- Email: [email protected]
Surface nanocrystallization and its properties of a rare earth magnesium alloy induced by HVOF–SMB
Current Applied Physics 11 (2011) 677e681
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Surface nanocrystallization and its properties of a rare earth magnesium alloy induced by HVOFeSMB Kai-dong Xu a, Ji-na Wang a, Ai-hua Wang b, *, Hua Yan b, Xiang-lin Zhang b, Zao-wen Huang b a
Department of Civil and Materials Engineering, Henan University of Urban Construction, Pingdingshan 467044, China State Key Laboratory of Material Processing and Die & Mould Technology, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2010 Received in revised form 19 October 2010 Accepted 9 November 2010 Available online 18 November 2010
The nanocrystalline microstructure in the surface of a rare earth magnesium alloy induced by a new process named HVOFeSMB (high velocity oxygen-fuel flame supersonic microparticles bombarding) has been characterized by means of X-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM). The effects of HVOFeSMB on mechanical properties and corrosion resistance of the magnesium alloy have been investigated by microhardness distribution and polarization curves, and the cross-sectional observation of the treated Mg alloy sample has been characterized by optical microscopy (OM). Results showed that HVOFeSMB can induce surface nanocrystallization with a grain size level less than 20 nm at topmost surface layer, and the microhardness at the top surface layer increases up to about 170 HV0.025 which is twice more than that of the substrate. The grain size increases as well as the microhardness decreases gradually with the depth varying from surface to substrate. The potentiodynamic polarization curves indicate that the HVOFeSMB treated surfaces show lower corrosion resistance in comparison with the untreated magnesium alloy in 3.5% NaCl solution. The underlying mechanism of the electrochemical properties on the nanocrystalline Mg-based alloy’s surface induced by severe plastic deformation in corrosion resistance is discussed. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
Keywords: Mg alloy Surface nanocrystallization Microstructure Microhardness Corrosion behavior
1. Introduction The low density and high specific stiffness of magnesium-based alloys make them potentially attractive for the transportation and electronic industries [1,2]. However, the poor wear resistance and poor corrosion performance extensively limits their practical applications. A number of techniques have been developed to modify the service behavior of Mg alloys through surface treatment. For example, electroplating [3] and anodizing treatment [4,5], but these methods have the disadvantage of involving multilayer coatings; laser alloying or clading [6e8] and ion beam treatment [9,10] are expensive. The surface modified methods above-mentioned aim at modifying the surface of magnesium alloys without affecting their bulk performance. However, all these techniques have certain inevitable disadvantages. Nanocrystalline materials with superior mechanical, physical and chemical properties have aroused great concern in recent years.
* Corresponding author. Tel./fax: þ8627 87540049. E-mail address: [email protected] (A.-h. Wang).
Currently, surface nanostructured materials, without changing the chemical composition of the material during grain refinement down to the nanometer scale with no porosity and contamination, induced by severe plastic deformation become a research hotspot owing to their excellent performance. Previous experiment results have shown that surface self-nanocrystallization of the metal or alloys can be achieved through many different methods such as surface mechanical attrition treatment (SMAT) [11e15], ultrasonic shot peening (USP) [16], sandblasting [17] and high pressure torsion (HPT) [18], etc. The nature of grain refinement down to nanometer scale induced by these techniques is severe plastic deformation in the surface layer. The microstructure and microhardness showed a gradient variation from surface to the substrate, and microhardness of the top surface layer reaches about twice that of the coarse-grained substrate. Nevertheless, the great discrepancy of corrosion resistance is subsistent for different materials after surface nanocrystallization. For instance, the nano-treated AISI-321 and 1Cr18Ni9Ti stainless steel demonstrate excellent corrosion resistance [19,20]; but the corrosion resistance of nanostructured 304 stainless steel [21] generated some deterioration. A large number of research works on the corrosion resistance of surface nanocrystallization has been carried out [19e22]. However, there
1567-1739/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.031
678
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681
are few reports in the open literature about the electrochemical properties of the nanocrystalline surface layer on Mg-based alloys induced by severe plastic deformation. The objective of this work is to investigate the microstructure, the mechanical and especially the electrochemical properties of surface nanocrystallization of Mg-based alloys produced by HVOFeSMB (high velocity oxygen-fuel flame supersonic microparticles bombarding) process [23]. The HVOFeSMB process has some unique advantages compared with the coating and deposition methods for surface self-nanocrystallization. For instance, there is no change in chemical compositions between the nanostructured surface layer and the substrate, as well as a gradual variation in grain size from nanoscale to coarse-grain, and no obvious interface between the surface layer and the substrate. Furthermore, the nanostructured surface layer is free from porosities and impurities. In particular, the underlying mechanism of the electrochemical properties on the nanocrystalline Mg-based alloy’s surface in corrosion resistance was investigated. 2. Experimental procedure The material used in this study was a Mge15Gde3Y (Mge15.26 Gde2.11 Y, wt.