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Surface vitrification of alloys by pulsed electrical discharge treatment
Journal of Alloys and Compounds xxx (2017) 1e7
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface vitrification of alloys by pulsed electrical discharge treatment Lei Zuo, Shujie Pang*, Shanfang Zou, Haifei Li, Tao Zhang** Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2016 Received in revised form 1 December 2016 Accepted 23 December 2016 Available online xxx
Pulsed electrical discharge (PED) treatment was applied as a novel method to synthesize amorphous or amorphous/crystalline composite layers on the surfaces of Zr-, Fe-, Ti- and Al-based crystalline alloys. The formation of amorphous or amorphous/crystalline composite layers was investigated in terms of the significant disparity in glass-forming ability of the alloys. The influences of PED processing parameters on the surface glass formation of Zr55Al10Ni5Cu30 alloy were further discussed and the PED-treated alloy exhibited a gradient structure: amorphous surface, amorphous/crystalline composite region and crystalline substrate from surface to inside. Besides, effects of processing parameters on the microhardness and corrosion behavior of the treated Zr55Al10Ni5Cu30 alloy were investigated and the results revealed that the PED treatment could improve the microhardness and corrosion resistance of the crystalline substrate alloy. This PED treatment is a promising method to synthesize amorphous or amorphous/ crystalline composite layers for novel mechanical, physical or chemical applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Amorphisation Composite materials Metallic glasses Microstructure Mechanical properties Corrosion
1. Introduction Metallic glasses are considered to be suitable candidates for wear- and corrosion-resistant coatings for crystalline metallic components, due to their high hardness and corrosion resistance resulting from their unique structural uniformity [1e8]. With a high cooling rate, laser surface treatment technique has been employed to prepare metallic glass surfaces/coatings [9e11]. In view of high cooling rate, pulsed electrical discharge (PED) treatment may also be a feasible method for preparing glassy metallic surfaces/coatings. During the PED treatment, alloy surfaces can be melted by electrical discharge and subsequently solidified under the cooling effect of alloy substrates during pulse intervals. Additionally, dielectric fluid can be introduced during the treatment, which may also contribute to the cooling effect. Electrical discharge machining has been used for machining workpieces for decades, based on the principle of the conversion of electrical energy into thermal energy by generating a succession of controlled electrical discharges between the electrode and the workpiece immersed in a dielectric fluid [12,13]. Thus, surface vitrification of alloys by PED
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Pang), [email protected] (T. Zhang).
treatment may be fulfilled by the electrical discharge machining. Attempts have been made to investigate the surface modifications of metallic materials by PED in the form of electrical discharge machining [14e18]. It has been reported that white layers were generated on a steel workpiece by PED treatment, leading to an increased resistance to corrosion and abrasion [14]. Besides, a crack-less layer of titanium carbide has also been formed by PED treatment to improve the surface properties [15]. Syntheses of coatings on alloy surfaces by PED treatment using powder metallurgy tool electrodes has also been reported [16e18]. However, PED treatment has not yet been used to synthesize amorphous or amorphous/crystalline composite layers on crystalline alloys. Different from laser surface treatment which is one-dimensional spot-scanning process, PED treatment is a two-dimensional process by applying the wire tool electrode to scan the alloy surface (Fig. 1). Thus, compared with laser surface treatment, PED treatment may be more efficiency to fabricate glassy metallic surfaces/ coatings. In the present study, surface treatments by PED using the wire electrical discharge machine were conducted on the Zr-, Ti-, Fe- and Al-based crystalline alloys with significant disparities in glassforming ability (GFA) as well as thermal properties (Table 1) [19e23]. The influences of GFA as well as melting temperatures and microstructural heterogeneity of the alloys on the surface glass formation were investigated. The effects of processing parameters of the PED treatment, including pulse duration (ton), pulse interval
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Fig. 1. (a) Schematic diagram of pulsed electrical discharge treatment on alloy surface, and (b) actual profile of a single PED pulse [13].
Table 1 Glass transition temperature (Tg), onset temperature of crystallization (Tx), solidus temperature (Tm), liquidus temperature (Tl) and critical diameters (Dc) of the Zr-, Ti-, Fe-, and Al-based metallic glasses. Alloy (at.%)
Tg (K)
Tx (K)
Tm (K)
Tl (K)
Dc (mm)
Ref.
