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Maximizing the benefit of aluminizing to AZ31 alloy by surface nanocrystallization for elevated resistance to wear and corrosive wear
Author’s Accepted Manuscript Maximizing the benefit of aluminizing to AZ31 alloy by surface nanocrystallization for elevated resistance to wear and corrosive wear Meisam Nouri, D.Y. Li www.elsevier.com/locate/jtri
PII: DOI: Reference:
S0301-679X(17)30126-3 http://dx.doi.org/10.1016/j.triboint.2017.03.009 JTRI4637
To appear in: Tribiology International Received date: 12 December 2016 Revised date: 1 March 2017 Accepted date: 6 March 2017 Cite this article as: Meisam Nouri and D.Y. Li, Maximizing the benefit of aluminizing to AZ31 alloy by surface nanocrystallization for elevated resistance to wear and corrosive wear, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2017.03.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Maximizing the benefit of aluminizing to AZ31 alloy by surface nanocrystallization for elevated resistance to wear and corrosive wear Meisam Nouri, D.Y. Li * Department of Chemical and Materials Engineering, University of Alberta Edmonton, T6G 1H9, Canada *
Correspondences to: Postal address: Department of Chemical and Materials Engineering, University of Alberta, 12th Floor - Donadeo ICE, 9211 - 116 Street, Edmonton, Alberta, Canada, T6G 1H9. Tel.: 780.492.6750, Fax: 780.492.2881. [email protected]
Abstract Surface modification is an effective approach for improving Mg alloys to corrosion and wear. Among various surface techniques, aluminizing is an economical one, which has been applied to various metallic materials for increased resistances to oxidation, corrosion, wear, and corrosive wear. The effectiveness of aluminizing could be further improved by surface nanocrystallization. In this work, effects of surface nanocrystallization on the resistance of aluminized Mg-3Al alloy to corrosion, wear, and corrosive wear were investigated. The results demonstrate that aluminizing increases the resistance of Mg-3Al alloy to wear and corrosion, resulting from introduced passivation capability and hard β-Mg17Al12 phase. Surface nanocrystallization maximizes the benefits, largely attributed to further strengthened surface layer with stronger passivation capability and a more protective film. Keywords: magnesium; aluminizing; wear; corrosion; nanocrystallization
Introduction The high strength-to-weight ratio of magnesium makes Mg alloys very attractive for automobile industry to develop fuel-efficient and environmental-friendly vehicles. However, high chemical reactivity of magnesium largely limits its applications in various environments. Compared to aluminum alloys that are widely used in the transportation industry, magnesium alloys show lower strength and poor resistance to corrosion and wear. Efforts have been made to improve the wear resistance of magnesium alloys through alloying with selected elements [1–5], making composites of Mg alloys with hard ceramic particles [6–10], developing special manufacturing processes [11–14], surface modification using laser [15–18], micro arc [19–21] and ion beam [22,23], or coating techniques [24–27]. 1
Even though those researches show some improvement in the wear resistance of magnesium alloys, when subjected to wear in corrosive environments, further improvement is required, since in this case, both wear and corrosion resistances need to be enhanced in order to resist the synergistic attack by wear and corrosion. Aluminum has a higher electrochemical potential than magnesium due to its protective passive oxide film which makes Al alloys much more corrosion-resistant than Mg alloys. For this reason, coating Mg alloys with Al or developing an Al-rich layer through diffusion is a promising approach to make Mg alloys more resistant to corrosion. According to previous studies reported in the literature, it is doable to coat Mg alloys with Al using pack powder diffusion [28–42], salt bath [43–48], cold and hot spray [49,50], and other coating processes [51–54]. Expected improvement in the corrosion resistance of Al-coated Mg alloys has been reported in the literature. However, very few studies [36,42] were conducted to investigate the effect of Al coating on wear and corrosive wear of Mg alloys. Surface nanocrystallization has been demonstrated to be an effective process to enhance the corrosion resistance of passive materials through promoting atomic diffusion along highdensity grain boundaries. The promoted diffusion helps reduce defects at the passive film/substrate interface and enhance oxide inward growth, resulting in faster passivation and a more protective passive film with stronger bonding with the substrate. Furthermore, refined grains increase the hardness, thus elevating the wear resistance [55–57]. It is thus expected that nanocrystallizing an aluminized Mg alloy surface would maximize its resistance to corrosion and that to wear and corrosive wear. The objectives of this study are 1) to evaluate the effect of aluminizing on resistance of AZ31 alloy to wear and corrosive wear, and 2) to further improve the aluminized AZ31 alloy by surface nanocrystallization. Corrosion and wear of the samples were evaluated in two environments: tap water and dilute salt solution. Although tap water is not very aggressive, it is present almost everywhere, e.g., humid environments, and can cause corrosion of Mg alloys and promote wear of materials [58–61]. This is the reason why the Mg alloy samples were tested in tap water, which helped assess how the aluminizing and surface nanocrystallization benefit the Mg alloy in commonly encountered environments. Corrosion and wear mutually accelerate each other, caused considerably increased material loss due to corrosion-wear synergy [62], even each of which may not be severe. The dilute salt solution was used to see how the treated Mg alloy performed in a more aggressive medium, which is common in marine environments.
