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Influence of high current pulsed electron beam on microstructure and properties of Ni–W alloy coatings
Journal Pre-proof Influence of high current pulsed electron beam on microstructure and properties of Ni–W alloy coatings Lingyan Zhang, Ching-Tun Peng, Jin Shi, Yunxue Jin, Ruifeng Lu PII:
S0925-8388(20)30823-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.154460
Reference:
JALCOM 154460
To appear in:
Journal of Alloys and Compounds
Received Date: 24 December 2019 Revised Date:
19 February 2020
Accepted Date: 21 February 2020
Please cite this article as: L. Zhang, C.-T. Peng, J. Shi, Y. Jin, R. Lu, Influence of high current pulsed electron beam on microstructure and properties of Ni–W alloy coatings, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154460. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Lingyan Zhang: Conceptualization, Data curation, Formal analysis, Writing- original draft, Software. Ching-Tun Peng: Resources, Writing- editing. Jin Shi: Supervision. Yunxue Jin: Resources. Ruifeng Lu: Formal analysis, Writing- review & editing.
Influence of high current pulsed electron beam on microstructure and properties of Ni-W alloy coatings Lingyan Zhang a, Ching-Tun Peng b,*, Jin Shi a, Yunxue Jin c, Ruifeng Lu d,* a
Research Center for Intelligent Information Technology, Nantong University, Nantong 226019, PR China b School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, PR China c School of Materials Science and Engineering, Jiangsu University of science and technology, Zhenjiang 212003, PR China d Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, PR China *
Corresponding author. E-mail address: [email protected] (R.F. Lu), [email protected] (C.-T. Peng).
Abstract Ni-W alloy coating layers were successfully materialized through an innovative surface alloying approach, high-current pulsed electron beam (HCPEB) irradiation. Two levels of HCPEB irradiation, 10 and 20 pulses, were applied to tungsten powders coated on nickel substrate. In the case of 10 pulses HCPEB irradiated sample, surface defects were formed on the Ni-W alloy surface layer. As the number of HCPEB pulse increased to 20, the surface became much smoother as compared to that of 10 pulses irradiated one. During irradiation, W particles were dissolved into the Ni substrate, and the newly formed Ni-W alloy layer was mainly composed of two major phases, NiW and Ni. After HCPEB irradiation, the microhardness was significantly increased, and the coefficient of friction (COF) and wear rate were reduced, which is attributed to the ultrafine W particles formed in the Ni substrate. Moreover, the corrosion property was enhanced after HCPEB irradiation. Keywords: Ni-W alloys; Intermetallic phase NiW; Microstructure; Microhardness; Tribological property; Corrosion resistance 1. Introduction Accompanied with high hardness, excellent wear and corrosion resistance, nickel-tungsten (Ni-W) alloys have been considered as attractive candidates for hard and wear-resistant coatings, especially for applications where corrosion resistance is needed [1-3]. Moreover, it was reported that Ni-W films possess desirable electrocatalytic properties [4-7]. Hence, Ni-W, Ni-Mo, and Ni-W-Mo alloy systems are known as proper catalysts for hydro-sulfuration and hydrogenation processes of various organic compounds and petroleum products [5].
It is well established that traditional alloying processes, such as direct current electrodeposition, cannot be facilitated in the production of nickel-tungsten alloys due to firstly the enormous difference in their melting point (nickel: 1445 °C and tungsten: 3410 °C) and secondly the low solubility (~12.5 at%) of W in Ni [8]. Therefore, electrodeposition of these alloys (via certain electrolytes) is a general method that adopted by researchers to apply a uniform and proper nickel-tungsten coating on different parts. However, some electrodeposition investigations showed that the bonding strength was found low between coating and substrate, so even a tiny corrosion in these coatings could develop into cracks or pores, resulting in the breakdown of the components [9]. In addition, it is also important to note that, during the process of nickel-tungsten electrodeposition, released hydrogen could diffuse through the substrate lattice, which could cause failure to specimens owing to hydrogen embrittlement [10]. In fact, many efforts have been devoted to the preparation of Ni-W alloys via melting methods [11-14]. Recently, Zhang et.al found that an innovative non-equilibrium surface modification technology, high-current pulsed electron beam (HCPEB) irradiation, could promote the solid solubility of alloy systems or even immiscible alloy systems, which encouraged the formation of supersaturated solid solution, and a great improvement of the surface strength, wear and corrosion resistances of the irradiated materials was reported [15-17]. During HCPEB irradiation, extremely short time of the high-density electron pulses can introduce various phenomena to the surface layer, such as rapid melting-solidification, evaporation-condensation, surface smoothing and annealing. Under such rapid melting-solidification conditions, several features, including phase transformations, supersaturated solid solution and ultra-fine grain etc., were induced, which might harden the top surface [18,19]. Meanwhile, during HCPEB the surface composition was homogenized, leading to the enhancement of corrosion properties [19,20]. In addition, microstructural characteristics, including dislocations, vacancies and deformation twins etc. were introduced by intense deformation [21,22], and these structures provided diffusing paths for immiscible atoms, which reduced the activation energy, improving the solid solubility for alloys [15,16]. Based on the abovementioned unique characters of HCPEB and Ni-W alloy, the surface alloying of Ni-W system were conducted by HCPEB in this investigation. 2. Experimental A commercial pure Ni was machined into square plates with dimensions of 10×10×10 mm. Before testing, all the Ni square plate samples were subjected to an annealing process in a high temperature furnace, and all the samples were ground using abrasive paper, polished with diamond paste and cleaned with anhydrous ethanol. For the alloying Tungsten (W) powder, a slurry was made up of mixing 3 grams of W powder and organic binder (nitrocellulose lacquer: diluents are 1:2), and which slurry was sprayed onto a surface of the square Ni samples using an air-pressurized spraying gun. Then, the Ni-W samples were subjected to a vacuum drying at room temperature. The thickness of the sprayed-on W powder coating was