Zr55Al10Ni5Cu30 (Zr0.55Al0.10Ni0.05Cu0.30)98Y2 Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1 Ti50Ni15Cu25Sn5Zr5 [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 Al86Si0.5Ni4.06Co2.94Y6Sc0.5
685 673 670 680 820 e
774 763 711 765 870 497
1090 e 1120 e e 903
1156 e 1205 e 1390 1055
10 12 4 5 5 ~1
[19] [19] [20] [21] [22] [23]
(toff), working voltage (V) and working current (I), on the surface structure and properties of the treated Zr55Al10Ni5Cu30 alloy with high glass-forming ability were further studied. 2. Experimental Alloy ingots with nominal compositions of Zr55Al10Ni5Cu30, (Zr0.55Al0.10Ni0.05Cu0.30)98Y2, Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1, Ti50Ni15Cu25Sn5Zr5, [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 and Al86Si0.5Ni4.06Co2.94Y6Sc0.5 (at.%) were prepared by arc-melting the mixture of the pure elements as well as an industry-grade FeB alloy in a Tigettered argon atmosphere. To guarantee the compositional homogeneity, the ingots were re-melted at least 5 times. The ingots were first cut into rectangular plates of 4 8 8 mm by a DK7716 type wire electrical discharge machine. Then, the square faces were polished with #2000 sand papers to eliminate the influences of the cutting. Finally, the polished surfaces of the rectangular plates were treated by PED using the wire electrical discharge machine with different processing parameters, and the schematic diagram and the actual profile of a single PED pulse [13] are shown in Fig. 1. The wire tool electrode is molybdenum wire. The central axis of the wire electrode (~120 mm in diameter) is 60 mm away from the alloy surface during the PED treatment. Structures of the treated alloys were examined by X-ray diffraction (XRD, Bruker D8-advance) with Cu-Ka radiation, scanning electron microscopy (SEM, JEOL JSM-7500F) coupled with an energy-dispersive spectrometer (EDS) and high-resolution transmission electron microscopy (HRTEM, JEOL-2100F). For the TEM observation, the PED-treated surfaces of the samples were bonded together, and then the cross section was mechanically ground to ~50 mm in thickness and finally electro-polished to perforation at the bonded region with a 25% HNO3-methanol solution at about
243 K. Thermal properties of the treated alloy surface layer were investigated by differential scanning calorimetry (DSC) at a heating rate of 0.33 K/s. The procedure to prepare DSC samples was as follows: a small piece was cut from the PED-treated alloy surface by diamond cutting machine, and then the small piece surface adjacent to the substrate was polished by sand papers to obtain a sample of several micrometers thick in order to reduce the impact of the substrate. Considering the wave surface of the recast layer, the DSC sample included both crests and troughs of the wave surface. Microhardness of the treated alloy surfaces was measured by Vickers hardness tester (450-SVD) under a load of 50 g for 15 s. Corrosion behavior of the treated surfaces in 3 mass% NaCl aqueous solution was characterized by electrochemical measurements conducted in a three-electrode cell using a PED-treated sample as the working electrode, a platinum foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. After the specimens were immersed in the solution at room temperature for 1 h to achieve a stable open-circuit potential, potentiodynamic polarization curves were measured at a scan rate of 0.833 mV/s. 3. Results The main parameters involved in the PED treatment includes pulse duration (ton), pulse interval (toff), working voltage (V) and working current (I). The surfaces of the Zr-, Ti-, Fe- and Al-based alloys were treated by PED at various pulse durations with fixed pulse intervals of 4 ms, working voltage of 82.5 V and working current of 1.2 A, and the optimum pulse durations for surface vitrification of each alloy (Table 2) were preliminarily determined according to the peak intensity and number in the XRD patterns. Fig. 2 shows the XRD patterns of the surfaces of the Zr-, Ti-, Fe- and Al-based alloys treated under their respective optimum ton. A broad Table 2 Optimum pulse durations for surface vitrification of the Zr-, Ti-, Feand Al-based alloys by PED treatment at fixed pulse interval of 4 ms, working voltage of 82.5 V and working current of 1.2 A. Alloy (at.%)
ton (ms)
Zr55Al10Ni5Cu30 (Zr0.55Al0.10Ni0.05Cu0.30)98Y2 Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1 Ti50Ni15Cu25Sn5Zr5 [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 Al86Si0.5Ni4.06Co2.94Y6Sc0.5
64 64 72 72 80 32
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Fig. 2. XRD patterns of the pulsed electrical discharge treated surfaces of the Zr-, Ti-, Fe-and Al-based alloys.
diffraction peak for typical amorphous structure superimposed with weak crystalline peaks is detected on the XRD patterns of Zr55Al10Ni5Cu30, (Zr0.55Al0.10Ni0.05Cu0.30)98Y2, Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1, Ti50Ni15Cu25Sn5Zr5 and [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 alloys. It indicates that the surfaces of these alloys consist of mainly amorphous phases and a small amount of crystalline phases. The XRD pattern of the treated Al86Si0.5Ni4.06Co2.94Y6Sc0.5 alloy shows only crystalline peaks, implying that amorphous surface could not be formed by the present PED treatment. It can be drawn that the surfaces of the alloys with higher GFA show a stronger glassforming tendency by the PED treatment. Zr55Al10Ni5Cu30 alloy was selected for the further study due to its high GFA. The existence of amorphous phase in the surface layer of the treated Zr55Al10Ni5Cu30 alloy was also confirmed by the DSC curve, which exhibited a distinct glass transition, followed by a supercooled liquid region prior to crystallization. The glass transition temperature (Tg) was 682 K and the onset temperature of crystallization (Tx) was 750 K for the treated Zr55Al10Ni5Cu30 alloy, which are similar to the reported values of the metallic glass with the same composition [19]. Fig. 3 shows the XRD patterns of the Zr55Al10Ni5Cu30 alloy treated at various pulse durations, pulse intervals and working powers (P ¼ V I). Since V and I are not independent variables, P is proposed to explain the joining effects of V and I. P values and the corresponding values of V and I are listed in Table 3. All the XRD patterns in Fig. 3 feature a broad diffraction peak typical for amorphous structure and some weak Bragg peaks corresponding to intermetallic phases CuZr2 and Cu10Zr7, suggesting that amorphous phase is the main phase in the surface of the treated alloy, though a small quantity of crystalline phases can also be detected. Especially,