%) rare earth magnesium alloy bar with 45 mm in diameter after extrusion, and the extrusion ratio is 10.6:1. The parallelepipedic specimens with dimensions of 15 mm 10 mm 10 mm were prepared by wire cutting from the extruded bar, and then one of the flat faces of the specimen was polished down to 1200 grit finish. After thorough washing in running water and 10 min ultrasonication in acetone, these specimens were treated through HVOFeSMB process. It should be noted that the treated surface is perpendicular to the extrusion direction, and the original grain size of the extruded bar is on average 20e50 mm. The details of HVOFeSMB set-up and process principles have been described in our previous work [23]. The main parameters of the HVOFeSMB process in the present work were chosen as follows: the pressure of gases was 1.5 MPa, and the O2eN2 ratio was 5:7; the bombarding distance was about 300 mm; the diameter of microparticles was about 0.5 mm; and the processing duration was 180 s. The potentiodynamic polarization measurements were carried out using a Zahner IM6ex set-up (Germany), Potentiodynamic polarization behavior of surfaces of as-received, HVOFeSMB treated and HVOFeSMB treatment þ annealed specimens was studied in a 3.5% NaCl solution at room temperature, and the potentiodynamic polarization curves were measured from 4 to 2 V with a scan rate of 1 mV/s. A saturated calomel electrode (SCE) and a platinum plate were used as a reference electrode and as the counter one, respectively. Insulated copper wire was soldered to the back of the specimen for electrical connection, specimens were mounted in epoxy resin, exposing 1 cm2 of the surface area, and then the surface was polished in order to get as smooth as possible. The epoxy-mounted specimens were washed thoroughly with lots of distilled water and sonicated in isopropyl alcohol for 10 min before introducing into the conventional three-electrode cell for electrochemical polarization investigations. An OPTEC MDS type optical microscope (OM) was used to examine the microstructural change along sections perpendicular to the treated surface. X-ray diffraction (XRD) analysis of the surface layer in the HVOFeSMB treated Mg alloy specimen was carried out on a Bruker AXS (Germany) D8 e Advance X-ray diffractometer with operation voltage at 35 kV using Cu Ka radiation. The step-scan mode with a step of 0.02 2q and a sampling time of 0.4 s/step in the range 15 2q 90 was used. Variation of microhardness along the sections perpendicular to the treated surface of the sample was determined by a Micromet-3 Vickers hardness tester, with a load of
25 g and a loading time of 15 s. FEI Tecnai G2 20 transmission electron microscope (TEM) and JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM) operating at 200 kV was used to investigate the microstructures at the different depths. The TEM and HRTEM foils of layers at different depths were obtained firstly by polishing the corresponding surface layer, then mechanically grinding the specimen on the untreated side until it was about 30 mm thick, finally the thin foils were ion milled in a Gatan PIPS-691 with a small incident angle till perforation. 3. Results and discussion 3.1. Microstructures and XRD characterization of the surface layer The XRD patterns of Mg alloy samples before and after HVOFeSMB treatment are shown in Fig. 1. It is found that there is an obvious broadening of the Bragg reflections and a slight shift in the centroid position of diffraction peaks relative to the coarse-grained sample after HVOFeSMB treatment. This might be attributed to grain refinement, microstrain and crystalline lattice distortion. This phenomenon is common in many nanocrystalline materials which refinement induced by severe plastic deformation [19,20,24]. Moreover, the new phases (such as GdMg3, Mg2Y, etc.) and diffraction peaks generated. It demonstrated that precipitation transformation took place in the surface layer during HVOFeSMB treatment. Quantitative XRD measurement shows that the average grain size is about 22 nm in the top surface layer of the treated sample, and the microstrain is neglectable. The average grain size determined from the XRD calculation is a little larger than that from TEM observation. It might be due to the fact that XRD results averaged the structure information of a surface layer about 10 mm thick, while the TEM sample was a very thin film (less than 1 mm thick) at the top surface layer. These indicate that HVOFeSMB treatment can produce surface nanocrystallization of Mg alloy and form new phases in the surface layer. The cross-sectional OM of the HVOFeSMB treated Mg alloy sample is shown in Fig. 2. It can be seen that the microstructure morphology of the deformed layer differs obviously from that in the substrate with equiaxed grains of 30e50 mm in diameter, and the thickness of distinct deformed layer is about 250 mm, an ultrafinegrained submicroscopic structure which is beyond the resolution of optical microscope in the top layer (about 70 mm in thick) of the treated surface. Large number of deformation twins are found in
Fig. 1. Comparison of the XRD patterns before and after HVOFeSMB treatment.
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681
Fig. 2. Cross-sectional observation of the HVOFeSMB treated Mg alloy sample.
the depth about 80e220 mm from the most top surface. In brief, the microstructures change gradually from the substrate to the top surface. The TEM dark-field image and corresponding selected area electron diffraction (SAED) pattern, and HRTEM examination of the microstructure in the most top surface layer of HVOFeSMB treated sample are shown in Fig. 3 (a) and (b), respectively. It can be seen that the crystalline grains in the near surface layer are equiaxed, and the grain size revealed by TEM dark-field image together with the HRTEM image observation ranges from 10 nm to 20 nm, which is a little smaller than that from the XRD calculation, and the nanosized crystallites with different misorientations could be clearly identified. The corresponding SAED pattern shows continuous, homogeneous and broadened concentric diffraction rings, illustrating that the grain size is very tiny and the grains are random in crystallographic orientation. The microstructural features in detail of nanostructured Mg alloy samples prepared by HVOFeSMB have already been characterized in our previous work [23].