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Experimental Procedure Sample Preparation Pure Magnesium rods (99.9%), pure Aluminum granules (99.9%) and pure Zinc powder (99.9%) with a weight ratio of 96%, 3%, and 1% were mixed and melted in an AGI induction furnace to make AZ31 alloy. For homogeneity, the ingot was re-melted four times (the ingot was turned over before each re-melting). Small disk-shape pieces were cut from the ingot, ground using SiC papers of up to 1200 mesh and cleaned in alcohol. The prepared disks were then aluminized using a pack aluminizing process. A mixture of pure Al powder (99.9%) and AlCl3 activator powder were used for the aluminizing treatment. AZ31 disks were embedded in the powder mixture in alumina crucibles, which were sealed and heated at 600 °C in a Thermolyne tube furnace in an Argon atmosphere for 50 minutes. The furnace was then turned off and let the samples cooled down in the furnace under a flow of Ar. After cooling, the treated samples were pulled out from the furnace and cleaned by light brushing and ultrasonic cleaning in alcohol. After cleaning the surface, some of the aluminized samples were hammered repeatedly for about 10 minutes using a roto-hammer (Robert Bosch Tool Corporation, USA) at a frequency of 50Hz. The head of the hammer had a semi-spherical shape of 5mm in diameter and was made of tool steel, which was much harder than both Al and AZ31. The impact energy of the hammering machine was 2.207Nm. During the hammering process, the target surface was hammered homogenously over the entire sample surface. Figure 1 schematically illustrates the hammering process. After hammering, as a recovery treatment, the sample was annealed at 250 °C in a Thermolyne tube furnace in an argon atmosphere for 40 minutes to develop nanocrystallized surface aluminized layer. Figure 2 illustrates the entire treatment and process conditions, including aluminizing and surface nanocrystallization.
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Figure 1. A schematic illustration of repeatedly hammering an aluminized sample.
Figure 2. Illustration of the entire treatment and process conditions, including aluminizing and surface nanocrystallization.
Wear and Micro-indentation Tests Mechanical properties of samples in different regions on the cross-section area were measured using a micro-indenter (Fischer Technology Inc.). The micro-indentation test was performed under a maximum load of 50 mN using a cone-shape diamond indenter tip. The diagonal of indent was about 10 micrometers and the distance between adjacent indents was larger than 30 micrometers in order to minimize the influence of work hardening on the minroindentation results. The measurement was repeated at least three times for each region. Dry sliding and corrosive wear tests were performed at ambient temperature using a THT High-Temperature pin-on-disk apparatus (CSEM). The pin was a Si3N4 ball with 3 mm in diameter and the sliding speed was 1 mm/s. The samples were tested under a load of 2 N for 2000 revolutions, corresponding to a sliding distance of about 13 m. Each wear test was repeated for three times and each test lasted about 20 minutes. Sample’ volume loss caused by wear was measured using a ZeGage 3D optical profiler (Zygo), which provided non-contact optical 3D-scaled images of wear track, from which the volume loss was automatically determined by the computerized profiler. Corrosive wear were performed under the same conditions but in tap water and a 3.5%NaCl solution, respectively. After wear tests or corrosive wear tests, samples were ultrasonically cleaned in alcohol and dried before measuring their volume loss.
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Corrosion Test The corrosion behavior of the samples was evaluated by performing polarization tests using a PC4-750 electrochemical testing apparatus (Gamry Instruments Inc). A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate (Pt) was used as the counter electrode. The corrosive medium was a 3.5 dilute NaCl solution made by dissolving 3.5 % NaCl in distilled water. The electrochemical tests were performed at the room temperature and polarization curves were obtained, from which corrosion potentials were determined and corrosion currents were automatically calculated by the computerized system based on the standard slope method.
Microstructural and Surface Analysis Microstructures of the samples were characterized using a Zeiss EVO MA 15 LaB6 filament scanning electron microscope. Backscattered images were taken using a Si diode detector and EDS (Energy Dispersive Spectroscopy) acquired with a peltier-cooled 10 mm2 Bruker Quantax 200 Silicon drift detector with 123 eV resolution. A Multimode 8 atomic force microscope (Bruker) was used to determine the size of nanosized grains in the aluminized layer. Before the AFM analysis, the hammered and annealed aluminized sample was sectioned and polished to a mirror-like surface using diamond suspensions, followed by etching using the Nital solution. The surface potential of the samples was measured using the same Multimode 8 atomic force microscope in tapping mode to determine the work functions of different areas on the sample. Electron Work Function (EWF) is the minimum energy required to move electrons from the inside a solid to its surface [63], which reflects the activity of electrons and thus the chemical stability of the solid.
Results and Discussion Microstructure Figure 3 presents a micrograph and a composition profile on the cross-section area of an aluminized sample. The line scan Energy-dispersive X-ray spectroscopy (EDS) analysis shows a gradual increase in Al content from inside the sample (~ 2.5 wt%) to its surface (> 45 wt%). This increase in Al content occurred within a thickness of about 300 µm, which is approximately the thickness of the aluminized layer. As shown, there are some aluminum rich domains (point 2) in
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the top surface layer, which have a different composition from that of the matrix (for details, see the table attached to Figure 3).
Figure 3. A line scan EDS analysis and a backscattered SEM picture of an aluminized sample. Figure 4 demonstrates the cross-sectional view of the aluminized sample at a higher magnification. In the surface layer and also in the vicinity of the Al-rich phase, lamellar Mg-Al eutectic microstructure was observed. According to the Mg-Al binary phase diagram (Figure 5), the eutectic micro-constituent consists of Mg and β-Mg17Al12 phase. In most casting processes of making Mg-Al alloys containing more than 3 wt% Al, the cooling rate is low enough to cause Mg-Al eutectic microstructure. The eutectic microstructure may have different morphologies, depending on composition and cooling rate during casting. Alloys that have a composition closer to the eutectic composition (33 wt% Al) tend to exhibit lamellar morphologies, while those with aluminum content less than 10 wt% (e.g., AZ31) display partially or fully divorced eutectic morphologies [64,65].