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the Bragg peaks of the intermetallic phase CuZr2 disappear for the alloy treated at ton ¼ 64 ms (Fig. 3(a)), toff ¼ 4 ms (Fig. 3(b)) and P ¼ 99 W (Fig. 3(c)). Thus, optimum processing parameters are suggested to be ton ¼ 64 ms, toff ¼ 4ms and P ¼ 99 W (V ¼ 82.5 V and I ¼ 1.2 A). It also indicates that proper processing parameters are crucial in the formation of amorphous phase. Fig. 4(a) is the SEM image of the Zr55Al10Ni5Cu30 alloy surface after the treatment with PED at ton ¼ 64 ms, toff ¼ 4 ms, V ¼ 82.5 V and I ¼ 1.2 A. The alloy surface is rough and may have been slightly oxidized. Fig. 4(b) shows the back-scattered electron (BSE) image of the cross section of the PED-treated Zr55Al10Ni5Cu30 alloy. The edge of the melted zone is clearly seen in the BSE image. The outmost surface shows featureless morphology, while a relatively dark region is found in the inner part where crystalline phases are observed. It indicates that the PED-treated Zr55Al10Ni5Cu30 alloy consists of three parts from surface to substrate, i.e., the recast surface layer, heat affected layer and crystalline substrate. The developed dendrites in the substrate and heat affected zone disappear at the border of the recast surface layer. The average thickness of the recast surface layer for the Zr55Al10Ni5Cu30 alloy is ~6.78 mm. TEM observation was carried out to study the structure of Zr55Al10Ni5Cu30 alloy treated by PED. As shown in Fig. 4(c), the HRTEM image of the recast surface layer shows a maze-like pattern and the corresponding selected area electron diffraction (SAED) pattern shows a halo ring, which confirms the amorphous structure of the recast surface layer. Fig. 4(d) and (e) are the HRTEM images and the corresponding SAED patterns of the heat affected layer. The observed area in Fig. 4(d) is close to the recast surface layer, while that in Fig. 4(e) is close to the crystalline substrate. As illustrated in Fig. 4(d), nano-crystallites with size of ~3 nm are detected in the amorphous matrix and diffraction spots begin to appear in the corresponding SAED pattern for the heat affected layer close to the recast surface layer. As shown in Fig. 4(e), grains with different crystallographic orientation, as marked by the white box and circle in the HRTEM image, for the heat affected layer close to the crystalline substrate can be seen. The size and fraction of the crystalline phases in the area shown in Fig. 4(e) are larger than that observed in Fig. 4(d). The TEM image of the crystalline substrate is shown in Fig. 4(f). The SAED pattern of the area marked with “A” shows diffraction rings typical for polycrystalline structure. It can be drawn from the BSE image and TEM results that the treated Zr55Al10Ni5Cu30 alloy exhibits a gradient microstructure: amorphous surface, amorphous-crystalline composite region and crystalline substrate from surface to inside. The formation of the amorphous recast surface layer for Zr55Al10Ni5Cu30 alloy after PED treatment indicates that PED treatment can be a feasible method to obtain amorphous layers on crystalline alloys with certain GFA. The surface properties including hardness and corrosion resistance of the PED-treated Zr55Al10Ni5Cu30 alloy were studied. Fig. 5(a) and (b) show the effects of pulse duration and working power on the microhardness of the PED-treated Zr55Al10Ni5Cu30 alloy, respectively. As shown in Fig. 5(a), the microhardness increases and then decreases with the increase in ton and reaches the highest value at ton ¼ 64 ms The variation of the microhardness with P is similar to the change of the microhardness with ton, and the highest microhardness is achieved at P ¼ 99 W. The microhardness of the crystalline substrate is HV537, while the microhardness of the Zr55Al10Ni5Cu30 alloy treated at ton ¼ 64 ms, toff ¼ 4 ms and P ¼ 99 W can reach HV631, which is comparable to that of the Zr55Al10Ni5Cu30 bulk metallic glass (HV648). It is indicated that the recast layer obtained by employing the optimum processing parameters exhibit the highest hardness value in the present work. Corrosion behaviors of Zr55Al10Ni5Cu30 master alloy and those treated by PED under different processing parameters were
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Fig. 3. XRD patterns of the Zr55Al10Ni5Cu30 alloy surface treated by pulsed electrical discharge at (a) different pulse duration ton with pulse interval toff ¼ 4 ms, working voltage V ¼ 82.5 V and working current I ¼ 1.2 A, (b) different toff with ton ¼ 64 ms, V ¼ 82.5 V and I ¼ 1.2 A, and (c) different working power P ¼ V I with ton ¼ 64 ms and toff ¼ 4 ms.
Table 3 P values and the corresponding values of V and I of the PED treatment for Zr55Al10Ni5Cu30 alloy. P (W)
V (V)
I (A)
52 64 80 99 120 139
74.5 80.5 89.0 82.5 109.5 115.5
0.7 0.8 0.9 1.2 1.1 1.2
examined by the potentiodynamic polarization measurement in 3 mass% NaCl solution, and the results are shown in Fig. 5(c) and (d). It is seen that the Zr55Al10Ni5Cu30 samples show typical characteristics of active dissolution in the NaCl solution. All the PEDtreated samples show a higher corrosion potential while a higher self-corrosion current density than the master alloy. The higher self-corrosion current density may be attributed to the rough surfaces after the PED treatment. Higher corrosion potential for the PED-treated Zr55Al10Ni5Cu30 samples indicates an improvement of
stability of the PED-treated surfaces. The corrosion potential increases to the highest value at ton ¼ 64 ms and subsequently decrease with the further increase of ton. As illustrated in Fig. 5(d), the highest corrosion potential corresponding to the best surface stability is achieved at P ¼ 99 W.
4. Discussion Many factors, including GFA, microstructural heterogeneity, melting temperature and processing parameters, show influences on the surface glass formation during the PED treatment of the Zr-, Ti-, Fe- and Al-based alloys. As shown in Fig. 2, in general, the higher GFA of the substrate alloy, the stronger glass-forming tendency by PED treatment. Amorphous layer could not be fabricated on the surface of Al-based alloy due to the low GFA, while amorphous or amorphous/crystalline composite layers were successfully formed on the surfaces of Zr-, Ti-, Fe-based alloys with higher GFA. It was revealed by the TEM results that the recast surface layer of the PED-treated Zr55Al10Ni5Cu30 alloy was fully amorphous. The weak crystalline peaks in the XRD patterns of the PED-treated
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Fig. 4. (a) SEM image of the surface and (b) back-scattered electron image of the cross-section of the Zr55Al10Ni5Cu30 alloy treated by pulsed electrical discharge at pulse duration ton ¼ 64 ms, pulse interval toff ¼ 4 ms, working voltage V ¼ 82.5 V and working current I ¼ 1.2 A. HRTEM images and SAED patterns (insets) of (c) the recast surface layer, (d) the heat affected layer closer to the recast surface layer and (e) the heat affected layer closer to the crystalline substrate, and (f) bright-field image and SAED pattern (inset) of the crystalline substrate for Zr55Al10Ni5Cu30 alloy treated at ton ¼ 64 ms, toff ¼ 4 ms, V ¼ 82.5 V and I ¼ 1.2 A.