679
about 170 HV0.025 with the thickness about 60 mm in the top surface nanostructured layer, and finally tends to a constant value of about 85 HV0.025 below a depth of 450 mm. It can be seen that the microhardness of the topmost surface layer is about twice in comparison with that of the substrate, which have a good consistency on the improvement degree of microhardness in contrast with the nanostructural AZ91 and AZ31 Mg alloy treated by SMAT and USP [12,24], respectively. Generally speaking, the distribution of microhardness value gradually decreases with the depth increasing. These results indicate that the microhardness near the treated surface has been significantly increased by the HVOFeSMB treatment, and the increase of microhardness from the substrate to the surface may be caused by the grain refinement. It can be interesting noted that the deformed layer is a gradient structure, which identified by the Figs. 2 and 4, resulting from a gradual decrease in the applied strain and strain rate as the depth of the treated surface layer increasing. Grain refinement even down to nanoscale in the surface layer of Mg alloy sample induced through severe plastic deformation, which caused by a large number of high-speed microparticles repeated multidirectional bombing onto the sample surface during a short period of time. Therefore, high residual stress must be existed inside deformed microstructure of the surface layer. Annealing experiment of the HVOFeSMB treated samples was carried out at 373 K and 673 K, respectively, in order to investigative the effect of residual stress on incremental microhardness. There is no obvious change in microhardness curve (Fig. 4) after the sample was annealed at 373 K for 2 h for residual stress relaxation comparison with the as-treated sample. Meanwhile, no significant change in grain size after low-temperature annealing (as shown in Fig. 5b), which showing that the incremental microhardness in the surface layer is not resulted from the residual stress induced by the HVOFeSMB treatment, and which confirmed by previous studies [25]. The microhardness in the surface layer drops close to that of the coarse-grained substrate after annealing at 673 K for 2 h for recrystallization of the nanostructures and forming coarse grains (as shown in Fig. 5c). The results indicate that the increment of microhardness is due to grain refinement down to the nanometer scale rather than the effect of high residual stress induced by severe plastic deformation.
3.2. Properties of the nanostructured surface layer 3.2.1. Microhardness measurements Fig. 4 shows the variation of microhardness from the treated surface to the substrate. The microhardness reaches a maximum
3.2.2. Effects of nanocrystallization on the corrosive behavior Fig. 6 shows potentiodynamic polarization curves obtained in 3.5% NaCl solution for as-received sample, HVOFeSMB treated sample and HVOFeSMB treatment þ annealed at 373 K for 2 h. The
Fig. 3. Dark-field image of the topmost surface layer with equiaxed nanograins and inset is the corresponding SAED pattern (a), and HRTEM image of the equiaxed nanograins (b).
680
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681
Fig. 4. Variation of microhardness of Mg alloy sample after HVOFeSMB treatment from the topmost surface to the substrate.
Fig. 6. Potentiodynamic polarization curves of different samples in a 3.5% NaCl solution.
corrosion potentials (Ecorr) and the corrosion currents (Icorr) of various samples calculated from Fig. 6 by Tafel analysis are listed in Table 1. The results listed in Table 1 allow comparison of the corrosion performance of samples with different treatment conditions. Significant difference in polarization behavior among the samples was observed. The Ecorr is a criterion for evaluating corrosion behavior. The materials with more negative potentials will generally tend to undergo more significant corrosion, while the others (with more positive potential values) will generally suffer less attack. The HVOFeSMB treatment þ annealed sample demonstrates much better behavior in this sense in comparison with as-annealed and HVOFeSMB treated samples. The potentiodynamic polarization curves revealed that the as-received, HVOFeSMB treated and HVOFeSMB treatment þ annealed samples could passivate under a certain level of potential. Although the free corrosion potential (Ecorr) of the treated and as-received samples were similar, the minimum passive current density of the HVOFeSMB treated sample was slightly less than that of the asreceived one. Meanwhile, the critical current density for passivation decreased under HVOFeSMB treated condition as compared to that of as-received condition. However, the surface layer of HVOFeSMB treated Mg alloy sample totally showed the poor polarization behavior in spite of it was harder than that of the asreceived sample. It is worth noting that the corrosion current
density (Icorr) of the HVOFeSMB treated sample was two magnitudes larger than that of the as-received one. The lower this value, the better the corrosion protection property, since the corrosion current is directly related to the corrosion rate. Additionally, the polarization behavior changed significantly for the Mg alloy sample with surface modification by HVOFeSMB treatment, not only markedly increasing the cathodic current density, but also having the tendency of shifting anodic current density and passivationmaintaining current density to lower value, yet slightly elevating the free corrosion potential. It should be pointed out that the influencing factors of electrochemical behavior are quite complex. A decrease in passive current density could be observed after HVOFeSMB treatment. On the one hand, as the surface was severely plastic deformed during HVOFeSMB treatment, the passive film formed on the surface also could be considered as highly disordered because of nanoscale dislocation structures. These defect structures induce a high number of localized electron levels close to the band edges and alter the forbidden energy gap [26]. On the other hand, resulting from the formation of the rare earth elements-enriched (e.g. Gd, Y) passive film, the results of XRD patterns before and after HVOFeSMB treatment are shown in Fig. 1, which protects the Mg alloy from corrosion attack [21,27]. When a surface layer is nanocrystalline induced by severe plastic deformation, the high density
Fig. 5. Cross-sectional microstructure of the HVOFeSMB treated samples with different annealing temperature (a) HVOFeSMB treatment; (b) annealing at 373 K for 2 h after HVOFeSMB treatment; (c) annealing at 673 K for 2 h after HVOFeSMB treatment.
