorr is improved with a large extent (from −738.74 mV to −313.62 mV) to the positive direction and Icorr decreased noticeably from 12.48 μA/cm2 to 2.53 μA/cm2. What's more, with increasing the processing time of EPT, corrosion potentials rise positively to −16.56 mV and the corrosion current densities decreased to 0.17 μA/cm2. The phenomenon in corrosion potentials and corrosion current evolution may be caused by the difference of the surface condition. On the other hand, all the samples were characterized in a partial stabilization of their current densities ranging from 1.53 μA/cm2 to 88.32 μA/cm2, which suggests that a protective passive film is formed within this current range. Ipass is as high 37.44 μA/cm2 (passive film formed late at the whole corrosion process) that numerous and largescale etching pit existed after the electrochemical corrosion. If the samples were treated by EPT, passive current densities decreased by the

Fig. 3. Potentiodynamic polarization curves for cold-rolled specimen and various EPT specimens in HF solution at 37 °C.


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Table 2 Corrosion potential (Ecorr), current density (Icorr), passivation current density (Ipass) and passivation potentials determined by the polarization curves. Sample number

Ecorr (mV/SCE)

Icorr (μA/cm2)

Ipass (μA/cm2)

Epass (V/SCE)


−738.74 −313.62 −196.59 −136.4 −98.77 −16.56

12.48 2.53 1.92 0.41 0.26 0.17

37.44 – 88.32 12.87 9.62 1.53

−0.46 – 0.36 0.18 0.25 0.18

improvement of anti-corrosion. However, at EPT5 passive current densities roared abnormally to 88.32 μA/cm2 because the oxidation layer is not thick and dense enough in undertaking the electrochemical corrosion so that corrosion cracks occurred and matrix titanium alloy was exposed to the solution under a relatively high scanning potential. The passive potentials shift toward the positive direction with the introduction of EPT. In addition, the absolute value of the passive potentials for EPT samples is decreased from that of the cold-rolled samples. Thus passive potentials demonstrate that the direction of passivation process is different in the cold-rolled specimen without oxide film and the EPT samples with oxide film. On the other hand, the EPT induced

oxide film can facilitate the passivation process with the less absolute passivation potentials. However, on the contrary the passivation disappears with the unstable oxide structure induced by the weak EPT2 process. 3.2. Effect of EPT on the microstructure, tensile fracture behavior of the Ti–6–4 strip Fig. 4 demonstrated the typical microstructure evolution of the coldrolled (CR) and EPT samples under various EPT parameters. The CR sample was characterized (Fig. 4a) as a generally inhomogeneous deformed microstructure with fragmented small grains and elongated grains with an average grain size of 50 μm. At the same time, the deformed twins also can be seen in the big grains. With increasing processing time to 6 s (EPT2), the early state of the recrystallization and recovery process can be shown in Fig. 4b. Polygonal grain boundaries and inhomogeneous nucleation of preliminary recrystallization occurred along the intensive strain area (grain boundaries and deformed twins). When persisting time went up to 9 s (Fig. 4c), the deformed microstructure and grain boundary gradually weakened accompanied by the augmented volume fraction of recrystallization and diminished average grain size (~36 μm). At 11 second-EPT figured as Fig. 4d, the microstructure which is full of complete recrystallization eliminated the deformed

Fig. 4. Typical microstructures of (a) cold-rolled sample and samples under different processing parameter-EPTs at (b) EPT2, (c) EPT5, (d) EPT7, (e) EPT9 and (f) EPT11.