measured as 150-200 µm. For HCPEB experiments, the Ni-W samples were irradiated at room temperature using HOPE-I type source with the following conditions: the electron energy 27 keV, the energy density 4 J/cm2, the current pulse duration 1.5 µs, the vacuum 5×10−3 Pa. Two levels (10 and 20) of irradiation pulse were conducted. Microstructures of the Ni-W samples were characterized by an LEICA DM-2500M optical microscope (OM) and a FEI NovaNano450 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) at 15 kV accelerating voltage. In order to further clarify the resultant phase and the HCPEB modified microstructure within the irradiated/alloyed layer, a JEM-2100 was utilized for transmission electron microscope (TEM) observation. Thin foils samples (about 100-300 nm in depth), were prepared by mechanical pre-thinning, dimpling and jet electrolytic thinning from the substrate side for TEM. Phase evolution was examined by a RigakuD/max-2500/pc X-ray diffractometer (XRD) with CuKα radiation, NaI crystal scintillation counter detector, 285 mm diffractometer radius, 0.02º step size and 5º/min step time. Microhardness was measured by a HVS-1000 testing instrument with a load of 0.098 N (10g) applied for 15 s, and in order to reduce random error, six test points were installed in each sample. The maximum and minimum values were removed, and then the average of the rest four readings were adopted as the microhardness value. The wear properties of the Ni-W alloy layer after HCPEB treatment were tested using a HT-1000 tribometer equipped with a computerized data acquisition system. The wear tests were conducted at room temperature without lubricant under a fixed load of 1.5 N, and the rotation speed of the wear tool was 300 r/min. In wear tests, the Ni-W samples slid against GCr15 balls of 6 mm in diameter with the sliding radius of 2 mm and the testing time of 10 min. The wear rate (W) was calculated using equation (1): W=
∆
(1)
Where ∆W is the wear weight, ρ is the density, L is the sliding distance and F is the load. In addition, wear morphologies and chemical composition of the samples were characterized by SEM (with EDS) to identify the wear mechanisms. The electrochemical corrosion study of the coatings was carried out in 3.5% NaCl solution. All samples were exposed to electrolyte solution under open circuit potential (OCP) at room temperature for 30 minutes. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were adopted to measure the corrosion behavior of Ni-W samples, which methods were practiced by connecting 1 cm2 of exposed sample surface area as working electrode, platinized platinum as counter electrode and saturated calomel as reference electrode. The cyclic polarization (CP) was done at a sweep rate of 0.333 mV/s. The corrosion morphology of Ni-W sample surface after HCPEB irradiation was also examined. 3. Results and discussion 3.1 Phase identification
Figure 1(a) showed the XRD patterns of Ni-W samples before and after two level (10 and 20) of HCPEB pulses irradiation. The initial sample was pure nickel sample, and only the diffraction peak of Ni was observed. After HCPEB, the Ni peaks were slightly shifted to lower angle, which shift is sensitive to a given change in plane spacing (d-placing). The relationship between the Ni lattice parameter (plane spacing parameter) and the HCPEB pulses was shown in Fig. 1(b), and from the figure, the Ni lattice parameter increased after HCPEB irradiation, which might be ascribed to that the W atoms were dissolved in Ni substrate (the radius of W atomic is larger than that of Ni atomic). The maximum value (0.35455 nm) of lattice parameter was found with the 10 pulses irradiated sample, and the value of lattice expansion was 0.00217 nm. However, in Fig. 1(a) the W peak was absent in the pattern of the 20 pulses HCPEB irradiated Ni-W sample, which implied that a small amount of W atoms might had been dissolved into Ni substrate during HCPEB irradiation.
Fig. 1. (a) XRD patterns of Ni-W samples before and after HCPEB irradiation, (b) the relationship between the Ni lattice parameter and HCPEB irradiation. 3.2 Microstructural characterization The secondary electron microscopy images of HCPEB irradiated samples are shown in Fig. 2(a-c). After 10 pulses of HCPEB irradiation, craters (indexed as W elements) were formed on the surface, as shown in Fig. 2(a), which is commonly observed in many HCPEB-irradiated metal surfaces [23,24]. The formation of Craters signified that the melting of Ni substrate occurred. In the smooth area (apart from the craters), a small amount of W elements was found, which indicated that W particles were melted into Ni substrate. Furthermore, after 20 pulses irradiation, shown in Fig. 2(b), the surface of Ni-W sample became much smoother (with fewer craters). With a higher magnification of SEM image, Fig. 2(c), tiny grains (with low angle grain boundaries) were observed, and the grain size was statistically averaged as 106 nm from 250 grains by measuring software, given in Fig. 2(d). During HCPEB, because of its inborn very rapid cooling process, the melted surface layer experienced a fast solidification, which cooling rate was evaluated as high as 108-9 K/s [25,26]. This unique phenomenon encouraged the nucleation rate but slowed down the grain growth.