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681 Table 1 Corrosion parameters of the samples studied in a 3.5% NaCl solution. Processing status
Ecorr (V)
Icorr (mA/cm2)
As-received sample HVOFeSMB treatment HVOFeSMB þ annealing at 373 K for 2 h
1.327 1.314 0.948
6.03 437 41.9
grain boundaries could easily promote the diffusion of Gd and Y to surface, owing to the nanocrystalline surface layer having higher distortion energy, thus forming a passive film containing richer Gd and Y that may strengthen the passive film. Those might be the reason why the decrease in passive current density on HVOFeSMB treated specimen. Moreover, the cross diffusion of metal vacancies and higher dislocation densities at the nanocrystalline surface layer could act as short circuit diffusion paths for passivation-promoting alloying elements to reach the surface and form a coherent passive film at the passive range of potentials [21]. For polycrystalline materials, especially the grain size was refined down to nanometer scale, several factors can influence significantly on the corrosion resistance of a sample, such as defect structures, grain boundaries, grain size, homogeneity of microstructure and stress state, etc. It is well known that the stress state of surface layer induced by severe plastic deformation through repeated multidirectional high velocity microparticles bombarding could be beneficial to improve the fatigue resistance, but it can induce/facilitate the stress corrosion and thereby accelerate corrosion rate. The potentiodynamic polarization curves (as shown in Fig. 6) with different treatment conditions revealed that HVOFeSMB treatment þ annealed sample (the stress was released through low-temperature annealing) has the most electrochemical behavior in general compared with asreceived and HVOFeSMB treated samples. 4. Conclusions Microstructure, mechanical and electrochemical properties of a rare earth Mg alloy nanocrystalline surface layer prepdprepared by severe plastic deformation via HVOFeSMB treatment were investigated, in comparison with as-received and HVOFeSMB treatment þ annealed samples. The XRD analysis, OM and HRTEM investigations of the microstructural evolution show that rare earth elements (e.g. Gd, Y) enriched in the top surface layer, and the microstructure changes gradually from the substrate to the surface. The grain refinement is prominent and the grain size increases gradually from about 10 nm at the topmost surface to about of 100e500 nm at a depth of 80 mm from the treated surface. The microhardness was enhanced about twice comparison with the
681
substrates and the distribution of microhardness value gradually decreases with the depth increasing. The microhardness behavior is shown to be mainly in correlation with the grain refinement down to nanoscale. The nanocrystallization prepdprepared through HVOFeSMB treatment þ low-temperature annealing considerably improved the polarization behavior of the material. In particular, the surface nanocrystallization of Mg alloy induced by severe plastic deformation although changed the polarization behavior significantly as compared to that of as-received condition, the corrosion resistance was deteriorated to a certain degree. Acknowledgments The authors are grateful for financial support from the Foundation of Huazhong University of Science and Technology. References [1] D.J. Li, X.Q. Zeng, J. Dong, C.Q. Zhai, W.J. Ding, J. Alloys Compd. 468 (2009) 164e169. [2] L.Q. Zhu, G.L. Song, Surf. Coat. Technol. 200 (2006) 2834e2840. [3] U. Erb, A.M. El-Sherik, G. Palumbo, K.T. Aust, Nanostruct. Mater. 2 (1993) 383e390. [4] A. Wolowik, M. Janik-Czachor, Mater. Sci. Eng. A 267 (1999) 301. [5] F.A. Bonilla, A. Berkani, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, C. John, K. Stevens, Corros. Sci. 44 (2002) 1941. [6] Manoj Masanta, P. Ganesh, Rakesh Kaul, A.K. Nath, A. Roy Choudhury, Mater. Sci. Eng. A 508 (2009) 134e140. [7] I. Manna, J. Dutta Majumdar, B. Ramesh Chandra, S. Nayak, Narendra B. Dahotre, Surf. Coat. Technol. 201 (2006) 434e440. [8] A.H. Wang, T.M. Yue, Compos. Sci. Technol. 61 (2001) 1549e1554. [9] Richard L.C. Wu, William Lanter, John Wrbanek, Charles DeJoseph, Surf. Coat. Technol. 140 (2001) 35e43. [10] T. Asmus, Gerhard K. Wolf, Nucl. Instrum. Meth. B 166e167 (2000) 732e736. [11] N.R. Tao, Z.B. Wang, W.P. Tong, et al., Acta Mater. 50 (2002) 4603e4616. [12] H.Q. Sun, Y.N. Shi, M.X. Zhang, et al., Acta Mater. 55 (2007) 975. [13] K.Y. Zhu, A. Vassel, F. Brisset, K. Lu, J. Lu, Acta Mater. 52 (2004) 4101e4110. [14] K. Wang, N.R. Tao, G. Liu, J. Lu, K. Lu, Acta Mater. 54 (2006) 5281e5291. [15] L. Huang, J. Lu, M. Troyon, Surf. Coat. Technol. 201 (2006) 208e213. [16] G. Liu, J. Lu, K. Lu, Mater. Sci. Eng. A 286 (2000) 91. [17] X.P. Jiang, X.Y. Wang, J.X. Li, D.Y. Li, C.S. Manc, M.J. Shepard, T. Zhai, Mater. Sci. Eng. A 429 (2006) 30. [18] K. Masaaki, Z.J. Horita, G. Terence, Mater. Sci. Eng. A 488 (2008) 117e124. [19] B.N. Mordyuk, G.I. Prokopenko, M.A. Vasylyev, M.O. Iefimov, Mater. Sci. Eng. A 458 (2007) 253e261. [20] T.S. Wang, J.K. Yu, B.F. Dong, Surf. Coat. Technol. 200 (2006) 4777e4781. [21] X.Y. Wang, D.Y. Li, Electrochim. Acta 47 (2002) 3939e3947. [22] K.S. Raja, S.A. Namjoshi, M. Misra, Mater. Lett. 59 (2005) 570e574. [23] K.D. Xu, A.H. Wang, Y. Wang, X.P. Dong, X.L. Zhang, Z.W. Huang, Appl. Surf. Sci. 256 (2009) 619e626. [24] Y.H. Wei, B.S. Liu, L.F. Hou, B.S. Xu, G. Liu, J. Alloys Compd. 452 (2008) 336e342. [25] K. Lu, J. Lu, Mater. Sci. Eng. A 375e377 (2004) 38. [26] P. Schmuki, H. Bohni, J. Electrochem. Soc. 139 (1992) 1908e1913. [27] J.B. Lumsden, Z. Szklarska-Smialowska, Corrosion 34 (1978) 169.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Surface nanocrystallization and its properties of a rare earth magnesium alloy induced by HVOFeSMB Kai-dong Xu a, Ji-na Wang a, Ai-hua Wang b, *, Hua Yan b, Xiang-lin Zhang b, Zao-wen Huang b a
