Vacuum 89 (2013) 233e237
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Microstructure and corrosion resistance of AZ91D magnesium alloy treated by hybrid ion implantation and heat treatment Liu Hongxi a, *, Xu Qian b, Xiong Damin a, Lin Bo a, Meng Chunlei a a b
Faculty of Materials Science and Engineering, Kunming University of Science and Technology, No. 253, Xuefu Road, Kunming 650093, Yunnan Province, China Faculty of Adult Education, Kunming University of Science and Technology, Kunming 650051, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2011 Received in revised form 19 May 2012 Accepted 19 May 2012
In order to improve the corrosion resistance of magnesium alloy, solid solution-aging treatment and then nitrogen and aluminum (N/Al) duplex ion implantation was conducted on AZ91D magnesium alloy surface. X-ray diffraction (XRD), scanning electronic microscope (SEM) with energy dispersive X-ray spectrometer (EDX or EDS) analysis, Auger electron spectroscopy (AES), electrochemical workstation and microhardness tester are used to characterize the surface microstructure, phase composition, atomic concentration-depth distribution, corrosion resistance and microhardness, respectively. XRD results show that the surface modified layer includes Mg, Mg17Al12, MgAl2O4 and AlN phase, the diffraction peak position and intensity of Mg and Mg17Al12 shift obviously. AES analysis indicates that 280 nm modified layer exists in the surface and near surface. Compared with the substrate, the microhardness of solid solution-aging sample and solid solution-aging combined with duplex ion implantation sample increases by 16.7% and 36.6%, the polarization resistance Rp increases by 20.6 and 107.3 times, respectively. The corrosion current intensity (icorr) decreases by two orders of magnitude in 0.56 M NaCl solution. SEM observation exhibits that duplex ion implantation sample shows less corrosion pit, but untreated and solution-aging coupons have a large amount of corrosion pit under the same corrosion conditions. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: Duplex ion implantation Magnesium alloy Solid solution-aging treatment Microstructure Corrosion resistance
1. Introduction Owing to their outstanding specific strength, good cast ability, high electric and thermal conductivities, excellent dimensional stability, strong shock-absorbing performance, distinguished electromagnetic shielding characteristics, and easy machinability and recycle ability, magnesium alloys are widely used in aerospace, automotive, computer, mobile phone, machinery and electronic fields as environmental friendly and advanced light materials in the 21st century [1,2]. However, their further application has been greatly limited due to their poor corrosion resistance especially in acidic environments or salt-water conditions [3,4]. Therefore, it is necessary to improve the corrosion performance of magnesium alloys for their future applications. The surface treatment of magnesium alloys such as electrochemical plating, chemical vapor deposition (CVD), physical vapor deposition (PVD) and ion implantation have been used to improve its corrosion and wear resistances [5e13]. Among these
* Corresponding author. Tel./fax: þ86 871 5136755. E-mail address:
[email protected] (L. Hongxi).
techniques, ion implantation is an effective technique to modify the surface behaviors without adversely affecting the bulk properties of materials. In addition to the change of chemical composition, ion implantation can also induce changes in the surface due to radiation damage [14,15]. Moreover, the dimensions of the materials have no variation after ion implantation. Single elements such as Ti, Al, Ag, Cr, Mo and N have been used to enhance the corrosion resistance of metals by ion implantation technique [16e19]. The composition and microstructure of single element modified processing is simple, however, it is difficult to meet the actual requirement in various harsh environment [20e25]. According to our knowledge, few papers have reported the effect of solid solution-aging treatment and then N/Al duplex ion implantation on the corrosion behavior of magnesium alloys. In this paper, solid solution-aging treatment combined with N/Al duplex ion implantation process was conducted on AZ91D magnesium alloy surface in order to obtain a more desirable surface modification layer, so that the corrosion resistance and the comprehensive mechanical property can be effectively improved. Depth-concentration distributions of different elements in the surface layer were also analyzed by Auger electron spectroscopy (AES).