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structure and micro-twins. At the same time, the microstructure consisted of dual size grains (5 μm and 20 μm). The microstructure presented in Fig. 4e is homogenized to the uniform equiaxed grains (11 μm) under the EPT9. Increasing the processing time to 14 s the grains grow up slightly to 15 μm within the low temperature and short time treatment. Three parts in the microstructure can be in agreement with the mechanical properties. During the (a) period, preliminary recovery statement can only enhance the ductility slightly as in Fig. 4b. In the next period, recrystallization process (Fig. 4c and d) facilitated the rapid increment of the plasticity. But when the processing time is longer than 12 s, the grain coarsening may cause the deterioration of UTS and EL at some extent as shown in Figs. 2 and 4e–f. Fig. 5 demonstrated the SEM fracto-graphs of the cold-rolled specimen and the other ones under EPT with different processing times. For the cold-rolled sample, typical brittle fracture behavior (Fig. 5a) characterized as the poor ductility consisted of about 15 μm wide river patterns, micro-cracks and smooth cleavage planes. At EPT2 (Fig. 5b), the improved ductility leads to the quasi-cleavage characterized as about 20 μm cleavage planes and shallow dimples. With increasing the processing time to 9 s (Fig. 5c), the fractograph comprising of more than 50 μm long tear ridge and some more big-sized dimples


represented the intermediate ductile fracture mode. When it came to EPT7, 20 μm and 5 μm deeper dimples presented the ductile fracture corresponding to the optimal ductility (EL can be 32.5 in Fig. 2). Increasing the lasting time of EPT to 13 s and 14 s (Fig. 5e and f) there appeared tear ridges again around the dimples and cleavage planes to represent the intermediate ductile fracture and quasi-cleavage fracture respectively with the decreased plasticity (EL — 27.9 and 26.5). After EPT the smooth edge fracture surface near the fracture of the cold-rolled specimen Fig. 6a has been transformed to the trans-granular fracture (Fig. 6b–f). Due to improved ductility of the EPT2 sample (Fig. 6b), there existed some micro-cracks near the leading edge of the fracture and small protrusions. With lengthening the persisting time to 9 s micro-cracks disappeared and local ductile regions appeared to present a better ductility state. Further increasing the time to 11 s as shown in Fig. 6d, complete recrystallization microstructure (Fig. 4d) induced large amounts of ductile regions and obvious protrusions representing the outstanding ductility. With the lasting time of EPT to more than 11 s, local ductile region (Fig. 6e) and smaller protrusions (Fig. 6f) occurred with plasticity drop induced by the uniform and coarsening grains as presented in Fig. 2. For the fracture surface (Figs. 5 and 6), they also explained the ductility evolution with increased processing time of EPT. For (a) period,

Fig. 5. SEM fractographs of (a) cold-rolled sample and samples under EPT with various processing parameters: (b) EPT2, (c) EPT5, (d) EPT7, (e) EPT9 and (f) EPT11.


X. Ye et al. / Materials Science and Engineering C 49 (2015) 851–860

Fig. 6. Optical micrographs of the (a) cold-rolled sample and other samples under different EPT processing parameters [(b) EPT2, (c) EPT5, (d) EPT7, (e) EPT9 and (f) EPT11] near the tensile fracture.

although the ductility slightly increased the river patterns disappeared and small dimples around the cleavage planes occurred. Great improvement of the ductility brought in intermediate ductile fracture and ductile fracture as shown in Fig. 5c and d. Due to grain coarsening, EL decreased a bit leading to the fractographs transforming into intermediate ductile fracture with tear ridges and even into quasi-cleavage fracture with cleavage planes. Similar phenomena occurred in the fracture surface near the tensile fracture presented in Fig. 6. 3.3. Effect of EPT on the oxidation layer and corrosion morphology of the Ti6-4 strip Lateral observation of the oxidation layers of the materials under EPT has been presented in Fig. 7. As a control Fig. 7a showed the transverse directional graph of the cold-rolled specimen with a clean and straight edge. Under EPT, obvious oxidation layer appeared outside the Ti alloy matrix as shown in Fig. 7b–f. With an increased persisting time of EPT, the oxidation layer grows to be denser and thicker. As shown in Fig. 7b, under 6 second-EPT sparse and thin oxidation layer (~10 μm) occurred outside the matrix metal. Also the oxidation rate for each