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Figure 4. A micrograph image near the top surface of an aluminized sample taken with a higher magnification (x2000).
Figure 5. A Mg-Al binary phase diagram, adapted by ASM International from [66]
Figure 6(a) represents a cross-sectional micrograph of an aluminized sample which was hammered followed by the recovery heat treatment. Figure 6(b) shows an AFM topography image in the area very close to the top surface of the sample. The maximum grain size is about 260 nm, confirming that the surface layer has been nanocrystallized. The severe plastic deformation of the surface layer caused by hammering generated dislocation cells which were turned into nano-sized grains by the subsequent recovery heat treatment. More details for surface nanocrystallization can be found in references [55,67]. 7
(b)
Figure 6. (a) A micrograph of an aluminized surface layer experienced surface nanocrystallization; (b) Corresponding AFM topographic image showing nano-sized features.
Hardness Figure 7(a) presents profiles of hardness versus distance from the top surface to the core of samples, including original (AZ31), aluminized, and aluminized with subsequent surface nanocrystallization. Corresponding loaddepth indentation curves of top surface layers of the
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(a)
(b)
Figure 7. (a) Microhardness profiles of different samples versus the distance from the top surface. The deviation is typically within 15% based on the measurements; (b) Load-depth indentation curves of the top surface of the samples.
samples are shown in Figure 7(b). As illustrated, the untreated AZ31 sample exhibits an approximately flat hardness line with no obvious changes. The aluminized sample, however, shows a greater hardness on the surface. The increase in hardness of the aluminizing layer results from the formation of Mg-Al(-Zn) phases [5,46,47], mainly β-Mg17Al12 phase in our case. Nanocrystallization of the aluminizing layer further hardened the aluminized surface. The increment in the hardness is attributed to the formation of very fine grains, which follows HallPetch relationship, σ= σ0+Kd-1/2, where σ is the yield stress, σ0 and K are material constants, and d is the grain diameter [68]. The equation is similarly applicable to hardness [69].
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Corrosion Behavior Figure 8 and Table 1 present polarization curves of the samples tested in the 3.5% NaCl solution and their calculated values for corrosion currents and corrosion potentials. The curves show an enhancement in corrosion resistance of AZ31 by aluminizing. This improvement is attributed to the Al-rich surface, which brought in the passivation capability to the aluminized surface, leading to higher corrosion potential. The work function in the region close to the surface of the aluminized sample also increased as Figure 9 illustrates, corresponding to a higher electrochemical stability and corrosion resistance.
Figure 8. Polarization curves of untreated, aluminized and nanocrystallized aluminized samples in the 3.5% NaCl solution. Table 1 ICorr and ECorr values determined from polarization curves
Sample
ICorr (Log A/cm^2) ECorr (V)
AZ31
1.6E-3
-1.46
Aluminized
1.19E-7
-0.53
10
Nano+ Aluminized
6.35E-9
-0.21
Figure 9. Work function measured in the vicinity of the aluminized surface of AZ31 alloy.
It should be noted that more β-Mg17Al12 domains formed in the aluminized surface. It was expected that the presence of β-Mg17Al12 domains could lower the corrosion resistance of Mg-Al alloys due to galvanic reaction between the magnesium matrix and the Mg17Al12 phase, which is cathodic relative to the Mg matrix. However, the role of the β-Mg17Al12 phase may depend on its volume fraction and domain size. It is suggested that a high volume fraction of the β phase might act as a barrier to corrosion of the alloy [70–72]. Recent studies on Al reinforced by SiC nanoparticles [73] and Ni respectively reinforced by TiC microparticles and nanoparticles [74] demonstrate that the corrosion potential and electron work function, which is a measure of surface stability, increase by adding the nanoparticles. However, the microparticles showed an opposite effect. In the present case, there is a high fraction of β phase in the aluminized surface layer with a fine lamellar eutectic morphology (Figure 4), which could result in a corrosion barrier for the base alloy underneath [75]. More investigation on the role of β-Mg17Al12 phase on the corrosion of the aluminized Mg alloy is being carried out. Hammering and annealing (recovery) further improved the effectiveness of aluminizing treatment with an elevated corrosion resistance. This improvement is a result of surface grain refinement, which enhanced the passivation capability due to accelerated atomistic diffusion at high-density grain boundaries. The faster diffusion also helped reduce the formation of defects, 11
e.g., pores and micro-voids, at the interface between the passive film and the substrate as well as inside the passive film. Besides, the existence of Mg17Al12 phase in a Mg alloy may promote nucleation of an oxide film, generating a thicker oxide film that lowers the corrosion rate [76– 78]. By hammering and annealing the aluminized sample, the β phase domains could also be refined, thus enhancing the above-mentioned effect. One more benefit of surface nanocrystallization to the corrosion resistance of the aluminized AZ31 alloys is the effect of oxide pegging. The high density of grain boundaries in the nanocrystallized surface may enhance inward growth of oxide film, leading to improved adherence of the passive film to the substrate [55–57].
Wear and Corrosive Wear Figure 10(a) illustrates volume losses of AZ31, aluminized and, surface nanocrystallized aluminized samples caused by sliding wear in air, tap water, and the 3.5% NaCl solution. As shown, aluminizing decreased the volume loss, which was further reduced by surface nanocrystallization, along with reduced friction as Figure 10 (b) illustrates.