0042-207X/$ e see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.vacuum.2012.05.022
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2. Experimental details
a
Commercially available extruded AZ91D magnesium alloy with dimensions of 15 mm 15 mm 4 mm were used as substrate in the experiment. The chemical composition is listed in Table 1. Prior to solid solution-aging and duplex ion implantation treatment, one side of each coupon was ground up to 1500 grit SiC abrasive paper, followed by polishing with fine diamond paste (average size 0.5 mm) to achieve a mirror-like surface, and then washed in pure acetone and alcohol for 10 min by ways of ultrasonic. The specimens were first conducted solution treatment at 400 C for 6 h and then aging treatment at 200 C for 8 h in a SX-4-10 box resistance furnace. After heat treatment, the samples were moved into an ion implanter (Beijing Normal University, Beijing, China) with a metal vapor vacuum arc (MEVVA) ion source. Prior to ion implantation, the substrates were sputter-cleaned with 6 kV argon ions excited by a capacitive coupling radio frequency glow discharge for 30 min. N and Al ions produced by 99.99% nitrogen gas and 99.99% aluminum target. The background pressure of the metal vapor vacuum arc implanter target chamber was 2 103 Pa. The aluminum and nitrogen ion implantation voltage was 45 kV for 3 h. During duplex ion implantation process, the samples were not cooled, and the implantation temperature depended on the beam current density which was 2.5 mA/cm2. The maximum implantation temperature was measured by Langmuir probe to be below 200 C. For convenience, symbol S0, S1 and S2 represents untreated sample, solid solution-aging sample and solid solution-aging combined with N/Al duplex ion implantation sample, respectively. The surface morphologies of hybrid solution-aging and N/Al duplex ion implantation specimens were observed by scanning electron microscopy (SEM, XL30ESEM-TM). Auger electron spectrometer (PHI-700 ESCA/SAM) was used to measure the composition and depth profiles of elements in the modified surface layer. The scanning voltage of argon ion beam coaxial electron gun is 5 kV. The energy resolution is 1& and electron beam incidence angle is 30 . Thermal oxidation SiO2/Si acted as standard sample, and the sputtering rate was 80 nm/min. The phase composition and microstructure of the modified layer were analyzed using a D/max2000PC type X-ray diffractometer (XRD), monochromatic CuKa radiation (wavelength 1.5406 nm), working voltage 40 kV and working current 30 mA. Surface microhardness of the specimens was characterized by HVS-1000A microhardness tester with a load of 0.1 N (HV0.01) and duration of 10 s. Corrosion behaviors of the substrate and treated samples were evaluated using PS-268A electrochemical measurement system with corresponding corrosion characteristic parameter fitting software. The experiment was carried out in 0.56 M NaCl solution at room temperature. A threeelectrode system was employed with treated samples as the working electrode, saturated calomel electrode (SCE) as the reference electrode and platinum as the counter electrode. Samples used in the electrochemical test were embedded within epoxy resin and an exposed area of 1.0 1.0 cm2 was reserved. The polarization in the anodic direction proceeded from a potential of 1800 mV to 800 mV at a scan rate of 10 mV/s. The electrochemical parameters of corrosion potential (Ecorr), corrosion current density (icorr) and polarization resistance (Rp) were analyzed. Tafel plot was constituted from the logarithm of current density value as a function of corrosion potential. After corrosion
b
c
Table 1 Chemical composition of AZ91D magnesium alloy (wt.%). Al
Mn
Zn
Si
Cu
Ni
Fe
Mg
8.5e9.5
0.17
0.4e0.9
0.05
0.015
0.001
0.004
Bal.
Fig. 1. XRD patterns of the sample S0 (a); sample S1 (b) and sample S2 (c).
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235
tests, the sample surfaces were observed using scanning electron microscope (SEM). 3. Results and discussion
Fig. 3. Microhardness for treated and untreated AZ91D samples.
-800
Sample S0 Sample S1 Sample S2
-900 -1000
Potential / mV vs.SCE
Fig. 1 shows the XRD patterns of treated and untreated samples. It can be determined that S0 and S1 are mainly composed of ɑ (ɑMg) and b (b-Mg17Al12) phases, but duplex ion implantation sample S2 produces a small amount of AlN phase. Compared with the untreated sample S0, the diffraction peak position of ɑ-Mg (100), (101), (102) crystal plane at 32.339 , 36.740 , 48.118 in sample S1, both peak position and intensity of 35.940 (002), 62.379 (103), 69.298 (201) are all changed. The peak intensity of b phase at 39.879 (332), 35.939 (411), the intensity and position at 57.799 (622), 70.315 (651) are also shifted to a higher and more broad peak direction. The main reason is that solution-aging treatment changed the solubility and precipitation morphology of b phase, so the peaks intensity and position eventually changed. Besides ɑ and b phases, duplex ion implantation sample produced AlN phase, the intensity and position of S2 also changed at the same crystalline face and diffraction angle. In addition, the Mg peak intensity and position of sample S2 at 77.637 (202) also be shifted in contrast with S0 and S1. Fig. 2 displays the AES depth profiles of Mg, N, Al, O and Zn acquired from the hybrid solid solution-aging and N/Al duplex ion implantation AZ91D magnesium alloy sample (S2). The near surface region has been heavily contaminated by oxygen, but the oxygen atomic concentration decreases from 38% to 6% with the depth increases from surface to about 120 nm. This suggests oxide layer existence in near surface 120 nm region. Owing to oxygen cannot be completely removed from the surface oxide in Ar ion sputtering pretreatment, and under our non-UHV (ultra high vacuum) conditions (about 2 103 Pa) in the MEVVA ion source chamber, surface oxidation is still inevitable. Mg atom concentration maintains about 20% before 120 nm, at depth over 120 nm, the Mg concentration increases gradually, indicating it began to appear transition layer. When the depth is about 280 nm (corresponding to sputtering 3.5 min), it tends toward stability and approach to substrate. So it can conclude that there are about 160 nm transition layer. Al atom keeps 30% before 160 nm from the surface, and then begins to decrease gradually. According to the position of concentration distribution and XRD pattern, we can infer that the main phases are MgAl2O4 and b- Mg17Al12. N atom shows a Gaussian distribution curve, the peak concentration is approximately 42% at a depth of 160 nm. The implanted layer depth is about 280 nm.