part of the surface is different as shown by the rough edge and oxidation denoted by the white arrow. When the lasting time of EPT is lengthened (Fig. 7c), a 15 μm thick layer with more compactness grows up. When the persisting EPT time is more than 10 s, the oxidation layer is getting denser and thicker (Fig. 7d–e) 20–50 μm which can be an effective way to hinder the electrochemical corrosion as shown in Fig. 3 and Table 2. The corrosion potentials (Ecorr) can be changed positively to −16.56 mV and corrosion current densities (Icorr) were lowered to 0.17 μA/cm2. Fig. 8 is the XRD pattern for the oxidation layer outside the substrate titanium alloy. Tested anatase-TiO2 explained the thicker and denser titanium dioxide layer under EPT which could be a protective layer from corrosion. After electrochemical corrosion, Fig. 9a showed the severe corrosion morphology with big-area etch pit and excessive inner corrosion without the protection of the oxidation layer. Under EPT2 (Fig. 9b), the stripped oxidation layer area occurred and naked matrix alloy presented the better corrosion resistance as shown in Fig. 9a. But on this condition the perfect anticorrosion has not been obtained owing to the thin and unsound oxidation layer (Fig. 7b). When the processing time of EPT is further lengthened, deep corrosion cracks evenly distributed on the material surface with about 20 μm naked matrix areas. Better anti-corrosion performance

X. Ye et al. / Materials Science and Engineering C 49 (2015) 851–860


Fig. 7. Transverse directional micrographs of the (a) cold-rolled sample and other samples under different EPT processing parameters [(b) EPT2, (c) EPT5, (d) EPT7, (e) EPT9 and (f) EPT11] near the tensile fracture.

Fig. 8. XRD pattern for the oxidation layer outside the substrate titanium alloy.

occurred in the EPT7 sample (Fig. 9d), instead of the naked matrix titanium alloy under electrochemical corrosion several etching points appeared on the surface observation. 13 second-EPT (Fig. 9e) brought in the tiny etching points on the surface of the material and thinner corrosion cracks surrounding them. EPT11 can be best anti-corrosion property of no etching points with tiny etching crevice as shown in Fig. 9f on the surface of the oxidation layer. Improvement of anti-corrosion by lengthening the processing time of EPT is the result of denser and thicker oxidation layers as shown in Fig. 7. It's the effective anatase-TiO2 layer that protects the matrix material from corrosion process as shown in Fig. 8. TiO2 layer greatly improve the anticorrosion compared to the pure titanium alloy. Li and Wang [35] utilized nano-TiO2 coatings doped with cerium nitrate by sol–gel method for corrosion protection of 316 L stainless steel. Shao and Nab [36] found that an incorporation of titania nanoparticles in nickel nanocomposite coating can achieve much improved corrosion resistance and mechanical properties of both hardness and wear resistance performances. Thus, the denser


X. Ye et al. / Materials Science and Engineering C 49 (2015) 851–860

and thicker oxidation layer brings the better anti-corrosion performance presented in Fig. 7b–f in the electrochemical corrosion of the HF dilute solution. Direct observation of the corrosion surface can be seen in Fig. 9. Obviously, the corrosion procedure is impeded by the help of relatively thinner oxidation layers under EPT2 and EPT5 as a comparison of excessive corrosion of the cold-rolled sample with a large-area etch pit as shown in Fig. 9a. Denser and thicker oxidation layers showed better anti-corrosion without any naked matrix in the process of electrochemical corrosion. There're only etching points and narrow crevices left on the surface after the corrosion (Fig. 9d–f). It's longer persisting time EPT that induced the formation of denser and thicker oxidation layers enhancing the corrosion resistance. 4. Discussion 4.1. The rapid temperature rising effect and selective effect in EPT processing

multiple pulse treatment (MPT) for the recrystallization of Mg–3Al–1Zn (AZ31) alloy strip and the surprising results that the complete recrystallization state of samples subjected to the electro-pulsing effect can be obtained rapidly similar to 10 s at relatively lower temperatures compared with conventional heat with this coupling of thermal and athermal effects. With the athermal effect, electron wind force induced by the drift electrons can exert a force on dislocation and acceleration of atom diffusion. By the previous research results, Joule heating effect is relative to the applied charge voltage and persisting time. Jiang and Tang [38] put forward a model for calculating the temperature field of a strip under EPT with the contributions of the coupling of the Joule heating effect and athermal effects. Yamaguchi and Nasu [39] found that high current density results in the Joule heating of the sample and the corresponding temperature rise ΔT considering the persisting time in an ambient condition that can be worked out based on the adiabatic condition: −1