Zr55Al10Ni5Cu30 alloys could correspond to the crystalline phases below the amorphous recast surface layer due to the larger penetration depth of the X-rays compared to the thickness of the recast layer. It is considered that microstructural heterogeneity and melting temperature may also play a role in the glass formation by PED treatment. As indicated by Fig. 4(b), the microstructure of the crystalline Zr55Al10Ni5Cu30 alloy substrate is inhomogeneous, where different phases can be detected. The EDS and XRD results indicated that the Zr55Al10Ni5Cu30 crystalline alloy consisted of three phases: CuZr, CuZr2 and Cu10Zr7. It is considered that, during the PED treatment, the mixing of these three phases may be not sufficient to obtain a homogeneous composition closing to the nominal composition Zr55Al10Ni5Cu30. The chemical composition of a spot in the amorphous recast layer was found to be Zr48.98Al8.69Ni7.36Cu34.97, which distinctly deviated from the nominal composition. Furthermore, an EDS line-scan analysis along a line parallel
to the alloy surface in the amorphous recast layer was performed, and the results indicated obvious fluctuation for all constituent elements. Different compositions may lead to different GFA, which may explain the effect of microstructural heterogeneity on surface vitrification. Though [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 alloy owns large Dc, the precipitation of crystalline a-Fe on the surface after PED treatment is found, as shown by the XRD patterns in Fig. 2. It is considered that some cells in the crystalline ingots possibly could not be melted and temporarily remained in the melted region due to the limited energy input, especially for the alloys with high melting temperatures, such as the [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 alloy. The un-melted cells could be the heterogeneous nucleation sites for crystalline phases to precipitate and grow with low nucleation energy barrier [9]. On the other hand, the thermal burden would increase if the PED energy input is too high, which is also detrimental to the surface glass formation. In addition, processing parameters of PED treatment show
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Fig. 5. Microhardness of Zr55Al10Ni5Cu30 alloy surfaces treated by pulsed electrical discharge at (a) different pulse duration ton with pulse interval toff ¼ 4 ms, working voltage V ¼ 82.5 V and working current I ¼ 1.2 A, and (b) different working power P ¼ V I with ton ¼ 64 ms and toff ¼ 4 ms Potentiodynamic polarization curves of Zr55Al10Ni5Cu30 master alloy and those treated at (c) different ton with toff ¼ 4 ms, V ¼ 82.5 V and I ¼ 1.2 A, and (d) different P with ton ¼ 64 ms and toff ¼ 4 ms.
significant effects on the glass formation and the surface properties. The actual profile of a single PED pulse and the processing parameters involved are shown in Fig. 1(b). In general, electrodischarge energy of a single pulse can be presented as:
Zt Wm ¼
VðtÞIðtÞdt
(1)
0
where Wm stands for the electro-discharge energy of a single pulse, t is the duration of a single pulse, and V(t) and I(t) are the timedependent voltage and current, respectively [12]. It can be inferred from eq. (1) that a higher V, I or ton would generate a higher Wm. Because V and I are not independent variables, working power P ¼ V I is put forward to study the joint effects of V and I on glass formation and surface properties, and P can be a simple measure of Wm when ton and toff are fixed. When the electro-discharge energy is too high, the melted layer possibly cannot be cooled timely below the glass transition temperature (Tg) and then crystallization occurs, which is detrimental to the surface glass formation. Low Wm can decrease the thermal burden. However, if Wm is too low, some crystalline phases in alloys possibly cannot be melted or different phases possibly cannot be melted sufficiently to obtain a homogeneous composition closing to the nominal composition, which may also be detrimental to surface glass formation. It is observed in Fig. 5 that both the microhardness and the corrosion potential reach the highest values for the Zr55Al10Ni5Cu30 amorphous surface treated at the optimum processing parameters for surface glass formation. In comparison, the alloy surfaces treated by using other parameters exhibit lower hardness and corrosion resistance, which
may be due to the existence of crystalline phases in the alloy surface or the smaller thickness of the amorphous layer judged from the relatively more intense crystalline peaks in the corresponding XRD patterns. These results indicate that proper processing parameters, including pulse duration, working voltage and working current, are beneficial to the surface glass formation by PED treatment, leading to a simultaneous improvement of microhardness and corrosion resistance. 5. Conclusions Surface glass formation by PED treatment is closely related to the GFA as well as the melting temperatures and microstructural heterogeneity of the alloys. Amorphous or amorphous/crystalline composite layers were formed on the Zr-, Fe- and Ti-based crystalline alloys with high GFA after PED treatment, while the PEDtreated Al-based crystalline alloy with low GFA is still crystalline. The PED-treated Zr55Al10Ni5Cu30 alloy exhibits a gradient structure from the surface to the substrate: amorphous surface layer, amorphous/crystalline composite layer and crystalline substrate. Effects of processing parameters on surface glass formation are attributed to the effects of electro-discharge energy Wm, which can be further attributed to the effects of working power P when ton and toff are fixed. Proper Wm or P is beneficial to glass formation. The surface microhardness of the Zr55Al10Ni5Cu30 alloy treated with proper processing parameters can reach HV631, which is comparable to that of Zr55Al10Ni5Cu30 bulk metallic glass. The Zr55Al10Ni5Cu30 alloy treated at proper processing parameters also shows a higher stability than the master alloy in 3 mass% NaCl solutions in terms of corrosion potential. It is suggested that the PED treatment can be
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface vitrification of alloys by pulsed electrical discharge treatment Lei Zuo, Shujie Pang*, Shanfang Zou, Haifei Li, Tao Zhang** Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
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a b s t r a c t
Article history: Received 30 July 2016 Received in revised form 1 December 2016 Accepted 23 December 2016 Available online xxx
Pulsed electrical discharge (PED) treatment was applied as a novel method to synthesize amorphous or amorphous/crystalline composite layers on the surfaces of Zr-, Fe-, Ti- and Al-based crystalline alloys. The formation of amorphous or amorphous/crystalline composite layers was investigated in terms of the significant disparity in glass-forming ability of the alloys. The influences of PED processing parameters on the surface glass formation of Zr55Al10Ni5Cu30 alloy were further discussed and the PED-treated alloy exhibited a gradient structure: amorphous surface, amorphous/crystalline composite region and crystalline substrate from surface to inside. Besides, effects of processing parameters on the microhardness and corrosion behavior of the treated Zr55Al10Ni5Cu30 alloy were investigated and the results revealed that the PED treatment could improve the microhardness and corrosion resistance of the crystalline substrate alloy. This PED treatment is a promising method to synthesize amorphous or amorphous/ crystalline composite layers for novel mechanical, physical or chemical applications. © 2016 Elsevier B.V. All rights reserved.