-1100 -1200 -1300 -1400 -1500 -1600 -1700 -1800 -1900
-5
-4
-3
-2
-1
0
1
2
3
2
Current density, lgi / (mA cm ) Fig. 4. Potentiodynamic polarization curves of untreated AZ91D substrate and treated samples in 0.56 M NaCl solution.
Because the solubility of N in magnesium alloy is very low, therefore N mainly exist not only solid solution but also bind Mg, Al and become Mg3N2 and AlN compounds in magnesium alloy substrate. Mg3N2 phase cannot be seen and only a small amount of AlN phase can be found from the above- mentioned XRD patterns. The main reason is that surface modified layer thickness is too thin to find Mg3N2 phase, but from the concentration distribution of Mg3N2 and AlN, we can conclude that Mg3N2 phase should include in the modified layer. Fig. 3 depicts the microhardness of treated and untreated samples in different surface position. The average microhardness of magnesium alloy substrate is about 714.5 MPa, solution-aging sample 834.1 MPa and solid solution-aging combined with N/Al duplex ion implantation sample 975.9 MPa. There is an obvious enhancement for hybrid solid solution-aging and N/Al duplex ion Table 2 Corrosion parameters of the untreated and treated AZ91D samples based on the polarization curves.
Fig. 2. AES spectra for hybrid solution-aging and N/Al duplex ion implantation sample.
Specimens
Ecorr (mV vs. SCE)
icorr (mA/cm2)
Rp (U cm2)
S0 S1 S2
1512 1450 1400
3.13 67.54 338.99
12.623 0.126 0.058
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implantation sample. Solution-aging treatment changed the solubility of Al, Zn in Mg phase, and b phase separated from the substrate, it caused the increase of microhardness. For hybrid solution-aging and N/Al duplex ion implantation sample, the hard
phase of Mg3N2 and AlN is the main reason to enhance the microhardness of the surface modified layer. Fig. 4 exhibits the polarization curve of untreated and treated AZ91D magnesium alloy samples in 0.56 M NaCl solution. The corresponding parameters fitting results of corrosion potential (Ecorr), corrosion current density (icorr) and polarization resistance (Rp) are given in Table 2. It can be seen that the polarization curve of the treated sample obviously shifted to positive potential in comparison with that of the untreated sample. The data in Table 2 show that the corrosion potential Ecorr shifted from 1512 mV (untreated sample S0) to 1450 mV (solution-aging sample S1) and 1400 mV (hybrid solid solution-aging and N/Al duplex ion implantation sample S2), with an increase of 62 mV and 112 mV, respectively. Corrosion current density icorr strongly decreases from 12.623 mA/cm2 to 0.126 mA/cm2 and 0.058 mA/cm2, with a decrease of over two orders of magnitude. The unit area polarization resistance Rp increase from 3.13 U to 67.54 U and 338.99 U, with an increase of 20.6 and 107.3 times in compared with the untreated sample. Therefore, hybrid solid solution-aging and N/Al duplex ion implantation can effectively improve the corrosion resistance of AZ91D magnesium alloy. The SEM surface morphology of the samples after corrosion in 0.56 M NaCl solution at room temperature is shown in Fig. 5. The predominant type of corrosion for treated and untreated samples is pitting corrosion [26]. It can be seen that the corroded surface of substrate magnesium had a great amount of pits with big size and deep depth, and cracks are also observed at some sites as an indication of intergranular corrosion. Beyond that, the structure of this surface has shown the characteristics of loose and porous, but the surface of hybrid solid solution-aging and N/Al duplex ion implantation sample still maintain uniform and a small amount of porous, there were only slightly less pits formed. We attributed the enhanced corrosion resistance of the long time treated samples against NaCl solution to increased MgeO and AleO bonding states, higher thickness of the oxide layer, as well as the more homogenous surface properties achieved by the N/Al duplex ion implantation process. Further, the more homogenous surface obtained by ion bombardment reduces the risk of pitting corrosion, the more it has uniform energy states. Heat accumulation during ion implantation process can lead to deeper diffusion of N and Al ions, higher film thickness and more complete oxidation of Mg and Al [27]. In addition, hybrid solid solutionaging and N/Al duplex ion implantation processing can change the surface microstructure and produce a transition layer of MgAl2O4 with a small amount of Mg3N2 and AlN. All these compounds can effectively improve the corrosion resistance of AZ91D magnesium alloy. 4. Conclusions
Fig. 5. Corrosion morphologies of the untreated AZ91D substrate S0 (a), treated sample S1 (b) and S2 (c).