(a)
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(b)
(c)
Figure 10. (a) Wear volume loss (in air) and corrosive wear volume loss (in 3.5% NaCl solution) of AZ31, aluminized and nanocrystallized aluminized samples; (b) Friction coefficient versus friction distance for samples worn in air; (c) Friction coefficient versus friction distance for samples worn in the 3.5% NaCl solution.
Regarding the friction coefficient, when tested in the 3.5%NaCl solution, after initial unstable range the frictional coefficients were relatively stable (see Figure 10(c)). All the three samples showed similar frictional coefficients. Since the dilute NaCl solution was not very aggressive, the solution could result in a lubrication effect, leading to lowered friction and minimizing the difference in friction al coefficient, compared to those measured during dry wear tests. Under 13
the dry wear testing condition, frictional coefficients of the samples were higher and the difference in friction coefficient among the three samples was lager with larger fluctuations. During the dry wear test without the solution, which acted more or less like a lubricant, the frictional coefficient largely depended on material hardness, resulting in distinguishable difference in frictional coefficient among the samples. Besides, raised adhesive force due to direct metal-metal contact should be responsible for the larger fluctuations in the frictional coefficient as Figure 10 (b) illustrates. Figure 11 illustrates cross-sectional profiles of wear tracks for AZ31, aluminized, and surface nanocrystallized aluminized samples, caused by dry wear and wear in water, respectively. The profiles show that AZ31 sample has a larger cross-sectional area of the wear track, compared to other two samples. This means that AZ31 sample has a larger volume loss. In addition, more material pile-up was observed at the edges of the wear track, indicating that more surface plastic deformation occurred in AZ31 sample during wear test, which came from its lower hardness. The aluminized sample with nanocrystallized surface showed the smallest crosssectional area of wear track with the least material pile-up at the track edges. In Figure 11, volume losses of the samples caused by wear in tap water are also presented for comparison purpose. The cross-sectional profiles are consistent with the wear measurement.
(a)
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(b)
(c)
Figure 11. Cross-sectional profiles of dry wear and wear in water: (a) AZ31, (b) Aluminized sample, and (c) nanocrystallized sample.
The enhancement in dry wear resistance by aluminizing should be credited to increased hardness, mainly resulting from the formation of second phases such as β-Mg17Al12 [5,46,47]. Solid-solution hardening by Al should also play a role. The presence of β-Mg17Al12 phase and eutectic microstructure in the surface layer considerably hardened the surface layer, as shown in Figure 7. The hardened surface also helped reduce the contact area, leading to lowered friction as Figure 10(b) illustrates. A lowered friction coefficient, in turn, benefits the wear resistance by reducing the tangential wearing force. The microstructure refinement by the surface nanocrystallization treatment further increased the surface hardness, rendering the surface more resistant to wear with further lowered coefficient of friction.
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Figure 12 shows the worn surface of dry wear tracks on the samples. As illustrated, wear of the samples mainly resulted from plowing, caused by abrasive wear. No significant difference in morphology of the wear track among the different samples is observed.
Figure 12. Dry wear tracks of samples. (a) AZ31, (b) Aluminized sample, and (c) nanocrystallized sample.
When tested in the solutions, as Figure 10(a) illustrates, aluminizing and surface nanocrystallization increased the wear resistance of the Mg alloy. With the aluminizing treatment, the introduced passivation capability, formed β-Mg17Al12 phase, and solid-solution hardening enhanced the alloy with elevated resistance to corrosion and wear as well as to the corrosion-wear synergy. When the aluminized surface was nanocrystallized, the surface was further improved with higher hardness, improved passivation and the formation of a stronger and more adherent surface oxide film. These improvements further raised the resistance of the Mg alloy to corrosive wear. Since the dilute NaCl solution is more aggressive than tap water, the wear test in the NaCl solution caused larger volume losses than the tests in the tap water as 16
illustrated in Figure 10 (a). However, the trends or rankings of the samples under different testing conditions are similar. It should be noted that in the wet environments, the volume losses are smaller than those caused by dry wear. This should be ascribed to the lubrication effect and the fact that the solutions under study may not result in strong wear-corrosion synergy, since tap water and the dilute NaCl solution are not very corrosive media, although the latter is relatively more aggressive than the former. In addition, the coefficients of friction of the three surfaces did not show large difference, which should be due to the lubrication effect as well (e.g., see Figure 10(c)).
Conclusions 1- Aluminizing surface of AZ31 alloy increased its resistance to corrosion, wear and corrosive wear by introducing passivation capability and hard β-Mg17Al12 phase. 2- The aluminized surface layer was nanocrystallized by repeated hammering and subsequent recovery heat treatment. The hammering introduced severe plastic deformation and the recovery treatment turned formed dislocation cells into nano-sized grains with the maximum grain size around 260 nm. 3- The surface nanocrystallization further improved the resistance to corrosion, wear and corrosive wear. The improvements are attributed to the grain or microstructure refinement, which resulted in increased hardness, stronger passivation capability and the formation of a more protective and adherent passive film. All the effects enhanced the Mg alloy against wear and corrosive wear.
Acknowledgement The authors are grateful for financial support from the Natural Science and Engineering Research Council of Canada and AUTO21.
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Highlights
Aluminizing surface of AZ31 alloy increases its resistance to corrosion, wear and corrosive wear by introducing passivation capability, hard β-Mg17Al12 phase and solidsolution hardened matrix. The aluminized surface layer is nanocrystallized by repeated hammering and subsequent recovery heat treatment. The hammering introduces severe plastic deformation and the recovery treatment turns dislocation cells into nano-sized grains with the maximum grain size of around 260 nm. The surface nanocrystallization further improves the resistance to corrosion, wear and corrosive wear. The improvements are attributed to the grain refinement, which results in increased hardness, stronger passivation capability and the formation of a more protective and adherent passive film. All the effects enhance the Mg alloy against wear and synergistic attack of wear and corrosion.