Keywords: Amorphisation Composite materials Metallic glasses Microstructure Mechanical properties Corrosion
1. Introduction Metallic glasses are considered to be suitable candidates for wear- and corrosion-resistant coatings for crystalline metallic components, due to their high hardness and corrosion resistance resulting from their unique structural uniformity [1e8]. With a high cooling rate, laser surface treatment technique has been employed to prepare metallic glass surfaces/coatings [9e11]. In view of high cooling rate, pulsed electrical discharge (PED) treatment may also be a feasible method for preparing glassy metallic surfaces/coatings. During the PED treatment, alloy surfaces can be melted by electrical discharge and subsequently solidified under the cooling effect of alloy substrates during pulse intervals. Additionally, dielectric fluid can be introduced during the treatment, which may also contribute to the cooling effect. Electrical discharge machining has been used for machining workpieces for decades, based on the principle of the conversion of electrical energy into thermal energy by generating a succession of controlled electrical discharges between the electrode and the workpiece immersed in a dielectric fluid [12,13]. Thus, surface vitrification of alloys by PED
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Pang), [email protected] (T. Zhang).
treatment may be fulfilled by the electrical discharge machining. Attempts have been made to investigate the surface modifications of metallic materials by PED in the form of electrical discharge machining [14e18]. It has been reported that white layers were generated on a steel workpiece by PED treatment, leading to an increased resistance to corrosion and abrasion [14]. Besides, a crack-less layer of titanium carbide has also been formed by PED treatment to improve the surface properties [15]. Syntheses of coatings on alloy surfaces by PED treatment using powder metallurgy tool electrodes has also been reported [16e18]. However, PED treatment has not yet been used to synthesize amorphous or amorphous/crystalline composite layers on crystalline alloys. Different from laser surface treatment which is one-dimensional spot-scanning process, PED treatment is a two-dimensional process by applying the wire tool electrode to scan the alloy surface (Fig. 1). Thus, compared with laser surface treatment, PED treatment may be more efficiency to fabricate glassy metallic surfaces/ coatings. In the present study, surface treatments by PED using the wire electrical discharge machine were conducted on the Zr-, Ti-, Fe- and Al-based crystalline alloys with significant disparities in glassforming ability (GFA) as well as thermal properties (Table 1) [19e23]. The influences of GFA as well as melting temperatures and microstructural heterogeneity of the alloys on the surface glass formation were investigated. The effects of processing parameters of the PED treatment, including pulse duration (ton), pulse interval
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Please cite this article in press as: L. Zuo, et al., Surface vitrification of alloys by pulsed electrical discharge treatment, Journal of Alloys and Compounds (2017), http://dx.doi.org/10.1016/j.jallcom.2016.12.312
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Fig. 1. (a) Schematic diagram of pulsed electrical discharge treatment on alloy surface, and (b) actual profile of a single PED pulse [13].
Table 1 Glass transition temperature (Tg), onset temperature of crystallization (Tx), solidus temperature (Tm), liquidus temperature (Tl) and critical diameters (Dc) of the Zr-, Ti-, Fe-, and Al-based metallic glasses. Alloy (at.%)
Tg (K)
Tx (K)
Tm (K)
Tl (K)
Dc (mm)
Ref.