Fig. 2. Surface morphology of Ni-W samples with (a) 10 pulses, (b-c) 20 pulses of HCPEB irradiation and (d) the distribution of grain size. TEM was practiced to further characterize the HCPEB irradiated microstructure within the Ni-W alloy layer, and Fig. 3 showed the TEM images of Ni-W alloy layers after 20 pulses irradiation. As presented in a bright-field image of Fig. 3(a), black nano-particles (3.75±2.25 nm in size) were observed in the alloy layer, and those nano-particles were indexed as NiW, given in Fig. 3(b). A dark-field image, Fig. 3(c), taken from the spots in the selected area electron diffraction (SAED, red circle) of the Fig. 3(b), revealed that there are numerous fine NiW particles (ICDD PDF 47-1172) uniformly dispersed in Ni substrate. Additionally, high resolution TEM (HR-TEM) observation was also conducted, shown in Fig. 3(d-f), to further study the atomic structure of the particles, and from Fig. 3(d), the analyses indicated that these particles are NiW intermetallic compounds. Furthermore, in Fig. 3(e), disc-like structures with a bright contrast were found, which were classified as GP zone [27]. From an investigation Peng’s [28], two different Cr phase structure (bcc and fcc) coexisted in the Cu substrate, in which the precipitation of the Cr-rich equilibrium bcc phase was initiated from the nucleation of fcc precipitates. Similar to Peng’s finding, in current study the W atoms were dissolved into the lattice {001} of the Ni, causing a slight variation of the Ni lattice constant, which phenomenon plays an important role in strengthening the Ni-W alloy layer. Fig. 3(f) showed a moire fringes, which is a kind of fringe contrast between two crystal structures (such as substrate and precipitate phase) with different lattice parameters [29]. The distance between each individual fringe was found about 1.14 nm, parallel to Ni (111) lattice plane. This means that the direction of moire
fringes is perpendicular to the g (111) reciprocal vector, suggesting that there is a coherent relationship between the moire fringes and the Ni lattice. On the other hand, the lattice plane spacing of the Ni-W sample is greater than that of pure nickel (111), as shown in Fig. 1(b), indicating that the W element mainly exists in the Ni substrate in the form of substitutional solid solution, and the effect of solid solution strengthening stayed consistent.
Fig. 3. TEM images of the Ni-W alloy sample after 20 pulses of HCPEB irradiation: (a) bright-field image, (b) the corresponding dark-field image, (c) the corresponding SAED, (d-f) high resolution TEM images. Numerous refined Ni boundaries, as seen in Fig. 2(c), provided a great deal of diffusing paths for W atoms since the activation energy needed for diffusion at the grain boundaries is lower than that within the grains [30-32]. Several defect structures
were demonstrated in the 20 HCPEB pulses irradiated Ni-W surface, as shown in Fig. 4. From Fig. 4(a-d), it was clearly observed that the deformation-induced dislocations were scattered around twin boundaries and dislocation cell/wall. From Fig. 4(b) and 4(c), the density of dislocations was found much higher along the boundaries as compared to that in the center, which could be explained by that when the applied stress (induced by HCPEB) exceeded the yield strength, the movement of dislocations was firstly initiated and then ended up with dislocation complexes or tangles. In other words, the increasing interactions of dislocations eventually resulted in barely movable dislocation complexes, and dislocation substructures was then formed, generating a minimum energy configuration [15]. Therefore, it can be observed from some local areas that small precipitations was preferentially precipitated near the subgrain boundary (Fig. 4(e)) and dislocation (Fig. 4(f)). This resulted in a slight reduction in the lattice parameters of the Ni-W sample after 20 pulses of HCPEB, as shown in Fig. 1(b).
Fig. 4. The TEM images of the 20 pulses HCPEB irradiated Ni-W sample surface: (a) twin boundaries, (b, c) dislocation cells, (d) dislocation walls, (e) grain boundaries, (f) dislocations. 3.3 Hardness measurements Microhardness values were plotted against initial and two levels (10 and 20) of HCPEB pulses irradiated samples, given in Fig. 5. The value of the initial sample was found as 1.57 GPa. After 10 pulses of HCPEB irradiation, microhardness slightly increased due to the lattice expansion (caused by W atoms in Ni lattice). As the pulse number rose to 20, the microhardness value increased again, which value was measured as 1.8 GPa, 14.6% greater than that of initial one. The hardening mechanism depends on several factors, which are discussed in the following.
As abovementioned in Fig. 4, the thermal stresses (induced by HCPEB irradiation) intensified the plastic deformation and increased dislocation cells/lines in quantity, and these dislocations acted as barriers for dislocation motion, further resulting in the microhardness enhancement. Besides, from Fig. 4(d), the dissolved W elements were evenly distributed in the vicinity of microstructural defects, which elements formed particles that could effectively pin grain boundaries and create strong obstacles for dislocation motion. That is to say, it is considered that the W atoms, which were diffused into Ni substrate lattice, acted as obstacles for dislocation motion, leading to the microhardness enhancement (so-called the solid solution strengthening). In addition, the solid solubility rose with the increasing number of HCPEB pulse [16], so the hardening/strengthening effect of solid solution increased accordingly. Consequently, the highest value of microhardness was obtained with the 20 pulses HCPEB irradiated Ni-W alloy coating. In short, the reinforcement for the surface alloyed Ni-W samples was due to sub-grain strengthening, precipitation hardening and/or the solid solution strengthening.