ΔT max ¼ tρjm t p ðCdÞ Instead of the single effect of traditional heating treatment, EPT is a fast increasing temperature procedure thanks to the coupling of thermal and athermal effects. Xu and Tang [37] reported the application of



where d = 4.44 × 103 kg/m3 is the density of the specimen, t is the processing time of EPT, jm is the maximum current density, tp = 80 μs stands

Fig. 9. Surface appearance of the (a) cold-rolled sample and other samples under different EPT processing parameters [(b) EPT2, (c) EPT5, (d) EPT7, (e) EPT9 and (f) EPT11] after the electrochemical corrosion.

X. Ye et al. / Materials Science and Engineering C 49 (2015) 851–860

for the EPT duration, C = 750 J/(kg·°C) means the thermal capacity of the specimen, ρ = 1.8 × 10−6Ω·m is the resistivity of the sample, k is the response factor of frequency and f is the frequency of EPT. Therefore, short persisting time EPT brought in obvious temperature rise in the treated sample. The current going through the specimen produced the heat in the view of selective effect, which produced the heat on the sample unevenly. The current pulses detour in the inhomogeneous physical field with the existence of many defects including micro-cracks, dislocations, voids and boundaries (grain boundaries and alpha-beta phase-boundaries) distributed unevenly. Thus uneven current detour makes uneven temperature distribution with local relative high temperature and local relative low temperature, which presents the “mosaic area”. In fact, local overheating by selective effect of EPT on the titanium alloy could supply enough temperature and input energy to drive the recrystallization to occur, while the whole apparent temperature of the specimen was relatively low as the processing temperature under EPT was 420 °C for EPT2 to start the recrystallization. 4.2. Effect of electropulse on the rapid recrystallization process and ductility of Ti alloy strip in EPT process The rapid recrystallization was accelerated by the dislocation motion, formation and growth of sub-grains under EPT. Meanwhile, a relatively low temperature kept the fine newly formed grain microstructure away from severe grain coarsening. In fact, the mechanism of the recrystallization is not currently fully clear. One possible mechanism is that acceleration of dislocation climbs into the sub-grain boundaries by the coupling effect of thermal and athermal effects. Yanbi and Guoyi [40] found that this climb process is greatly related to the atomic flux in the research of EPT treated AZ61 magnesium alloy strip. During the EPT process, atomic diffusions noticeably are sped up by the hybrid effect of the thermal effect and athermal effect, as can be denoted by the following style [41]: F ¼ Ft þ Fa ¼

  2πD1 C  1þ þ 2N  D1  Z  e  ρ  f  jm  τ p =ðπkT Þ Ω ln ðR=r Þ C0

ð4Þ  .  −Q D1 ¼ D0  exp RT


where Ft means the flux of atoms induced by the thermal activation effect during EPT and Fa is the flux of atoms resulting from athermal effect of EPT, D1 means the lattice diffusion coefficient, C0 means the average concentration of vacancies, Ω denotes the atomic volume, r and R are distances from dislocation where the vacancy concentrations are C0 and C0 + C, N is the atomic density, Z means an effective chemical valence, e is the carried charge of an atom, ρ means the electrical resistivity, k means the Boltzmann constant, T is the thermodynamic temperature, jm, f and τp are the amplitudes of the current density, the frequency and the duration of EPT, respectively; D0 means the diffusion base factor, Q means the activation energy, and R is the thermodynamic gas constant. According to the above Eq. (4), athermal effect of the EPT made a contribution of the atomic flux for the dislocation motion to facilitate recrystallization. But temperature is another important factor which can't be neglected by Eqs. (4) and (5). These formulas showed that the thermal effect in the atomic diffusion for the recrystallization process and the lattice diffusion coefficient are both relative to the temperature. Additionally, athermal effect is also positive to the lattice diffusion coefficient based on the Eq. (4) so that the athermal effect also depends on the temperature. The titanium alloy can be heated to a very high temperature in a short persisting time by the EPT referring to Eq. (3). Therefore rapid increase in temperature induced a strong coupling effect of thermal and athermal effects of