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PII: DOI: Reference:
S0301-679X(17)30126-3 http://dx.doi.org/10.1016/j.triboint.2017.03.009 JTRI4637
To appear in: Tribiology International Received date: 12 December 2016 Revised date: 1 March 2017 Accepted date: 6 March 2017 Cite this article as: Meisam Nouri and D.Y. Li, Maximizing the benefit of aluminizing to AZ31 alloy by surface nanocrystallization for elevated resistance to wear and corrosive wear, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2017.03.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. 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.
Maximizing the benefit of aluminizing to AZ31 alloy by surface nanocrystallization for elevated resistance to wear and corrosive wear Meisam Nouri, D.Y. Li * Department of Chemical and Materials Engineering, University of Alberta Edmonton, T6G 1H9, Canada *
Correspondences to: Postal address: Department of Chemical and Materials Engineering, University of Alberta, 12th Floor - Donadeo ICE, 9211 - 116 Street, Edmonton, Alberta, Canada, T6G 1H9. Tel.: 780.492.6750, Fax: 780.492.2881. [email protected]
Abstract Surface modification is an effective approach for improving Mg alloys to corrosion and wear. Among various surface techniques, aluminizing is an economical one, which has been applied to various metallic materials for increased resistances to oxidation, corrosion, wear, and corrosive wear. The effectiveness of aluminizing could be further improved by surface nanocrystallization. In this work, effects of surface nanocrystallization on the resistance of aluminized Mg-3Al alloy to corrosion, wear, and corrosive wear were investigated. The results demonstrate that aluminizing increases the resistance of Mg-3Al alloy to wear and corrosion, resulting from introduced passivation capability and hard β-Mg17Al12 phase. Surface nanocrystallization maximizes the benefits, largely attributed to further strengthened surface layer with stronger passivation capability and a more protective film. Keywords: magnesium; aluminizing; wear; corrosion; nanocrystallization
Introduction The high strength-to-weight ratio of magnesium makes Mg alloys very attractive for automobile industry to develop fuel-efficient and environmental-friendly vehicles. However, high chemical reactivity of magnesium largely limits its applications in various environments. Compared to aluminum alloys that are widely used in the transportation industry, magnesium alloys show lower strength and poor resistance to corrosion and wear. Efforts have been made to improve the wear resistance of magnesium alloys through alloying with selected elements [1–5], making composites of Mg alloys with hard ceramic particles [6–10], developing special manufacturing processes [11–14], surface modification using laser [15–18], micro arc [19–21] and ion beam [22,23], or coating techniques [24–27]. 1
Even though those researches show some improvement in the wear resistance of magnesium alloys, when subjected to wear in corrosive environments, further improvement is required, since in this case, both wear and corrosion resistances need to be enhanced in order to resist the synergistic attack by wear and corrosion. Aluminum has a higher electrochemical potential than magnesium due to its protective passive oxide film which makes Al alloys much more corrosion-resistant than Mg alloys. For this reason, coating Mg alloys with Al or developing an Al-rich layer through diffusion is a promising approach to make Mg alloys more resistant to corrosion. According to previous studies reported in the literature, it is doable to coat Mg alloys with Al using pack powder diffusion [28–42], salt bath [43–48], cold and hot spray [49,50], and other coating processes [51–54]. Expected improvement in the corrosion resistance of Al-coated Mg alloys has been reported in the literature. However, very few studies [36,42] were conducted to investigate the effect of Al coating on wear and corrosive wear of Mg alloys. Surface nanocrystallization has been demonstrated to be an effective process to enhance the corrosion resistance of passive materials through promoting atomic diffusion along highdensity grain boundaries. The promoted diffusion helps reduce defects at the passive film/substrate interface and enhance oxide inward growth, resulting in faster passivation and a more protective passive film with stronger bonding with the substrate. Furthermore, refined grains increase the hardness, thus elevating the wear resistance [55–57]. It is thus expected that nanocrystallizing an aluminized Mg alloy surface would maximize its resistance to corrosion and that to wear and corrosive wear. The objectives of this study are 1) to evaluate the effect of aluminizing on resistance of AZ31 alloy to wear and corrosive wear, and 2) to further improve the aluminized AZ31 alloy by surface nanocrystallization. Corrosion and wear of the samples were evaluated in two environments: tap water and dilute salt solution. Although tap water is not very aggressive, it is present almost everywhere, e.g., humid environments, and can cause corrosion of Mg alloys and promote wear of materials [58–61]. This is the reason why the Mg alloy samples were tested in tap water, which helped assess how the aluminizing and surface nanocrystallization benefit the Mg alloy in commonly encountered environments. Corrosion and wear mutually accelerate each other, caused considerably increased material loss due to corrosion-wear synergy [62], even each of which may not be severe. The dilute salt solution was used to see how the treated Mg alloy performed in a more aggressive medium, which is common in marine environments.