Zr55Al10Ni5Cu30 (Zr0.55Al0.10Ni0.05Cu0.30)98Y2 Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1 Ti50Ni15Cu25Sn5Zr5 [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 Al86Si0.5Ni4.06Co2.94Y6Sc0.5
685 673 670 680 820 e
774 763 711 765 870 497
1090 e 1120 e e 903
1156 e 1205 e 1390 1055
10 12 4 5 5 ~1
[19] [19] [20] [21] [22] [23]
(toff), working voltage (V) and working current (I), on the surface structure and properties of the treated Zr55Al10Ni5Cu30 alloy with high glass-forming ability were further studied. 2. Experimental Alloy ingots with nominal compositions of Zr55Al10Ni5Cu30, (Zr0.55Al0.10Ni0.05Cu0.30)98Y2, Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1, Ti50Ni15Cu25Sn5Zr5, [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 and Al86Si0.5Ni4.06Co2.94Y6Sc0.5 (at.%) were prepared by arc-melting the mixture of the pure elements as well as an industry-grade FeB alloy in a Tigettered argon atmosphere. To guarantee the compositional homogeneity, the ingots were re-melted at least 5 times. The ingots were first cut into rectangular plates of 4 8 8 mm by a DK7716 type wire electrical discharge machine. Then, the square faces were polished with #2000 sand papers to eliminate the influences of the cutting. Finally, the polished surfaces of the rectangular plates were treated by PED using the wire electrical discharge machine with different processing parameters, and the schematic diagram and the actual profile of a single PED pulse [13] are shown in Fig. 1. The wire tool electrode is molybdenum wire. The central axis of the wire electrode (~120 mm in diameter) is 60 mm away from the alloy surface during the PED treatment. Structures of the treated alloys were examined by X-ray diffraction (XRD, Bruker D8-advance) with Cu-Ka radiation, scanning electron microscopy (SEM, JEOL JSM-7500F) coupled with an energy-dispersive spectrometer (EDS) and high-resolution transmission electron microscopy (HRTEM, JEOL-2100F). For the TEM observation, the PED-treated surfaces of the samples were bonded together, and then the cross section was mechanically ground to ~50 mm in thickness and finally electro-polished to perforation at the bonded region with a 25% HNO3-methanol solution at about
243 K. Thermal properties of the treated alloy surface layer were investigated by differential scanning calorimetry (DSC) at a heating rate of 0.33 K/s. The procedure to prepare DSC samples was as follows: a small piece was cut from the PED-treated alloy surface by diamond cutting machine, and then the small piece surface adjacent to the substrate was polished by sand papers to obtain a sample of several micrometers thick in order to reduce the impact of the substrate. Considering the wave surface of the recast layer, the DSC sample included both crests and troughs of the wave surface. Microhardness of the treated alloy surfaces was measured by Vickers hardness tester (450-SVD) under a load of 50 g for 15 s. Corrosion behavior of the treated surfaces in 3 mass% NaCl aqueous solution was characterized by electrochemical measurements conducted in a three-electrode cell using a PED-treated sample as the working electrode, a platinum foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. After the specimens were immersed in the solution at room temperature for 1 h to achieve a stable open-circuit potential, potentiodynamic polarization curves were measured at a scan rate of 0.833 mV/s. 3. Results The main parameters involved in the PED treatment includes pulse duration (ton), pulse interval (toff), working voltage (V) and working current (I). The surfaces of the Zr-, Ti-, Fe- and Al-based alloys were treated by PED at various pulse durations with fixed pulse intervals of 4 ms, working voltage of 82.5 V and working current of 1.2 A, and the optimum pulse durations for surface vitrification of each alloy (Table 2) were preliminarily determined according to the peak intensity and number in the XRD patterns. Fig. 2 shows the XRD patterns of the surfaces of the Zr-, Ti-, Fe- and Al-based alloys treated under their respective optimum ton. A broad Table 2 Optimum pulse durations for surface vitrification of the Zr-, Ti-, Feand Al-based alloys by PED treatment at fixed pulse interval of 4 ms, working voltage of 82.5 V and working current of 1.2 A. Alloy (at.%)
ton (ms)
Zr55Al10Ni5Cu30 (Zr0.55Al0.10Ni0.05Cu0.30)98Y2 Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1 Ti50Ni15Cu25Sn5Zr5 [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 Al86Si0.5Ni4.06Co2.94Y6Sc0.5
64 64 72 72 80 32
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Fig. 2. XRD patterns of the pulsed electrical discharge treated surfaces of the Zr-, Ti-, Fe-and Al-based alloys.
diffraction peak for typical amorphous structure superimposed with weak crystalline peaks is detected on the XRD patterns of Zr55Al10Ni5Cu30, (Zr0.55Al0.10Ni0.05Cu0.30)98Y2, Ti45.5Zr6.5Cu39.9Ni5.1Sn2Si1, Ti50Ni15Cu25Sn5Zr5 and [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 alloys. It indicates that the surfaces of these alloys consist of mainly amorphous phases and a small amount of crystalline phases. The XRD pattern of the treated Al86Si0.5Ni4.06Co2.94Y6Sc0.5 alloy shows only crystalline peaks, implying that amorphous surface could not be formed by the present PED treatment. It can be drawn that the surfaces of the alloys with higher GFA show a stronger glassforming tendency by the PED treatment. Zr55Al10Ni5Cu30 alloy was selected for the further study due to its high GFA. The existence of amorphous phase in the surface layer of the treated Zr55Al10Ni5Cu30 alloy was also confirmed by the DSC curve, which exhibited a distinct glass transition, followed by a supercooled liquid region prior to crystallization. The glass transition temperature (Tg) was 682 K and the onset temperature of crystallization (Tx) was 750 K for the treated Zr55Al10Ni5Cu30 alloy, which are similar to the reported values of the metallic glass with the same composition [19]. Fig. 3 shows the XRD patterns of the Zr55Al10Ni5Cu30 alloy treated at various pulse durations, pulse intervals and working powers (P ¼ V I). Since V and I are not independent variables, P is proposed to explain the joining effects of V and I. P values and the corresponding values of V and I are listed in Table 3. All the XRD patterns in Fig. 3 feature a broad diffraction peak typical for amorphous structure and some weak Bragg peaks corresponding to intermetallic phases CuZr2 and Cu10Zr7, suggesting that amorphous phase is the main phase in the surface of the treated alloy, though a small quantity of crystalline phases can also be detected. Especially,