atomic diffusion so that the recrystallization process can be accomplished in an ultrafast process. At the same time, thanks to the coupling effect of the thermal and athermal effects the apparent temperature under which the recrystallized process began is relative low (less than 500°C). On the other part, the selective effect by the EPT also leads to local overheating to fulfill the temperature for starting the recrystallization. Based on above analysis, a short persisting time EPT could speed up the recrystallization process due to increasing atomic flux by the thermal and athermal effects. EPTs supplied multi-momentum energy pulses to speed up the atom diffusion to fulfill the recrystallization under low temperature and short persistence. However, less than 6 second-EPT can't obtain the phenomenon so that it's necessary to take the temperature for consideration in rapid recrystallization process. Certain temperature is totally necessary because temperature is an important factor for the atomic flux in the EPT process as discussed in the previous part. 4.3. Effect of electro-pulses on the oxidation layer and corrosion resistance of Ti alloy strip in EPT process EPTs facilitated the oxidation layer growing up to be dense and thick enough to sustain the electrochemical corrosion within about 10 s. At the same time, the corrosion resistance of the TiO2 is better than that of the titanium alloy, so EPTs effectively enhance the corrosion resistance of the materials. The reaction equation of the oxidation layer in the air condition is shown as the following style: Ti þ O2 → TiO2 θ


ð6Þ θ

G298 ¼ Δ f H 298 −T  S298 ¼ −889:3kJ  mol




where the thermodynamic parameters for TiO2 (s) are ΔfHθ298 = − 944.6 kJ ⋅ mol− 1 and Sθ298 = 50.28 J ⋅ mol− 1 ⋅ K−1, for O2 (g) ΔfHθ298 = 0 kJ ⋅ mol−1 and Sθ298 = 205.14 J ⋅ mol−1 ⋅ K−1 and for Ti(s) ΔfHθ298 = 0 kJ ⋅ mol−1 and Sθ298 = 30.69 J ⋅ mol−1 ⋅ K−1. The reaction of Eq. (6) can proceed to the positive direction automatically on the standard state fulfilling the thermodynamic condition from Eq. (7). In the process of EPT, temperature and pressure are higher than that of the standard state, so the oxidation process occurred from the view of thermodynamics. Furthermore, the oxidation layer grows up on the surface of the sample in roughly four procedures: (a) oxygen molecule adsorption on the surface of the titanium alloy, (b) nucleation of titanium dioxide, (c) nucleus grow laterally and (d) formation of the compact oxidation layer. When the nucleus grew along the side direction to cover the surface of the material, the layer kept the matrix alloy away from air condition, and the oxidation process slowed down by the control of the mass transfer of oxygen. The air in the subsequent oxidation process came into reaction through the cracks or crevice, grain boundary diffusion and volume diffusion. Therefore, in general the oxidation formation kinetics is presented in the following style: dδ k ¼ dt δ 2

δ ¼ kt þ C:

ð8Þ ð9Þ

In the above equation, δ is the thickness of the oxidation layer, C is the integration constant and k is the factor of the oxidation reaction. Eq. (9) is the kinetics model of oxidation process of EPT. On the account of Arrhenius equation: k ¼ k0 expð−Q =RT Þ:


In the above formula, Q is effective activation energy, R is the gas constant, T is the temperature. The titanium alloy under EPT can reach