Department of Civil and Materials Engineering, Henan University of Urban Construction, Pingdingshan 467044, China State Key Laboratory of Material Processing and Die & Mould Technology, Department of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2010 Received in revised form 19 October 2010 Accepted 9 November 2010 Available online 18 November 2010
The nanocrystalline microstructure in the surface of a rare earth magnesium alloy induced by a new process named HVOFeSMB (high velocity oxygen-fuel flame supersonic microparticles bombarding) has been characterized by means of X-ray diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM). The effects of HVOFeSMB on mechanical properties and corrosion resistance of the magnesium alloy have been investigated by microhardness distribution and polarization curves, and the cross-sectional observation of the treated Mg alloy sample has been characterized by optical microscopy (OM). Results showed that HVOFeSMB can induce surface nanocrystallization with a grain size level less than 20 nm at topmost surface layer, and the microhardness at the top surface layer increases up to about 170 HV0.025 which is twice more than that of the substrate. The grain size increases as well as the microhardness decreases gradually with the depth varying from surface to substrate. The potentiodynamic polarization curves indicate that the HVOFeSMB treated surfaces show lower corrosion resistance in comparison with the untreated magnesium alloy in 3.5% NaCl solution. The underlying mechanism of the electrochemical properties on the nanocrystalline Mg-based alloy’s surface induced by severe plastic deformation in corrosion resistance is discussed. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
Keywords: Mg alloy Surface nanocrystallization Microstructure Microhardness Corrosion behavior
1. Introduction The low density and high specific stiffness of magnesium-based alloys make them potentially attractive for the transportation and electronic industries [1,2]. However, the poor wear resistance and poor corrosion performance extensively limits their practical applications. A number of techniques have been developed to modify the service behavior of Mg alloys through surface treatment. For example, electroplating [3] and anodizing treatment [4,5], but these methods have the disadvantage of involving multilayer coatings; laser alloying or clading [6e8] and ion beam treatment [9,10] are expensive. The surface modified methods above-mentioned aim at modifying the surface of magnesium alloys without affecting their bulk performance. However, all these techniques have certain inevitable disadvantages. Nanocrystalline materials with superior mechanical, physical and chemical properties have aroused great concern in recent years.
* Corresponding author. Tel./fax: þ8627 87540049. E-mail address: [email protected] (A.-h. Wang).
Currently, surface nanostructured materials, without changing the chemical composition of the material during grain refinement down to the nanometer scale with no porosity and contamination, induced by severe plastic deformation become a research hotspot owing to their excellent performance. Previous experiment results have shown that surface self-nanocrystallization of the metal or alloys can be achieved through many different methods such as surface mechanical attrition treatment (SMAT) [11e15], ultrasonic shot peening (USP) [16], sandblasting [17] and high pressure torsion (HPT) [18], etc. The nature of grain refinement down to nanometer scale induced by these techniques is severe plastic deformation in the surface layer. The microstructure and microhardness showed a gradient variation from surface to the substrate, and microhardness of the top surface layer reaches about twice that of the coarse-grained substrate. Nevertheless, the great discrepancy of corrosion resistance is subsistent for different materials after surface nanocrystallization. For instance, the nano-treated AISI-321 and 1Cr18Ni9Ti stainless steel demonstrate excellent corrosion resistance [19,20]; but the corrosion resistance of nanostructured 304 stainless steel [21] generated some deterioration. A large number of research works on the corrosion resistance of surface nanocrystallization has been carried out [19e22]. However, there
1567-1739/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.11.031
678
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681
are few reports in the open literature about the electrochemical properties of the nanocrystalline surface layer on Mg-based alloys induced by severe plastic deformation. The objective of this work is to investigate the microstructure, the mechanical and especially the electrochemical properties of surface nanocrystallization of Mg-based alloys produced by HVOFeSMB (high velocity oxygen-fuel flame supersonic microparticles bombarding) process [23]. The HVOFeSMB process has some unique advantages compared with the coating and deposition methods for surface self-nanocrystallization. For instance, there is no change in chemical compositions between the nanostructured surface layer and the substrate, as well as a gradual variation in grain size from nanoscale to coarse-grain, and no obvious interface between the surface layer and the substrate. Furthermore, the nanostructured surface layer is free from porosities and impurities. In particular, the underlying mechanism of the electrochemical properties on the nanocrystalline Mg-based alloy’s surface in corrosion resistance was investigated. 2. Experimental procedure The material used in this study was a Mge15Gde3Y (Mge15.26 Gde2.11 Y, wt.