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the Bragg peaks of the intermetallic phase CuZr2 disappear for the alloy treated at ton ¼ 64 ms (Fig. 3(a)), toff ¼ 4 ms (Fig. 3(b)) and P ¼ 99 W (Fig. 3(c)). Thus, optimum processing parameters are suggested to be ton ¼ 64 ms, toff ¼ 4ms and P ¼ 99 W (V ¼ 82.5 V and I ¼ 1.2 A). It also indicates that proper processing parameters are crucial in the formation of amorphous phase. Fig. 4(a) is the SEM image of the Zr55Al10Ni5Cu30 alloy surface after the treatment with PED at ton ¼ 64 ms, toff ¼ 4 ms, V ¼ 82.5 V and I ¼ 1.2 A. The alloy surface is rough and may have been slightly oxidized. Fig. 4(b) shows the back-scattered electron (BSE) image of the cross section of the PED-treated Zr55Al10Ni5Cu30 alloy. The edge of the melted zone is clearly seen in the BSE image. The outmost surface shows featureless morphology, while a relatively dark region is found in the inner part where crystalline phases are observed. It indicates that the PED-treated Zr55Al10Ni5Cu30 alloy consists of three parts from surface to substrate, i.e., the recast surface layer, heat affected layer and crystalline substrate. The developed dendrites in the substrate and heat affected zone disappear at the border of the recast surface layer. The average thickness of the recast surface layer for the Zr55Al10Ni5Cu30 alloy is ~6.78 mm. TEM observation was carried out to study the structure of Zr55Al10Ni5Cu30 alloy treated by PED. As shown in Fig. 4(c), the HRTEM image of the recast surface layer shows a maze-like pattern and the corresponding selected area electron diffraction (SAED) pattern shows a halo ring, which confirms the amorphous structure of the recast surface layer. Fig. 4(d) and (e) are the HRTEM images and the corresponding SAED patterns of the heat affected layer. The observed area in Fig. 4(d) is close to the recast surface layer, while that in Fig. 4(e) is close to the crystalline substrate. As illustrated in Fig. 4(d), nano-crystallites with size of ~3 nm are detected in the amorphous matrix and diffraction spots begin to appear in the corresponding SAED pattern for the heat affected layer close to the recast surface layer. As shown in Fig. 4(e), grains with different crystallographic orientation, as marked by the white box and circle in the HRTEM image, for the heat affected layer close to the crystalline substrate can be seen. The size and fraction of the crystalline phases in the area shown in Fig. 4(e) are larger than that observed in Fig. 4(d). The TEM image of the crystalline substrate is shown in Fig. 4(f). The SAED pattern of the area marked with “A” shows diffraction rings typical for polycrystalline structure. It can be drawn from the BSE image and TEM results that the treated Zr55Al10Ni5Cu30 alloy exhibits a gradient microstructure: amorphous surface, amorphous-crystalline composite region and crystalline substrate from surface to inside. The formation of the amorphous recast surface layer for Zr55Al10Ni5Cu30 alloy after PED treatment indicates that PED treatment can be a feasible method to obtain amorphous layers on crystalline alloys with certain GFA. The surface properties including hardness and corrosion resistance of the PED-treated Zr55Al10Ni5Cu30 alloy were studied. Fig. 5(a) and (b) show the effects of pulse duration and working power on the microhardness of the PED-treated Zr55Al10Ni5Cu30 alloy, respectively. As shown in Fig. 5(a), the microhardness increases and then decreases with the increase in ton and reaches the highest value at ton ¼ 64 ms The variation of the microhardness with P is similar to the change of the microhardness with ton, and the highest microhardness is achieved at P ¼ 99 W. The microhardness of the crystalline substrate is HV537, while the microhardness of the Zr55Al10Ni5Cu30 alloy treated at ton ¼ 64 ms, toff ¼ 4 ms and P ¼ 99 W can reach HV631, which is comparable to that of the Zr55Al10Ni5Cu30 bulk metallic glass (HV648). It is indicated that the recast layer obtained by employing the optimum processing parameters exhibit the highest hardness value in the present work. Corrosion behaviors of Zr55Al10Ni5Cu30 master alloy and those treated by PED under different processing parameters were
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Fig. 3. XRD patterns of the Zr55Al10Ni5Cu30 alloy surface treated by pulsed electrical discharge at (a) different pulse duration ton with pulse interval toff ¼ 4 ms, working voltage V ¼ 82.5 V and working current I ¼ 1.2 A, (b) different toff with ton ¼ 64 ms, V ¼ 82.5 V and I ¼ 1.2 A, and (c) different working power P ¼ V I with ton ¼ 64 ms and toff ¼ 4 ms.
Table 3 P values and the corresponding values of V and I of the PED treatment for Zr55Al10Ni5Cu30 alloy. P (W)
V (V)
I (A)
52 64 80 99 120 139
74.5 80.5 89.0 82.5 109.5 115.5
0.7 0.8 0.9 1.2 1.1 1.2
examined by the potentiodynamic polarization measurement in 3 mass% NaCl solution, and the results are shown in Fig. 5(c) and (d). It is seen that the Zr55Al10Ni5Cu30 samples show typical characteristics of active dissolution in the NaCl solution. All the PEDtreated samples show a higher corrosion potential while a higher self-corrosion current density than the master alloy. The higher self-corrosion current density may be attributed to the rough surfaces after the PED treatment. Higher corrosion potential for the PED-treated Zr55Al10Ni5Cu30 samples indicates an improvement of
stability of the PED-treated surfaces. The corrosion potential increases to the highest value at ton ¼ 64 ms and subsequently decrease with the further increase of ton. As illustrated in Fig. 5(d), the highest corrosion potential corresponding to the best surface stability is achieved at P ¼ 99 W.
4. Discussion Many factors, including GFA, microstructural heterogeneity, melting temperature and processing parameters, show influences on the surface glass formation during the PED treatment of the Zr-, Ti-, Fe- and Al-based alloys. As shown in Fig. 2, in general, the higher GFA of the substrate alloy, the stronger glass-forming tendency by PED treatment. Amorphous layer could not be fabricated on the surface of Al-based alloy due to the low GFA, while amorphous or amorphous/crystalline composite layers were successfully formed on the surfaces of Zr-, Ti-, Fe-based alloys with higher GFA. It was revealed by the TEM results that the recast surface layer of the PED-treated Zr55Al10Ni5Cu30 alloy was fully amorphous. The weak crystalline peaks in the XRD patterns of the PED-treated
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Fig. 4. (a) SEM image of the surface and (b) back-scattered electron image of the cross-section of the Zr55Al10Ni5Cu30 alloy treated by pulsed electrical discharge at pulse duration ton ¼ 64 ms, pulse interval toff ¼ 4 ms, working voltage V ¼ 82.5 V and working current I ¼ 1.2 A. HRTEM images and SAED patterns (insets) of (c) the recast surface layer, (d) the heat affected layer closer to the recast surface layer and (e) the heat affected layer closer to the crystalline substrate, and (f) bright-field image and SAED pattern (inset) of the crystalline substrate for Zr55Al10Ni5Cu30 alloy treated at ton ¼ 64 ms, toff ¼ 4 ms, V ¼ 82.5 V and I ¼ 1.2 A.