Fig. 5. Microhardness measurements of Ni-W samples before and after HCPEB irradiation. 3.4 Tribological property Friction coefficient and the calculated wear rate of the Ni-W alloy coatings before and after HCPEB irradiation were presented in Fig. 6(a) and 6(b), respectively. The COF of initial sample was found much higher than those of HCPEB irradiated ones, as shown in Fig. 6(a). The wear rate of the initial sample, in Fig. 6(b), was found as 2.781⨯10-3 mm3/Nm, and the value slightly decreased to 1.391⨯10-3 mm3/Nm with the 10 pulses irradiated Ni-W sample. The lowest wear rate (0.595⨯10-3 mm3/Nm) was found with the 20 pulses irradiated sample. In brief, as the number of HCPEB pulse irradiation increased, the friction coefficient and wear rate decreased. The remarkable improvement in tribological property was primarily attributed to the ultrafine particles residing in the nickel matrices.
Fig. 6. Friction coefficient and wear rate of Ni-W samples before and after HCPEB irradiation. After tribological experiments, the surface morphologies of Ni-W samples before and after HCPEB irradiation were observed with SEM, shown in Fig. 7. Firstly, Fig. 7(a, d) showed the worn surface of the initial sample, exhibiting a rough surface with deep grooves and cracks, which features were classified as the abrasive wear. The degree of wear on the 10 pulses irradiated Ni-W sample decreased, shown in Fig. 7(b, e), and the surface appeared a crush-like flaking phenomenon. A certain amount of plastic deformation occurred on both sides of the furrow along the direction of sliding. At the same time, a peeling phenomenon was observed on the worn surface, which is mainly due to that hard particles generated and attached onto the surface during sliding wear test, and this resulted in a micro-cutting on the Ni-W layer, further accelerating the abrasive wear. In contrast, the worn surface of the 20 pulses HCPEB irradiated sample was found relatively smooth with shallow scratches, shown in in Fig. 7(c, f), and the minimum width of the scratch (295.7 µm) was observed. Avient and Rabinowicz suggested that the wear resistance of an alloy is proportional to its hardness [33,34], which are in good agreement with our findings.
Fig. 7. The worn surface of Ni-W samples before and after HCPEB irradiation: (a, d) initial, (b, e) after 10 pulses HCPEB, and (c, f) after 20 pulses HCPEB samples. 3.5 Corrosion resistance For corrosion tests, typical polarization curves of the Ni-W samples before and after HCPEB irradiation were given in Fig. 8(a). It revealed that the corrosion current density dropped after HCPEB irradiation, and the lowest value was found with 20 pulses irradiated sample. These polarization curves were then analyzed by Tafel extrapolation, and the parameters such as corrosion current (Icorr) density and corrosion potential (Ecorr) were calculated and plotted in Fig. 8(b). From Fig. 8(b), the Ecorr increased significantly with the increase of the HCPEB pulse number, so a enhancement of the corrosion resistance was obtained for the 20 pulses irradiated sample. In a HCPEB related investigation [35], it was reported that a homogeneous layer was formed on top of the irradiated surface, which made a great
contribution to the improvement of corrosion resistance. Additionally, studies on corrosion resistance of pure metals, such as commercial pure copper and Ti [22,36], suggested that microstructural defects and ultra-fine grains were in favor of forming a thicker and denser passive layer on surfaces.
Fig. 8. (a) The CP curves with 3.5 wt% NaCl solution, (b) variation of corrosion current density (Icorr) and corrosion potential (Ecorr) of Ni-W samples before and after HCPEB irradiation. Figure 9(a) shows the surface morphology of the initial pure nickel sample after corrosion test. Fig. 9(b, c) presented corrosion tested morphologies of Ni-W sample surface after 10 and 20 HCPEB pulses irradiation, and it was observed that the number and size of corrosion pits on the 20 pulses irradiated sample were minimized. From earlier discussion on microstructure characterization analysis, fine intermetallic compounds (particles) were formed dispersively in the Ni-W alloy layer, and due to the potential difference between the particles and the substrate, many small corrosion cells would be formed while the small particles would be consumed during anode reaction. Thus, the substrate material was protected from corrosion, which is called sacrificial anode protection method [37]. In a word, the intermetallic compounds delayed the surface corrosion, so the corrosion resistance of the Ni-W alloy samples was significantly improved.
Fig. 9. The corrosion tested morphologies of (a) pure nickel and Ni-W sample surface after (b) 10 pulses and (c) 20 pulses of HCPEB irradiation. 4. Conclusions Tungsten (W) powders were chosen as alloying elements to be alloyed onto Ni substrate, and which alloying treatment was successfully materialized by HCPEB irradiation in current study. The main results were summarized as follows. 1. Through HCPEB irradiation, part of the W powders were dissolved into the Ni
substrate, forming Ni-W alloy layers. The newly formed Ni-W alloy layer was mainly composed of Ni (W) solid solution, NiW ultra-fine intermetallic compound and Ni phase. 2. After HCPEB irradiation, numerous dislocation cell/wall were formed in the Ni-W alloy layers, which provided a large amount of diffusing paths for W atoms, so the effect of solid solution was encouraged. 3. After HCPEB irradiation, the surface hardness and wear resistance were increased, and which improvement was mainly attributed to the induced microstructural defects, solid solution effect, Ni subgrains and ultra-fine W particles. 4. After HCPEB irradiation, the enhanced corrosion resistance was ascribed to the joint effects of the induced microstructure defects and the W addition, which formed a more stable passive film on the Ni-W alloy coatings. Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Highlights: 1. Ni-W alloy layer formed on the surface of nickel surface after irradiation of refractory metal tungsten. 2. Microstructure defects, NiW intermetallic compounds, W particles and other
structures formed in the alloying layer. 3. Changes in microstructure affect performance. 4. Treatment of refractory metal alloying by non-equilibrium process.
Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
S0925-8388(20)30823-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.154460
Reference:
JALCOM 154460
To appear in:
Journal of Alloys and Compounds
Received Date: 24 December 2019 Revised Date:
19 February 2020
Accepted Date: 21 February 2020
Please cite this article as: L. Zhang, C.-T. Peng, J. Shi, Y. Jin, R. Lu, Influence of high current pulsed electron beam on microstructure and properties of Ni–W alloy coatings, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154460. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit Author Statement Lingyan Zhang: Conceptualization, Data curation, Formal analysis, Writing- original draft, Software. Ching-Tun Peng: Resources, Writing- editing. Jin Shi: Supervision. Yunxue Jin: Resources. Ruifeng Lu: Formal analysis, Writing- review & editing.
Influence of high current pulsed electron beam on microstructure and properties of Ni-W alloy coatings Lingyan Zhang a, Ching-Tun Peng b,*, Jin Shi a, Yunxue Jin c, Ruifeng Lu d,* a
Research Center for Intelligent Information Technology, Nantong University, Nantong 226019, PR China b School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, PR China c School of Materials Science and Engineering, Jiangsu University of science and technology, Zhenjiang 212003, PR China d Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, PR China *
Corresponding author. E-mail address: [email protected] (R.F. Lu), [email protected] (C.-T. Peng).
Abstract Ni-W alloy coating layers were successfully materialized through an innovative surface alloying approach, high-current pulsed electron beam (HCPEB) irradiation. Two levels of HCPEB irradiation, 10 and 20 pulses, were applied to tungsten powders coated on nickel substrate. In the case of 10 pulses HCPEB irradiated sample, surface defects were formed on the Ni-W alloy surface layer. As the number of HCPEB pulse increased to 20, the surface became much smoother as compared to that of 10 pulses irradiated one. During irradiation, W particles were dissolved into the Ni substrate, and the newly formed Ni-W alloy layer was mainly composed of two major phases, NiW and Ni. After HCPEB irradiation, the microhardness was significantly increased, and the coefficient of friction (COF) and wear rate were reduced, which is attributed to the ultrafine W particles formed in the Ni substrate. Moreover, the corrosion property was enhanced after HCPEB irradiation. Keywords: Ni-W alloys; Intermetallic phase NiW; Microstructure; Microhardness; Tribological property; Corrosion resistance 1. Introduction Accompanied with high hardness, excellent wear and corrosion resistance, nickel-tungsten (Ni-W) alloys have been considered as attractive candidates for hard and wear-resistant coatings, especially for applications where corrosion resistance is needed [1-3]. Moreover, it was reported that Ni-W films possess desirable electrocatalytic properties [4-7]. Hence, Ni-W, Ni-Mo, and Ni-W-Mo alloy systems are known as proper catalysts for hydro-sulfuration and hydrogenation processes of various organic compounds and petroleum products [5].
It is well established that traditional alloying processes, such as direct current electrodeposition, cannot be facilitated in the production of nickel-tungsten alloys due to firstly the enormous difference in their melting point (nickel: 1445 °C and tungsten: 3410 °C) and secondly the low solubility (~12.5 at%) of W in Ni [8]. Therefore, electrodeposition of these alloys (via certain electrolytes) is a general method that adopted by researchers to apply a uniform and proper nickel-tungsten coating on different parts. However, some electrodeposition investigations showed that the bonding strength was found low between coating and substrate, so even a tiny corrosion in these coatings could develop into cracks or pores, resulting in the breakdown of the components [9]. In addition, it is also important to note that, during the process of nickel-tungsten electrodeposition, released hydrogen could diffuse through the substrate lattice, which could cause failure to specimens owing to hydrogen embrittlement [10]. In fact, many efforts have been devoted to the preparation of Ni-W alloys via melting methods [11-14]. Recently, Zhang et.al found that an innovative non-equilibrium surface modification technology, high-current pulsed electron beam (HCPEB) irradiation, could promote the solid solubility of alloy systems or even immiscible alloy systems, which encouraged the formation of supersaturated solid solution, and a great improvement of the surface strength, wear and corrosion resistances of the irradiated materials was reported [15-17]. During HCPEB irradiation, extremely short time of the high-density electron pulses can introduce various phenomena to the surface layer, such as rapid melting-solidification, evaporation-condensation, surface smoothing and annealing. Under such rapid melting-solidification conditions, several features, including phase transformations, supersaturated solid solution and ultra-fine grain etc., were induced, which might harden the top surface [18,19]. Meanwhile, during HCPEB the surface composition was homogenized, leading to the enhancement of corrosion properties [19,20]. In addition, microstructural characteristics, including dislocations, vacancies and deformation twins etc. were introduced by intense deformation [21,22], and these structures provided diffusing paths for immiscible atoms, which reduced the activation energy, improving the solid solubility for alloys [15,16]. Based on the abovementioned unique characters of HCPEB and Ni-W alloy, the surface alloying of Ni-W system were conducted by HCPEB in this investigation. 2. Experimental A commercial pure Ni was machined into square plates with dimensions of 10×10×10 mm. Before testing, all the Ni square plate samples were subjected to an annealing process in a high temperature furnace, and all the samples were ground using abrasive paper, polished with diamond paste and cleaned with anhydrous ethanol. For the alloying Tungsten (W) powder, a slurry was made up of mixing 3 grams of W powder and organic binder (nitrocellulose lacquer: diluents are 1:2), and which slurry was sprayed onto a surface of the square Ni samples using an air-pressurized spraying gun. Then, the Ni-W samples were subjected to a vacuum drying at room temperature. The thickness of the sprayed-on W powder coating was