2
Experimental Procedure Sample Preparation Pure Magnesium rods (99.9%), pure Aluminum granules (99.9%) and pure Zinc powder (99.9%) with a weight ratio of 96%, 3%, and 1% were mixed and melted in an AGI induction furnace to make AZ31 alloy. For homogeneity, the ingot was re-melted four times (the ingot was turned over before each re-melting). Small disk-shape pieces were cut from the ingot, ground using SiC papers of up to 1200 mesh and cleaned in alcohol. The prepared disks were then aluminized using a pack aluminizing process. A mixture of pure Al powder (99.9%) and AlCl3 activator powder were used for the aluminizing treatment. AZ31 disks were embedded in the powder mixture in alumina crucibles, which were sealed and heated at 600 °C in a Thermolyne tube furnace in an Argon atmosphere for 50 minutes. The furnace was then turned off and let the samples cooled down in the furnace under a flow of Ar. After cooling, the treated samples were pulled out from the furnace and cleaned by light brushing and ultrasonic cleaning in alcohol. After cleaning the surface, some of the aluminized samples were hammered repeatedly for about 10 minutes using a roto-hammer (Robert Bosch Tool Corporation, USA) at a frequency of 50Hz. The head of the hammer had a semi-spherical shape of 5mm in diameter and was made of tool steel, which was much harder than both Al and AZ31. The impact energy of the hammering machine was 2.207Nm. During the hammering process, the target surface was hammered homogenously over the entire sample surface. Figure 1 schematically illustrates the hammering process. After hammering, as a recovery treatment, the sample was annealed at 250 °C in a Thermolyne tube furnace in an argon atmosphere for 40 minutes to develop nanocrystallized surface aluminized layer. Figure 2 illustrates the entire treatment and process conditions, including aluminizing and surface nanocrystallization.
3
Figure 1. A schematic illustration of repeatedly hammering an aluminized sample.
Figure 2. Illustration of the entire treatment and process conditions, including aluminizing and surface nanocrystallization.
Wear and Micro-indentation Tests Mechanical properties of samples in different regions on the cross-section area were measured using a micro-indenter (Fischer Technology Inc.). The micro-indentation test was performed under a maximum load of 50 mN using a cone-shape diamond indenter tip. The diagonal of indent was about 10 micrometers and the distance between adjacent indents was larger than 30 micrometers in order to minimize the influence of work hardening on the minroindentation results. The measurement was repeated at least three times for each region. Dry sliding and corrosive wear tests were performed at ambient temperature using a THT High-Temperature pin-on-disk apparatus (CSEM). The pin was a Si3N4 ball with 3 mm in diameter and the sliding speed was 1 mm/s. The samples were tested under a load of 2 N for 2000 revolutions, corresponding to a sliding distance of about 13 m. Each wear test was repeated for three times and each test lasted about 20 minutes. Sample’ volume loss caused by wear was measured using a ZeGage 3D optical profiler (Zygo), which provided non-contact optical 3D-scaled images of wear track, from which the volume loss was automatically determined by the computerized profiler. Corrosive wear were performed under the same conditions but in tap water and a 3.5%NaCl solution, respectively. After wear tests or corrosive wear tests, samples were ultrasonically cleaned in alcohol and dried before measuring their volume loss.
4
Corrosion Test The corrosion behavior of the samples was evaluated by performing polarization tests using a PC4-750 electrochemical testing apparatus (Gamry Instruments Inc). A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum plate (Pt) was used as the counter electrode. The corrosive medium was a 3.5 dilute NaCl solution made by dissolving 3.5 % NaCl in distilled water. The electrochemical tests were performed at the room temperature and polarization curves were obtained, from which corrosion potentials were determined and corrosion currents were automatically calculated by the computerized system based on the standard slope method.
Microstructural and Surface Analysis Microstructures of the samples were characterized using a Zeiss EVO MA 15 LaB6 filament scanning electron microscope. Backscattered images were taken using a Si diode detector and EDS (Energy Dispersive Spectroscopy) acquired with a peltier-cooled 10 mm2 Bruker Quantax 200 Silicon drift detector with 123 eV resolution. A Multimode 8 atomic force microscope (Bruker) was used to determine the size of nanosized grains in the aluminized layer. Before the AFM analysis, the hammered and annealed aluminized sample was sectioned and polished to a mirror-like surface using diamond suspensions, followed by etching using the Nital solution. The surface potential of the samples was measured using the same Multimode 8 atomic force microscope in tapping mode to determine the work functions of different areas on the sample. Electron Work Function (EWF) is the minimum energy required to move electrons from the inside a solid to its surface [63], which reflects the activity of electrons and thus the chemical stability of the solid.
Results and Discussion Microstructure Figure 3 presents a micrograph and a composition profile on the cross-section area of an aluminized sample. The line scan Energy-dispersive X-ray spectroscopy (EDS) analysis shows a gradual increase in Al content from inside the sample (~ 2.5 wt%) to its surface (> 45 wt%). This increase in Al content occurred within a thickness of about 300 µm, which is approximately the thickness of the aluminized layer. As shown, there are some aluminum rich domains (point 2) in
5
the top surface layer, which have a different composition from that of the matrix (for details, see the table attached to Figure 3).
Figure 3. A line scan EDS analysis and a backscattered SEM picture of an aluminized sample. Figure 4 demonstrates the cross-sectional view of the aluminized sample at a higher magnification. In the surface layer and also in the vicinity of the Al-rich phase, lamellar Mg-Al eutectic microstructure was observed. According to the Mg-Al binary phase diagram (Figure 5), the eutectic micro-constituent consists of Mg and β-Mg17Al12 phase. In most casting processes of making Mg-Al alloys containing more than 3 wt% Al, the cooling rate is low enough to cause Mg-Al eutectic microstructure. The eutectic microstructure may have different morphologies, depending on composition and cooling rate during casting. Alloys that have a composition closer to the eutectic composition (33 wt% Al) tend to exhibit lamellar morphologies, while those with aluminum content less than 10 wt% (e.g., AZ31) display partially or fully divorced eutectic morphologies [64,65].
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Figure 4. A micrograph image near the top surface of an aluminized sample taken with a higher magnification (x2000).