%) rare earth magnesium alloy bar with 45 mm in diameter after extrusion, and the extrusion ratio is 10.6:1. The parallelepipedic specimens with dimensions of 15 mm 10 mm 10 mm were prepared by wire cutting from the extruded bar, and then one of the flat faces of the specimen was polished down to 1200 grit finish. After thorough washing in running water and 10 min ultrasonication in acetone, these specimens were treated through HVOFeSMB process. It should be noted that the treated surface is perpendicular to the extrusion direction, and the original grain size of the extruded bar is on average 20e50 mm. The details of HVOFeSMB set-up and process principles have been described in our previous work [23]. The main parameters of the HVOFeSMB process in the present work were chosen as follows: the pressure of gases was 1.5 MPa, and the O2eN2 ratio was 5:7; the bombarding distance was about 300 mm; the diameter of microparticles was about 0.5 mm; and the processing duration was 180 s. The potentiodynamic polarization measurements were carried out using a Zahner IM6ex set-up (Germany), Potentiodynamic polarization behavior of surfaces of as-received, HVOFeSMB treated and HVOFeSMB treatment þ annealed specimens was studied in a 3.5% NaCl solution at room temperature, and the potentiodynamic polarization curves were measured from 4 to 2 V with a scan rate of 1 mV/s. A saturated calomel electrode (SCE) and a platinum plate were used as a reference electrode and as the counter one, respectively. Insulated copper wire was soldered to the back of the specimen for electrical connection, specimens were mounted in epoxy resin, exposing 1 cm2 of the surface area, and then the surface was polished in order to get as smooth as possible. The epoxy-mounted specimens were washed thoroughly with lots of distilled water and sonicated in isopropyl alcohol for 10 min before introducing into the conventional three-electrode cell for electrochemical polarization investigations. An OPTEC MDS type optical microscope (OM) was used to examine the microstructural change along sections perpendicular to the treated surface. X-ray diffraction (XRD) analysis of the surface layer in the HVOFeSMB treated Mg alloy specimen was carried out on a Bruker AXS (Germany) D8 e Advance X-ray diffractometer with operation voltage at 35 kV using Cu Ka radiation. The step-scan mode with a step of 0.02 2q and a sampling time of 0.4 s/step in the range 15 2q 90 was used. Variation of microhardness along the sections perpendicular to the treated surface of the sample was determined by a Micromet-3 Vickers hardness tester, with a load of
25 g and a loading time of 15 s. FEI Tecnai G2 20 transmission electron microscope (TEM) and JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM) operating at 200 kV was used to investigate the microstructures at the different depths. The TEM and HRTEM foils of layers at different depths were obtained firstly by polishing the corresponding surface layer, then mechanically grinding the specimen on the untreated side until it was about 30 mm thick, finally the thin foils were ion milled in a Gatan PIPS-691 with a small incident angle till perforation. 3. Results and discussion 3.1. Microstructures and XRD characterization of the surface layer The XRD patterns of Mg alloy samples before and after HVOFeSMB treatment are shown in Fig. 1. It is found that there is an obvious broadening of the Bragg reflections and a slight shift in the centroid position of diffraction peaks relative to the coarse-grained sample after HVOFeSMB treatment. This might be attributed to grain refinement, microstrain and crystalline lattice distortion. This phenomenon is common in many nanocrystalline materials which refinement induced by severe plastic deformation [19,20,24]. Moreover, the new phases (such as GdMg3, Mg2Y, etc.) and diffraction peaks generated. It demonstrated that precipitation transformation took place in the surface layer during HVOFeSMB treatment. Quantitative XRD measurement shows that the average grain size is about 22 nm in the top surface layer of the treated sample, and the microstrain is neglectable. The average grain size determined from the XRD calculation is a little larger than that from TEM observation. It might be due to the fact that XRD results averaged the structure information of a surface layer about 10 mm thick, while the TEM sample was a very thin film (less than 1 mm thick) at the top surface layer. These indicate that HVOFeSMB treatment can produce surface nanocrystallization of Mg alloy and form new phases in the surface layer. The cross-sectional OM of the HVOFeSMB treated Mg alloy sample is shown in Fig. 2. It can be seen that the microstructure morphology of the deformed layer differs obviously from that in the substrate with equiaxed grains of 30e50 mm in diameter, and the thickness of distinct deformed layer is about 250 mm, an ultrafinegrained submicroscopic structure which is beyond the resolution of optical microscope in the top layer (about 70 mm in thick) of the treated surface. Large number of deformation twins are found in
Fig. 1. Comparison of the XRD patterns before and after HVOFeSMB treatment.
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681
Fig. 2. Cross-sectional observation of the HVOFeSMB treated Mg alloy sample.
the depth about 80e220 mm from the most top surface. In brief, the microstructures change gradually from the substrate to the top surface. The TEM dark-field image and corresponding selected area electron diffraction (SAED) pattern, and HRTEM examination of the microstructure in the most top surface layer of HVOFeSMB treated sample are shown in Fig. 3 (a) and (b), respectively. It can be seen that the crystalline grains in the near surface layer are equiaxed, and the grain size revealed by TEM dark-field image together with the HRTEM image observation ranges from 10 nm to 20 nm, which is a little smaller than that from the XRD calculation, and the nanosized crystallites with different misorientations could be clearly identified. The corresponding SAED pattern shows continuous, homogeneous and broadened concentric diffraction rings, illustrating that the grain size is very tiny and the grains are random in crystallographic orientation. The microstructural features in detail of nanostructured Mg alloy samples prepared by HVOFeSMB have already been characterized in our previous work [23].