measured as 150-200 µm. For HCPEB experiments, the Ni-W samples were irradiated at room temperature using HOPE-I type source with the following conditions: the electron energy 27 keV, the energy density 4 J/cm2, the current pulse duration 1.5 µs, the vacuum 5×10−3 Pa. Two levels (10 and 20) of irradiation pulse were conducted. Microstructures of the Ni-W samples were characterized by an LEICA DM-2500M optical microscope (OM) and a FEI NovaNano450 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) at 15 kV accelerating voltage. In order to further clarify the resultant phase and the HCPEB modified microstructure within the irradiated/alloyed layer, a JEM-2100 was utilized for transmission electron microscope (TEM) observation. Thin foils samples (about 100-300 nm in depth), were prepared by mechanical pre-thinning, dimpling and jet electrolytic thinning from the substrate side for TEM. Phase evolution was examined by a RigakuD/max-2500/pc X-ray diffractometer (XRD) with CuKα radiation, NaI crystal scintillation counter detector, 285 mm diffractometer radius, 0.02º step size and 5º/min step time. Microhardness was measured by a HVS-1000 testing instrument with a load of 0.098 N (10g) applied for 15 s, and in order to reduce random error, six test points were installed in each sample. The maximum and minimum values were removed, and then the average of the rest four readings were adopted as the microhardness value. The wear properties of the Ni-W alloy layer after HCPEB treatment were tested using a HT-1000 tribometer equipped with a computerized data acquisition system. The wear tests were conducted at room temperature without lubricant under a fixed load of 1.5 N, and the rotation speed of the wear tool was 300 r/min. In wear tests, the Ni-W samples slid against GCr15 balls of 6 mm in diameter with the sliding radius of 2 mm and the testing time of 10 min. The wear rate (W) was calculated using equation (1): W=
∆
(1)
Where ∆W is the wear weight, ρ is the density, L is the sliding distance and F is the load. In addition, wear morphologies and chemical composition of the samples were characterized by SEM (with EDS) to identify the wear mechanisms. The electrochemical corrosion study of the coatings was carried out in 3.5% NaCl solution. All samples were exposed to electrolyte solution under open circuit potential (OCP) at room temperature for 30 minutes. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods were adopted to measure the corrosion behavior of Ni-W samples, which methods were practiced by connecting 1 cm2 of exposed sample surface area as working electrode, platinized platinum as counter electrode and saturated calomel as reference electrode. The cyclic polarization (CP) was done at a sweep rate of 0.333 mV/s. The corrosion morphology of Ni-W sample surface after HCPEB irradiation was also examined. 3. Results and discussion 3.1 Phase identification
Figure 1(a) showed the XRD patterns of Ni-W samples before and after two level (10 and 20) of HCPEB pulses irradiation. The initial sample was pure nickel sample, and only the diffraction peak of Ni was observed. After HCPEB, the Ni peaks were slightly shifted to lower angle, which shift is sensitive to a given change in plane spacing (d-placing). The relationship between the Ni lattice parameter (plane spacing parameter) and the HCPEB pulses was shown in Fig. 1(b), and from the figure, the Ni lattice parameter increased after HCPEB irradiation, which might be ascribed to that the W atoms were dissolved in Ni substrate (the radius of W atomic is larger than that of Ni atomic). The maximum value (0.35455 nm) of lattice parameter was found with the 10 pulses irradiated sample, and the value of lattice expansion was 0.00217 nm. However, in Fig. 1(a) the W peak was absent in the pattern of the 20 pulses HCPEB irradiated Ni-W sample, which implied that a small amount of W atoms might had been dissolved into Ni substrate during HCPEB irradiation.
Fig. 1. (a) XRD patterns of Ni-W samples before and after HCPEB irradiation, (b) the relationship between the Ni lattice parameter and HCPEB irradiation. 3.2 Microstructural characterization The secondary electron microscopy images of HCPEB irradiated samples are shown in Fig. 2(a-c). After 10 pulses of HCPEB irradiation, craters (indexed as W elements) were formed on the surface, as shown in Fig. 2(a), which is commonly observed in many HCPEB-irradiated metal surfaces [23,24]. The formation of Craters signified that the melting of Ni substrate occurred. In the smooth area (apart from the craters), a small amount of W elements was found, which indicated that W particles were melted into Ni substrate. Furthermore, after 20 pulses irradiation, shown in Fig. 2(b), the surface of Ni-W sample became much smoother (with fewer craters). With a higher magnification of SEM image, Fig. 2(c), tiny grains (with low angle grain boundaries) were observed, and the grain size was statistically averaged as 106 nm from 250 grains by measuring software, given in Fig. 2(d). During HCPEB, because of its inborn very rapid cooling process, the melted surface layer experienced a fast solidification, which cooling rate was evaluated as high as 108-9 K/s [25,26]. This unique phenomenon encouraged the nucleation rate but slowed down the grain growth.