The hybrid solid solution-aging treatment and N/Al duplex ion implantation process was carried out on AZ91D magnesium alloy substrate surface. The surface modified layer was mainly composed of Mg, MgO, Mg17Al12, MgAl2O4 and AlN phases. The surface oxide layer depth was about 120 nm and ion implantation modified layer depth 280 nm. Comparing with AZ91D magnesium alloy substrate, the microhardness of hybrid solid solution-aging and N/Al duplex ion implantation coupons increased by 16.7% and 36.6%, the polarization resistance increased by 20.6 and 107.3 times, and the corrosion current intensity decreased by two orders of magnitude, respectively. Hybrid solution-aging and duplex ion implantation sample produced less corrosion pit, but untreated and solutionaging coupons had a large amount of corrosion pit. The corrosion resistance of hybrid treatment sample has been significantly improved.
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Acknowledgments This work was supported by the National Nature Science Foundation of China (Grant No. 51165015), the Nature Science Foundation of Yunnan Province (Grant No. 2008ZC021M) and the Analysis and Testing Foundation of Kunming University of Science and Technology (2011008). The authors would also express their sincere appreciation to the SEM support from the Analysis & Testing Research Center of Yunnan Province, Kunming University of Science and Technology. References [1] Shadanbaz S, Dias GJ. Calcium phosphate coatings on magnesium alloys for biomedical applications: a review. Acta Biomater 2012;8:20e30. [2] Mu SJ, Samman TA, Mohles V, Gottstein G. Cluster type grain interaction model including twinning for texture prediction: application to magnesium alloys. Acta Mater 2011;59:6938e48. [3] Wang L, Zhang BP, Shinohara T. Corrosion behavior of AZ91 magnesium alloy in dilute NaCl solutions. Mater Design 2010;31:857e63. [4] Chen J, Wang JQ, Han EH, Dong JH, Ke W. States and transport of hydrogen in the corrosion process of an AZ91 magnesium alloy in aqueous solution. Corros Sci 2008;50:1292e305. [5] Li YB, Chen YB, Cui H, Xiong BQ, Zhang JS. Microstructure and mechanical properties of spray-formed AZ91 magnesium alloy. Mater Charact 2009;60:240e5. ski J, Rudnicki J, Borowski T, Trzaska M, Wierzchon T. The [6] Tacikowski M, Kamin effect of the diffusive, composite chromium nitride layers produced by a hybrid surface treatment on the corrosion behavior of AZ91D magnesium alloy. Vacuum 2011;85:938e42. [7] Song YL, Liu YH, Yu SR, Zhu XY, Wang Q. Plasma electrolytic oxidation coating on AZ91 magnesium alloy modified by neodymium and its corrosion resistance. Appl Surf Sci 2008;254:3014e20. [8] Majumdar JD, Manna I. Mechanical properties of a laser-surface-alloyed magnesium-based alloy (AZ91) with nickel. Scripta Mater 2010;62:579e81. [9] Majumdar JD, Bhattacharyya U, Manna I. Studies on thermal oxidation of Mgalloy (AZ91) for improving corrosion and wear resistance. Surf Coat Technol 2008;202:3638e41. [10] Tang JW, Azumi K. Influence of zincate pretreatment on adhesion strength of a copper electroplating layer on AZ91 D magnesium alloy. Surf Coat Technol 2011;205:3050e7. [11] Shi YJ, Pan FS, Long SY, Wang WQ, Zhu GJ. Structure and tribological property of MoSx-CrTiAlN film by unbalanced magnetron sputtering. Vacuum 2011;86: 171e7.
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