679
about 170 HV0.025 with the thickness about 60 mm in the top surface nanostructured layer, and finally tends to a constant value of about 85 HV0.025 below a depth of 450 mm. It can be seen that the microhardness of the topmost surface layer is about twice in comparison with that of the substrate, which have a good consistency on the improvement degree of microhardness in contrast with the nanostructural AZ91 and AZ31 Mg alloy treated by SMAT and USP [12,24], respectively. Generally speaking, the distribution of microhardness value gradually decreases with the depth increasing. These results indicate that the microhardness near the treated surface has been significantly increased by the HVOFeSMB treatment, and the increase of microhardness from the substrate to the surface may be caused by the grain refinement. It can be interesting noted that the deformed layer is a gradient structure, which identified by the Figs. 2 and 4, resulting from a gradual decrease in the applied strain and strain rate as the depth of the treated surface layer increasing. Grain refinement even down to nanoscale in the surface layer of Mg alloy sample induced through severe plastic deformation, which caused by a large number of high-speed microparticles repeated multidirectional bombing onto the sample surface during a short period of time. Therefore, high residual stress must be existed inside deformed microstructure of the surface layer. Annealing experiment of the HVOFeSMB treated samples was carried out at 373 K and 673 K, respectively, in order to investigative the effect of residual stress on incremental microhardness. There is no obvious change in microhardness curve (Fig. 4) after the sample was annealed at 373 K for 2 h for residual stress relaxation comparison with the as-treated sample. Meanwhile, no significant change in grain size after low-temperature annealing (as shown in Fig. 5b), which showing that the incremental microhardness in the surface layer is not resulted from the residual stress induced by the HVOFeSMB treatment, and which confirmed by previous studies [25]. The microhardness in the surface layer drops close to that of the coarse-grained substrate after annealing at 673 K for 2 h for recrystallization of the nanostructures and forming coarse grains (as shown in Fig. 5c). The results indicate that the increment of microhardness is due to grain refinement down to the nanometer scale rather than the effect of high residual stress induced by severe plastic deformation.
3.2. Properties of the nanostructured surface layer 3.2.1. Microhardness measurements Fig. 4 shows the variation of microhardness from the treated surface to the substrate. The microhardness reaches a maximum
3.2.2. Effects of nanocrystallization on the corrosive behavior Fig. 6 shows potentiodynamic polarization curves obtained in 3.5% NaCl solution for as-received sample, HVOFeSMB treated sample and HVOFeSMB treatment þ annealed at 373 K for 2 h. The
Fig. 3. Dark-field image of the topmost surface layer with equiaxed nanograins and inset is the corresponding SAED pattern (a), and HRTEM image of the equiaxed nanograins (b).
680
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681
Fig. 4. Variation of microhardness of Mg alloy sample after HVOFeSMB treatment from the topmost surface to the substrate.
Fig. 6. Potentiodynamic polarization curves of different samples in a 3.5% NaCl solution.
corrosion potentials (Ecorr) and the corrosion currents (Icorr) of various samples calculated from Fig. 6 by Tafel analysis are listed in Table 1. The results listed in Table 1 allow comparison of the corrosion performance of samples with different treatment conditions. Significant difference in polarization behavior among the samples was observed. The Ecorr is a criterion for evaluating corrosion behavior. The materials with more negative potentials will generally tend to undergo more significant corrosion, while the others (with more positive potential values) will generally suffer less attack. The HVOFeSMB treatment þ annealed sample demonstrates much better behavior in this sense in comparison with as-annealed and HVOFeSMB treated samples. The potentiodynamic polarization curves revealed that the as-received, HVOFeSMB treated and HVOFeSMB treatment þ annealed samples could passivate under a certain level of potential. Although the free corrosion potential (Ecorr) of the treated and as-received samples were similar, the minimum passive current density of the HVOFeSMB treated sample was slightly less than that of the asreceived one. Meanwhile, the critical current density for passivation decreased under HVOFeSMB treated condition as compared to that of as-received condition. However, the surface layer of HVOFeSMB treated Mg alloy sample totally showed the poor polarization behavior in spite of it was harder than that of the asreceived sample. It is worth noting that the corrosion current
density (Icorr) of the HVOFeSMB treated sample was two magnitudes larger than that of the as-received one. The lower this value, the better the corrosion protection property, since the corrosion current is directly related to the corrosion rate. Additionally, the polarization behavior changed significantly for the Mg alloy sample with surface modification by HVOFeSMB treatment, not only markedly increasing the cathodic current density, but also having the tendency of shifting anodic current density and passivationmaintaining current density to lower value, yet slightly elevating the free corrosion potential. It should be pointed out that the influencing factors of electrochemical behavior are quite complex. A decrease in passive current density could be observed after HVOFeSMB treatment. On the one hand, as the surface was severely plastic deformed during HVOFeSMB treatment, the passive film formed on the surface also could be considered as highly disordered because of nanoscale dislocation structures. These defect structures induce a high number of localized electron levels close to the band edges and alter the forbidden energy gap [26]. On the other hand, resulting from the formation of the rare earth elements-enriched (e.g. Gd, Y) passive film, the results of XRD patterns before and after HVOFeSMB treatment are shown in Fig. 1, which protects the Mg alloy from corrosion attack [21,27]. When a surface layer is nanocrystalline induced by severe plastic deformation, the high density
Fig. 5. Cross-sectional microstructure of the HVOFeSMB treated samples with different annealing temperature (a) HVOFeSMB treatment; (b) annealing at 373 K for 2 h after HVOFeSMB treatment; (c) annealing at 673 K for 2 h after HVOFeSMB treatment.