Fig. 2. Surface morphology of Ni-W samples with (a) 10 pulses, (b-c) 20 pulses of HCPEB irradiation and (d) the distribution of grain size. TEM was practiced to further characterize the HCPEB irradiated microstructure within the Ni-W alloy layer, and Fig. 3 showed the TEM images of Ni-W alloy layers after 20 pulses irradiation. As presented in a bright-field image of Fig. 3(a), black nano-particles (3.75±2.25 nm in size) were observed in the alloy layer, and those nano-particles were indexed as NiW, given in Fig. 3(b). A dark-field image, Fig. 3(c), taken from the spots in the selected area electron diffraction (SAED, red circle) of the Fig. 3(b), revealed that there are numerous fine NiW particles (ICDD PDF 47-1172) uniformly dispersed in Ni substrate. Additionally, high resolution TEM (HR-TEM) observation was also conducted, shown in Fig. 3(d-f), to further study the atomic structure of the particles, and from Fig. 3(d), the analyses indicated that these particles are NiW intermetallic compounds. Furthermore, in Fig. 3(e), disc-like structures with a bright contrast were found, which were classified as GP zone [27]. From an investigation Peng’s [28], two different Cr phase structure (bcc and fcc) coexisted in the Cu substrate, in which the precipitation of the Cr-rich equilibrium bcc phase was initiated from the nucleation of fcc precipitates. Similar to Peng’s finding, in current study the W atoms were dissolved into the lattice {001} of the Ni, causing a slight variation of the Ni lattice constant, which phenomenon plays an important role in strengthening the Ni-W alloy layer. Fig. 3(f) showed a moire fringes, which is a kind of fringe contrast between two crystal structures (such as substrate and precipitate phase) with different lattice parameters [29]. The distance between each individual fringe was found about 1.14 nm, parallel to Ni (111) lattice plane. This means that the direction of moire
fringes is perpendicular to the g (111) reciprocal vector, suggesting that there is a coherent relationship between the moire fringes and the Ni lattice. On the other hand, the lattice plane spacing of the Ni-W sample is greater than that of pure nickel (111), as shown in Fig. 1(b), indicating that the W element mainly exists in the Ni substrate in the form of substitutional solid solution, and the effect of solid solution strengthening stayed consistent.
Fig. 3. TEM images of the Ni-W alloy sample after 20 pulses of HCPEB irradiation: (a) bright-field image, (b) the corresponding dark-field image, (c) the corresponding SAED, (d-f) high resolution TEM images. Numerous refined Ni boundaries, as seen in Fig. 2(c), provided a great deal of diffusing paths for W atoms since the activation energy needed for diffusion at the grain boundaries is lower than that within the grains [30-32]. Several defect structures
were demonstrated in the 20 HCPEB pulses irradiated Ni-W surface, as shown in Fig. 4. From Fig. 4(a-d), it was clearly observed that the deformation-induced dislocations were scattered around twin boundaries and dislocation cell/wall. From Fig. 4(b) and 4(c), the density of dislocations was found much higher along the boundaries as compared to that in the center, which could be explained by that when the applied stress (induced by HCPEB) exceeded the yield strength, the movement of dislocations was firstly initiated and then ended up with dislocation complexes or tangles. In other words, the increasing interactions of dislocations eventually resulted in barely movable dislocation complexes, and dislocation substructures was then formed, generating a minimum energy configuration [15]. Therefore, it can be observed from some local areas that small precipitations was preferentially precipitated near the subgrain boundary (Fig. 4(e)) and dislocation (Fig. 4(f)). This resulted in a slight reduction in the lattice parameters of the Ni-W sample after 20 pulses of HCPEB, as shown in Fig. 1(b).
Fig. 4. The TEM images of the 20 pulses HCPEB irradiated Ni-W sample surface: (a) twin boundaries, (b, c) dislocation cells, (d) dislocation walls, (e) grain boundaries, (f) dislocations. 3.3 Hardness measurements Microhardness values were plotted against initial and two levels (10 and 20) of HCPEB pulses irradiated samples, given in Fig. 5. The value of the initial sample was found as 1.57 GPa. After 10 pulses of HCPEB irradiation, microhardness slightly increased due to the lattice expansion (caused by W atoms in Ni lattice). As the pulse number rose to 20, the microhardness value increased again, which value was measured as 1.8 GPa, 14.6% greater than that of initial one. The hardening mechanism depends on several factors, which are discussed in the following.
As abovementioned in Fig. 4, the thermal stresses (induced by HCPEB irradiation) intensified the plastic deformation and increased dislocation cells/lines in quantity, and these dislocations acted as barriers for dislocation motion, further resulting in the microhardness enhancement. Besides, from Fig. 4(d), the dissolved W elements were evenly distributed in the vicinity of microstructural defects, which elements formed particles that could effectively pin grain boundaries and create strong obstacles for dislocation motion. That is to say, it is considered that the W atoms, which were diffused into Ni substrate lattice, acted as obstacles for dislocation motion, leading to the microhardness enhancement (so-called the solid solution strengthening). In addition, the solid solubility rose with the increasing number of HCPEB pulse [16], so the hardening/strengthening effect of solid solution increased accordingly. Consequently, the highest value of microhardness was obtained with the 20 pulses HCPEB irradiated Ni-W alloy coating. In short, the reinforcement for the surface alloyed Ni-W samples was due to sub-grain strengthening, precipitation hardening and/or the solid solution strengthening.