Figure 5. A Mg-Al binary phase diagram, adapted by ASM International from [66]
Figure 6(a) represents a cross-sectional micrograph of an aluminized sample which was hammered followed by the recovery heat treatment. Figure 6(b) shows an AFM topography image in the area very close to the top surface of the sample. The maximum grain size is about 260 nm, confirming that the surface layer has been nanocrystallized. The severe plastic deformation of the surface layer caused by hammering generated dislocation cells which were turned into nano-sized grains by the subsequent recovery heat treatment. More details for surface nanocrystallization can be found in references [55,67]. 7
(b)
Figure 6. (a) A micrograph of an aluminized surface layer experienced surface nanocrystallization; (b) Corresponding AFM topographic image showing nano-sized features.
Hardness Figure 7(a) presents profiles of hardness versus distance from the top surface to the core of samples, including original (AZ31), aluminized, and aluminized with subsequent surface nanocrystallization. Corresponding loaddepth indentation curves of top surface layers of the
8
(a)
(b)
Figure 7. (a) Microhardness profiles of different samples versus the distance from the top surface. The deviation is typically within 15% based on the measurements; (b) Load-depth indentation curves of the top surface of the samples.
samples are shown in Figure 7(b). As illustrated, the untreated AZ31 sample exhibits an approximately flat hardness line with no obvious changes. The aluminized sample, however, shows a greater hardness on the surface. The increase in hardness of the aluminizing layer results from the formation of Mg-Al(-Zn) phases [5,46,47], mainly β-Mg17Al12 phase in our case. Nanocrystallization of the aluminizing layer further hardened the aluminized surface. The increment in the hardness is attributed to the formation of very fine grains, which follows HallPetch relationship, σ= σ0+Kd-1/2, where σ is the yield stress, σ0 and K are material constants, and d is the grain diameter [68]. The equation is similarly applicable to hardness [69].
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Corrosion Behavior Figure 8 and Table 1 present polarization curves of the samples tested in the 3.5% NaCl solution and their calculated values for corrosion currents and corrosion potentials. The curves show an enhancement in corrosion resistance of AZ31 by aluminizing. This improvement is attributed to the Al-rich surface, which brought in the passivation capability to the aluminized surface, leading to higher corrosion potential. The work function in the region close to the surface of the aluminized sample also increased as Figure 9 illustrates, corresponding to a higher electrochemical stability and corrosion resistance.
Figure 8. Polarization curves of untreated, aluminized and nanocrystallized aluminized samples in the 3.5% NaCl solution. Table 1 ICorr and ECorr values determined from polarization curves
Sample
ICorr (Log A/cm^2) ECorr (V)
AZ31
1.6E-3
-1.46
Aluminized
1.19E-7
-0.53
10
Nano+ Aluminized
6.35E-9
-0.21
Figure 9. Work function measured in the vicinity of the aluminized surface of AZ31 alloy.
It should be noted that more β-Mg17Al12 domains formed in the aluminized surface. It was expected that the presence of β-Mg17Al12 domains could lower the corrosion resistance of Mg-Al alloys due to galvanic reaction between the magnesium matrix and the Mg17Al12 phase, which is cathodic relative to the Mg matrix. However, the role of the β-Mg17Al12 phase may depend on its volume fraction and domain size. It is suggested that a high volume fraction of the β phase might act as a barrier to corrosion of the alloy [70–72]. Recent studies on Al reinforced by SiC nanoparticles [73] and Ni respectively reinforced by TiC microparticles and nanoparticles [74] demonstrate that the corrosion potential and electron work function, which is a measure of surface stability, increase by adding the nanoparticles. However, the microparticles showed an opposite effect. In the present case, there is a high fraction of β phase in the aluminized surface layer with a fine lamellar eutectic morphology (Figure 4), which could result in a corrosion barrier for the base alloy underneath [75]. More investigation on the role of β-Mg17Al12 phase on the corrosion of the aluminized Mg alloy is being carried out. Hammering and annealing (recovery) further improved the effectiveness of aluminizing treatment with an elevated corrosion resistance. This improvement is a result of surface grain refinement, which enhanced the passivation capability due to accelerated atomistic diffusion at high-density grain boundaries. The faster diffusion also helped reduce the formation of defects, 11
e.g., pores and micro-voids, at the interface between the passive film and the substrate as well as inside the passive film. Besides, the existence of Mg17Al12 phase in a Mg alloy may promote nucleation of an oxide film, generating a thicker oxide film that lowers the corrosion rate [76– 78]. By hammering and annealing the aluminized sample, the β phase domains could also be refined, thus enhancing the above-mentioned effect. One more benefit of surface nanocrystallization to the corrosion resistance of the aluminized AZ31 alloys is the effect of oxide pegging. The high density of grain boundaries in the nanocrystallized surface may enhance inward growth of oxide film, leading to improved adherence of the passive film to the substrate [55–57].
Wear and Corrosive Wear Figure 10(a) illustrates volume losses of AZ31, aluminized and, surface nanocrystallized aluminized samples caused by sliding wear in air, tap water, and the 3.5% NaCl solution. As shown, aluminizing decreased the volume loss, which was further reduced by surface nanocrystallization, along with reduced friction as Figure 10 (b) illustrates.
(a)
12
(b)
(c)
Figure 10. (a) Wear volume loss (in air) and corrosive wear volume loss (in 3.5% NaCl solution) of AZ31, aluminized and nanocrystallized aluminized samples; (b) Friction coefficient versus friction distance for samples worn in air; (c) Friction coefficient versus friction distance for samples worn in the 3.5% NaCl solution.