Zr55Al10Ni5Cu30 alloys could correspond to the crystalline phases below the amorphous recast surface layer due to the larger penetration depth of the X-rays compared to the thickness of the recast layer. It is considered that microstructural heterogeneity and melting temperature may also play a role in the glass formation by PED treatment. As indicated by Fig. 4(b), the microstructure of the crystalline Zr55Al10Ni5Cu30 alloy substrate is inhomogeneous, where different phases can be detected. The EDS and XRD results indicated that the Zr55Al10Ni5Cu30 crystalline alloy consisted of three phases: CuZr, CuZr2 and Cu10Zr7. It is considered that, during the PED treatment, the mixing of these three phases may be not sufficient to obtain a homogeneous composition closing to the nominal composition Zr55Al10Ni5Cu30. The chemical composition of a spot in the amorphous recast layer was found to be Zr48.98Al8.69Ni7.36Cu34.97, which distinctly deviated from the nominal composition. Furthermore, an EDS line-scan analysis along a line parallel
to the alloy surface in the amorphous recast layer was performed, and the results indicated obvious fluctuation for all constituent elements. Different compositions may lead to different GFA, which may explain the effect of microstructural heterogeneity on surface vitrification. Though [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 alloy owns large Dc, the precipitation of crystalline a-Fe on the surface after PED treatment is found, as shown by the XRD patterns in Fig. 2. It is considered that some cells in the crystalline ingots possibly could not be melted and temporarily remained in the melted region due to the limited energy input, especially for the alloys with high melting temperatures, such as the [(Fe0.5Co0.5)0.75Si0.05B0.2]96Nb4 alloy. The un-melted cells could be the heterogeneous nucleation sites for crystalline phases to precipitate and grow with low nucleation energy barrier [9]. On the other hand, the thermal burden would increase if the PED energy input is too high, which is also detrimental to the surface glass formation. In addition, processing parameters of PED treatment show
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Fig. 5. Microhardness of Zr55Al10Ni5Cu30 alloy surfaces treated by pulsed electrical discharge at (a) different pulse duration ton with pulse interval toff ¼ 4 ms, working voltage V ¼ 82.5 V and working current I ¼ 1.2 A, and (b) different working power P ¼ V I with ton ¼ 64 ms and toff ¼ 4 ms Potentiodynamic polarization curves of Zr55Al10Ni5Cu30 master alloy and those treated at (c) different ton with toff ¼ 4 ms, V ¼ 82.5 V and I ¼ 1.2 A, and (d) different P with ton ¼ 64 ms and toff ¼ 4 ms.
significant effects on the glass formation and the surface properties. The actual profile of a single PED pulse and the processing parameters involved are shown in Fig. 1(b). In general, electrodischarge energy of a single pulse can be presented as:
Zt Wm ¼
VðtÞIðtÞdt
(1)
0
where Wm stands for the electro-discharge energy of a single pulse, t is the duration of a single pulse, and V(t) and I(t) are the timedependent voltage and current, respectively [12]. It can be inferred from eq. (1) that a higher V, I or ton would generate a higher Wm. Because V and I are not independent variables, working power P ¼ V I is put forward to study the joint effects of V and I on glass formation and surface properties, and P can be a simple measure of Wm when ton and toff are fixed. When the electro-discharge energy is too high, the melted layer possibly cannot be cooled timely below the glass transition temperature (Tg) and then crystallization occurs, which is detrimental to the surface glass formation. Low Wm can decrease the thermal burden. However, if Wm is too low, some crystalline phases in alloys possibly cannot be melted or different phases possibly cannot be melted sufficiently to obtain a homogeneous composition closing to the nominal composition, which may also be detrimental to surface glass formation. It is observed in Fig. 5 that both the microhardness and the corrosion potential reach the highest values for the Zr55Al10Ni5Cu30 amorphous surface treated at the optimum processing parameters for surface glass formation. In comparison, the alloy surfaces treated by using other parameters exhibit lower hardness and corrosion resistance, which
may be due to the existence of crystalline phases in the alloy surface or the smaller thickness of the amorphous layer judged from the relatively more intense crystalline peaks in the corresponding XRD patterns. These results indicate that proper processing parameters, including pulse duration, working voltage and working current, are beneficial to the surface glass formation by PED treatment, leading to a simultaneous improvement of microhardness and corrosion resistance. 5. Conclusions Surface glass formation by PED treatment is closely related to the GFA as well as the melting temperatures and microstructural heterogeneity of the alloys. Amorphous or amorphous/crystalline composite layers were formed on the Zr-, Fe- and Ti-based crystalline alloys with high GFA after PED treatment, while the PEDtreated Al-based crystalline alloy with low GFA is still crystalline. The PED-treated Zr55Al10Ni5Cu30 alloy exhibits a gradient structure from the surface to the substrate: amorphous surface layer, amorphous/crystalline composite layer and crystalline substrate. Effects of processing parameters on surface glass formation are attributed to the effects of electro-discharge energy Wm, which can be further attributed to the effects of working power P when ton and toff are fixed. Proper Wm or P is beneficial to glass formation. The surface microhardness of the Zr55Al10Ni5Cu30 alloy treated with proper processing parameters can reach HV631, which is comparable to that of Zr55Al10Ni5Cu30 bulk metallic glass. The Zr55Al10Ni5Cu30 alloy treated at proper processing parameters also shows a higher stability than the master alloy in 3 mass% NaCl solutions in terms of corrosion potential. It is suggested that the PED treatment can be
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