Fig. 5. Microhardness measurements of Ni-W samples before and after HCPEB irradiation. 3.4 Tribological property Friction coefficient and the calculated wear rate of the Ni-W alloy coatings before and after HCPEB irradiation were presented in Fig. 6(a) and 6(b), respectively. The COF of initial sample was found much higher than those of HCPEB irradiated ones, as shown in Fig. 6(a). The wear rate of the initial sample, in Fig. 6(b), was found as 2.781⨯10-3 mm3/Nm, and the value slightly decreased to 1.391⨯10-3 mm3/Nm with the 10 pulses irradiated Ni-W sample. The lowest wear rate (0.595⨯10-3 mm3/Nm) was found with the 20 pulses irradiated sample. In brief, as the number of HCPEB pulse irradiation increased, the friction coefficient and wear rate decreased. The remarkable improvement in tribological property was primarily attributed to the ultrafine particles residing in the nickel matrices.
Fig. 6. Friction coefficient and wear rate of Ni-W samples before and after HCPEB irradiation. After tribological experiments, the surface morphologies of Ni-W samples before and after HCPEB irradiation were observed with SEM, shown in Fig. 7. Firstly, Fig. 7(a, d) showed the worn surface of the initial sample, exhibiting a rough surface with deep grooves and cracks, which features were classified as the abrasive wear. The degree of wear on the 10 pulses irradiated Ni-W sample decreased, shown in Fig. 7(b, e), and the surface appeared a crush-like flaking phenomenon. A certain amount of plastic deformation occurred on both sides of the furrow along the direction of sliding. At the same time, a peeling phenomenon was observed on the worn surface, which is mainly due to that hard particles generated and attached onto the surface during sliding wear test, and this resulted in a micro-cutting on the Ni-W layer, further accelerating the abrasive wear. In contrast, the worn surface of the 20 pulses HCPEB irradiated sample was found relatively smooth with shallow scratches, shown in in Fig. 7(c, f), and the minimum width of the scratch (295.7 µm) was observed. Avient and Rabinowicz suggested that the wear resistance of an alloy is proportional to its hardness [33,34], which are in good agreement with our findings.
Fig. 7. The worn surface of Ni-W samples before and after HCPEB irradiation: (a, d) initial, (b, e) after 10 pulses HCPEB, and (c, f) after 20 pulses HCPEB samples. 3.5 Corrosion resistance For corrosion tests, typical polarization curves of the Ni-W samples before and after HCPEB irradiation were given in Fig. 8(a). It revealed that the corrosion current density dropped after HCPEB irradiation, and the lowest value was found with 20 pulses irradiated sample. These polarization curves were then analyzed by Tafel extrapolation, and the parameters such as corrosion current (Icorr) density and corrosion potential (Ecorr) were calculated and plotted in Fig. 8(b). From Fig. 8(b), the Ecorr increased significantly with the increase of the HCPEB pulse number, so a enhancement of the corrosion resistance was obtained for the 20 pulses irradiated sample. In a HCPEB related investigation [35], it was reported that a homogeneous layer was formed on top of the irradiated surface, which made a great
contribution to the improvement of corrosion resistance. Additionally, studies on corrosion resistance of pure metals, such as commercial pure copper and Ti [22,36], suggested that microstructural defects and ultra-fine grains were in favor of forming a thicker and denser passive layer on surfaces.
Fig. 8. (a) The CP curves with 3.5 wt% NaCl solution, (b) variation of corrosion current density (Icorr) and corrosion potential (Ecorr) of Ni-W samples before and after HCPEB irradiation. Figure 9(a) shows the surface morphology of the initial pure nickel sample after corrosion test. Fig. 9(b, c) presented corrosion tested morphologies of Ni-W sample surface after 10 and 20 HCPEB pulses irradiation, and it was observed that the number and size of corrosion pits on the 20 pulses irradiated sample were minimized. From earlier discussion on microstructure characterization analysis, fine intermetallic compounds (particles) were formed dispersively in the Ni-W alloy layer, and due to the potential difference between the particles and the substrate, many small corrosion cells would be formed while the small particles would be consumed during anode reaction. Thus, the substrate material was protected from corrosion, which is called sacrificial anode protection method [37]. In a word, the intermetallic compounds delayed the surface corrosion, so the corrosion resistance of the Ni-W alloy samples was significantly improved.
Fig. 9. The corrosion tested morphologies of (a) pure nickel and Ni-W sample surface after (b) 10 pulses and (c) 20 pulses of HCPEB irradiation. 4. Conclusions Tungsten (W) powders were chosen as alloying elements to be alloyed onto Ni substrate, and which alloying treatment was successfully materialized by HCPEB irradiation in current study. The main results were summarized as follows. 1. Through HCPEB irradiation, part of the W powders were dissolved into the Ni
substrate, forming Ni-W alloy layers. The newly formed Ni-W alloy layer was mainly composed of Ni (W) solid solution, NiW ultra-fine intermetallic compound and Ni phase. 2. After HCPEB irradiation, numerous dislocation cell/wall were formed in the Ni-W alloy layers, which provided a large amount of diffusing paths for W atoms, so the effect of solid solution was encouraged. 3. After HCPEB irradiation, the surface hardness and wear resistance were increased, and which improvement was mainly attributed to the induced microstructural defects, solid solution effect, Ni subgrains and ultra-fine W particles. 4. After HCPEB irradiation, the enhanced corrosion resistance was ascribed to the joint effects of the induced microstructure defects and the W addition, which formed a more stable passive film on the Ni-W alloy coatings. Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
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Highlights: 1. Ni-W alloy layer formed on the surface of nickel surface after irradiation of refractory metal tungsten. 2. Microstructure defects, NiW intermetallic compounds, W particles and other
structures formed in the alloying layer. 3. Changes in microstructure affect performance. 4. Treatment of refractory metal alloying by non-equilibrium process.
Declaration of competing interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.