Regarding the friction coefficient, when tested in the 3.5%NaCl solution, after initial unstable range the frictional coefficients were relatively stable (see Figure 10(c)). All the three samples showed similar frictional coefficients. Since the dilute NaCl solution was not very aggressive, the solution could result in a lubrication effect, leading to lowered friction and minimizing the difference in friction al coefficient, compared to those measured during dry wear tests. Under 13
the dry wear testing condition, frictional coefficients of the samples were higher and the difference in friction coefficient among the three samples was lager with larger fluctuations. During the dry wear test without the solution, which acted more or less like a lubricant, the frictional coefficient largely depended on material hardness, resulting in distinguishable difference in frictional coefficient among the samples. Besides, raised adhesive force due to direct metal-metal contact should be responsible for the larger fluctuations in the frictional coefficient as Figure 10 (b) illustrates. Figure 11 illustrates cross-sectional profiles of wear tracks for AZ31, aluminized, and surface nanocrystallized aluminized samples, caused by dry wear and wear in water, respectively. The profiles show that AZ31 sample has a larger cross-sectional area of the wear track, compared to other two samples. This means that AZ31 sample has a larger volume loss. In addition, more material pile-up was observed at the edges of the wear track, indicating that more surface plastic deformation occurred in AZ31 sample during wear test, which came from its lower hardness. The aluminized sample with nanocrystallized surface showed the smallest crosssectional area of wear track with the least material pile-up at the track edges. In Figure 11, volume losses of the samples caused by wear in tap water are also presented for comparison purpose. The cross-sectional profiles are consistent with the wear measurement.
(a)
14
(b)
(c)
Figure 11. Cross-sectional profiles of dry wear and wear in water: (a) AZ31, (b) Aluminized sample, and (c) nanocrystallized sample.
The enhancement in dry wear resistance by aluminizing should be credited to increased hardness, mainly resulting from the formation of second phases such as β-Mg17Al12 [5,46,47]. Solid-solution hardening by Al should also play a role. The presence of β-Mg17Al12 phase and eutectic microstructure in the surface layer considerably hardened the surface layer, as shown in Figure 7. The hardened surface also helped reduce the contact area, leading to lowered friction as Figure 10(b) illustrates. A lowered friction coefficient, in turn, benefits the wear resistance by reducing the tangential wearing force. The microstructure refinement by the surface nanocrystallization treatment further increased the surface hardness, rendering the surface more resistant to wear with further lowered coefficient of friction.
15
Figure 12 shows the worn surface of dry wear tracks on the samples. As illustrated, wear of the samples mainly resulted from plowing, caused by abrasive wear. No significant difference in morphology of the wear track among the different samples is observed.
Figure 12. Dry wear tracks of samples. (a) AZ31, (b) Aluminized sample, and (c) nanocrystallized sample.
When tested in the solutions, as Figure 10(a) illustrates, aluminizing and surface nanocrystallization increased the wear resistance of the Mg alloy. With the aluminizing treatment, the introduced passivation capability, formed β-Mg17Al12 phase, and solid-solution hardening enhanced the alloy with elevated resistance to corrosion and wear as well as to the corrosion-wear synergy. When the aluminized surface was nanocrystallized, the surface was further improved with higher hardness, improved passivation and the formation of a stronger and more adherent surface oxide film. These improvements further raised the resistance of the Mg alloy to corrosive wear. Since the dilute NaCl solution is more aggressive than tap water, the wear test in the NaCl solution caused larger volume losses than the tests in the tap water as 16
illustrated in Figure 10 (a). However, the trends or rankings of the samples under different testing conditions are similar. It should be noted that in the wet environments, the volume losses are smaller than those caused by dry wear. This should be ascribed to the lubrication effect and the fact that the solutions under study may not result in strong wear-corrosion synergy, since tap water and the dilute NaCl solution are not very corrosive media, although the latter is relatively more aggressive than the former. In addition, the coefficients of friction of the three surfaces did not show large difference, which should be due to the lubrication effect as well (e.g., see Figure 10(c)).
Conclusions 1- Aluminizing surface of AZ31 alloy increased its resistance to corrosion, wear and corrosive wear by introducing passivation capability and hard β-Mg17Al12 phase. 2- The aluminized surface layer was nanocrystallized by repeated hammering and subsequent recovery heat treatment. The hammering introduced severe plastic deformation and the recovery treatment turned formed dislocation cells into nano-sized grains with the maximum grain size around 260 nm. 3- The surface nanocrystallization further improved the resistance to corrosion, wear and corrosive wear. The improvements are attributed to the grain or microstructure refinement, which resulted in increased hardness, stronger passivation capability and the formation of a more protective and adherent passive film. All the effects enhanced the Mg alloy against wear and corrosive wear.
Acknowledgement The authors are grateful for financial support from the Natural Science and Engineering Research Council of Canada and AUTO21.
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Highlights
Aluminizing surface of AZ31 alloy increases its resistance to corrosion, wear and corrosive wear by introducing passivation capability, hard β-Mg17Al12 phase and solidsolution hardened matrix. The aluminized surface layer is nanocrystallized by repeated hammering and subsequent recovery heat treatment. The hammering introduces severe plastic deformation and the recovery treatment turns dislocation cells into nano-sized grains with the maximum grain size of around 260 nm. The surface nanocrystallization further improves the resistance to corrosion, wear and corrosive wear. The improvements are attributed to the grain refinement, which results in increased hardness, stronger passivation capability and the formation of a more protective and adherent passive film. All the effects enhance the Mg alloy against wear and synergistic attack of wear and corrosion.
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