X. Ye et al. / Materials Science and Engineering C 49 (2015) 851–860

a relative high temperature in a short persisting time according to Eq. (3). Thus the reaction rate of the oxidation on the high temperature can be great to lead a rapid process on the basis of Eqs. (8) and (10). This phenomenon is in agreement with Fig. 7b, in just 6 second-EPT a thin oxidation film formed on the surface of the material. Oxygen came to the surface to continue the reaction easily so that the EPT7 oxidation layer (Fig. 7d) can be formed as 20 μm thick within just 11 s. Of course, the oxidation process is a much more complex procedure than imagined. Zhang and Hu [42] found that electromagnetic waves radiate from the surface and form colinear diffraction-limited electromagnetic beams in the inward and outward directions of semiconductors and metals. Thus, the electromagnetic field caused by EPT may affect the ionization and oxidation of the gas molecules surrounding the treated specimen. Whether this effect can also accelerate the formation of the oxidation layer is our next research work in the future. Further lengthening the process time of EPT to 13 s and 14 s, with the 50 μm thick oxidation layer the oxidation rate decreased by the difficulty of oxygen coming into contact with the inner alloy just as is shown in Fig. 7e and f. The oxidation layer formed more slowly with the thicker film that can be explained by Eq. (8). That means the oxidation rate is inversely proportional to the thickness of the oxidation layer. If the oxidation time is longer and the thickness of the layer is greater, the oxidation layer of the surface grew more slowly. This slow surface oxidation layer formation can be illustrated in Fig. 7e and f. With the help of protective titanium dioxide film, the corrosion rate is decreased to improve the anti-corrosion performance as presented by the previous part 3.1. The electrochemical corrosion potential shifted positively and the corrosion current decreased after EPT. The corrosive solution could simulate hostile environment in chemical enterprise and navigation. Additionally, H+ and F− can be made as an analogy of the inner human body environment like body fluid and mouth. So there are not any problems in applying the material in these fields for the long lasting time. From the aspects of the corrosion resistance, EPT11 is the best treatment technique in this group of experiments as shown in Figs. 3, 7 and 9. However, the mechanical property (Fig. 2) pointed out that this is not the best selection yet. On the other hand, excessive oxidation may lead to the deterioration of other material performance and loss of the material. So EPT7 (11 s) may be the better selection from the overall consideration of mechanical property and anti-corrosion performance. What's more abstracting is that the near net shape the EPT processing technique is a low temperature and fast procedure to produce the final materials with the demanding high strength–ductility and corrosion resistance performance. According to the oxidation kinetics of metallic materials, Tao et al. [43] put forward the expression of the oxidation layer as the following style:   ΔW n ¼ kn t þ c: A


In the equation, ΔW means the mass difference between before and after the oxidation process; A is the cross sectional area of the material; n is the index number of the oxidation rate; kn is the coefficient of oxidation velocity; t is the time and c is the reaction constant. It's easy to produce thick oxidation layer on the surface under high temperature for long-lasting times. While, processing temperature and persisting time could be reduced by the athermal effect to limit the thickness of the oxide with less than 100 μm as shown in Fig. 7. Therefore, EPT could produce the protective film on the surface of the titanium alloy strip without much weight loss. 5. Conclusions The high-energy electro-pulsing treatment (EPT) technique is a promising and environment-friendly method to improve the deformability and

anti-corrosion concurrently of treated titanium materials. What's more abstracting is low temperature and short processing time of this technique. With increasing the EPT time, elongation to failure (EL) of the coldrolled titanium alloy can be greatly improved with only sacrificing a slight strength. At 11 second-EPT, EL can be increased to maximum value (32.5%) with the fine recrystallized grain microstructure and ductile fracture observation. Enhancing nucleation rate and accelerating atomic diffusion by the coupling effect of EPT was proposed to discuss the mechanism of rapid recrystallization and mechanical performance. Meanwhile, the anatase-TiO2 layer is formed on the substrate titanium alloy under EPT. With increasing persisting time the oxidation layer gradually becomes thicker and denser to impede the electrochemical corrosion. Electrochemical corrosion parameters and corrosion surface both demonstrate better anti-corrosion property of the EPT samples than that of the cold-rolled sample. The coupling effect of ionization and Joule heating under EPT was put forward to explain the rapid formation of the oxide layer. Due to outstanding composite performances of treated titanium alloys with low processing temperature and high efficiency, the electro-pulsing treatment (EPT) technique can be widely utilized in the aerospace, navigation, chemical engineering and biomedical fields. Acknowledgments The work is supported by National Natural Science Foundation of China (No. 50571048) and Shenzhen Science and Technology Research Funding Project of China (No. SGLH20121008144756946). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]

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Effects of high-energy electro-pulsing treatment on microstructure, mechanical properties and corrosion behavior of Ti-6Al-4V alloy.

The effect of electro-pulsing treatment (EPT) on the microstructure, mechanical properties and corrosion behavior of cold-rolled Ti-6Al-4V alloy strip...
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