K.-d. Xu et al. / Current Applied Physics 11 (2011) 677e681 Table 1 Corrosion parameters of the samples studied in a 3.5% NaCl solution. Processing status
Ecorr (V)
Icorr (mA/cm2)
As-received sample HVOFeSMB treatment HVOFeSMB þ annealing at 373 K for 2 h
1.327 1.314 0.948
6.03 437 41.9
grain boundaries could easily promote the diffusion of Gd and Y to surface, owing to the nanocrystalline surface layer having higher distortion energy, thus forming a passive film containing richer Gd and Y that may strengthen the passive film. Those might be the reason why the decrease in passive current density on HVOFeSMB treated specimen. Moreover, the cross diffusion of metal vacancies and higher dislocation densities at the nanocrystalline surface layer could act as short circuit diffusion paths for passivation-promoting alloying elements to reach the surface and form a coherent passive film at the passive range of potentials [21]. For polycrystalline materials, especially the grain size was refined down to nanometer scale, several factors can influence significantly on the corrosion resistance of a sample, such as defect structures, grain boundaries, grain size, homogeneity of microstructure and stress state, etc. It is well known that the stress state of surface layer induced by severe plastic deformation through repeated multidirectional high velocity microparticles bombarding could be beneficial to improve the fatigue resistance, but it can induce/facilitate the stress corrosion and thereby accelerate corrosion rate. The potentiodynamic polarization curves (as shown in Fig. 6) with different treatment conditions revealed that HVOFeSMB treatment þ annealed sample (the stress was released through low-temperature annealing) has the most electrochemical behavior in general compared with asreceived and HVOFeSMB treated samples. 4. Conclusions Microstructure, mechanical and electrochemical properties of a rare earth Mg alloy nanocrystalline surface layer prepdprepared by severe plastic deformation via HVOFeSMB treatment were investigated, in comparison with as-received and HVOFeSMB treatment þ annealed samples. The XRD analysis, OM and HRTEM investigations of the microstructural evolution show that rare earth elements (e.g. Gd, Y) enriched in the top surface layer, and the microstructure changes gradually from the substrate to the surface. The grain refinement is prominent and the grain size increases gradually from about 10 nm at the topmost surface to about of 100e500 nm at a depth of 80 mm from the treated surface. The microhardness was enhanced about twice comparison with the
681
substrates and the distribution of microhardness value gradually decreases with the depth increasing. The microhardness behavior is shown to be mainly in correlation with the grain refinement down to nanoscale. The nanocrystallization prepdprepared through HVOFeSMB treatment þ low-temperature annealing considerably improved the polarization behavior of the material. In particular, the surface nanocrystallization of Mg alloy induced by severe plastic deformation although changed the polarization behavior significantly as compared to that of as-received condition, the corrosion resistance was deteriorated to a certain degree. Acknowledgments The authors are grateful for financial support from the Foundation of Huazhong University of Science and Technology. References [1] D.J. Li, X.Q. Zeng, J. Dong, C.Q. Zhai, W.J. Ding, J. Alloys Compd. 468 (2009) 164e169. [2] L.Q. Zhu, G.L. Song, Surf. Coat. Technol. 200 (2006) 2834e2840. [3] U. Erb, A.M. El-Sherik, G. Palumbo, K.T. Aust, Nanostruct. Mater. 2 (1993) 383e390. [4] A. Wolowik, M. Janik-Czachor, Mater. Sci. Eng. A 267 (1999) 301. [5] F.A. Bonilla, A. Berkani, P. Skeldon, G.E. Thompson, H. Habazaki, K. Shimizu, C. John, K. Stevens, Corros. Sci. 44 (2002) 1941. [6] Manoj Masanta, P. Ganesh, Rakesh Kaul, A.K. Nath, A. Roy Choudhury, Mater. Sci. Eng. A 508 (2009) 134e140. [7] I. Manna, J. Dutta Majumdar, B. Ramesh Chandra, S. Nayak, Narendra B. Dahotre, Surf. Coat. Technol. 201 (2006) 434e440. [8] A.H. Wang, T.M. Yue, Compos. Sci. Technol. 61 (2001) 1549e1554. [9] Richard L.C. Wu, William Lanter, John Wrbanek, Charles DeJoseph, Surf. Coat. Technol. 140 (2001) 35e43. [10] T. Asmus, Gerhard K. Wolf, Nucl. Instrum. Meth. B 166e167 (2000) 732e736. [11] N.R. Tao, Z.B. Wang, W.P. Tong, et al., Acta Mater. 50 (2002) 4603e4616. [12] H.Q. Sun, Y.N. Shi, M.X. Zhang, et al., Acta Mater. 55 (2007) 975. [13] K.Y. Zhu, A. Vassel, F. Brisset, K. Lu, J. Lu, Acta Mater. 52 (2004) 4101e4110. [14] K. Wang, N.R. Tao, G. Liu, J. Lu, K. Lu, Acta Mater. 54 (2006) 5281e5291. [15] L. Huang, J. Lu, M. Troyon, Surf. Coat. Technol. 201 (2006) 208e213. [16] G. Liu, J. Lu, K. Lu, Mater. Sci. Eng. A 286 (2000) 91. [17] X.P. Jiang, X.Y. Wang, J.X. Li, D.Y. Li, C.S. Manc, M.J. Shepard, T. Zhai, Mater. Sci. Eng. A 429 (2006) 30. [18] K. Masaaki, Z.J. Horita, G. Terence, Mater. Sci. Eng. A 488 (2008) 117e124. [19] B.N. Mordyuk, G.I. Prokopenko, M.A. Vasylyev, M.O. Iefimov, Mater. Sci. Eng. A 458 (2007) 253e261. [20] T.S. Wang, J.K. Yu, B.F. Dong, Surf. Coat. Technol. 200 (2006) 4777e4781. [21] X.Y. Wang, D.Y. Li, Electrochim. Acta 47 (2002) 3939e3947. [22] K.S. Raja, S.A. Namjoshi, M. Misra, Mater. Lett. 59 (2005) 570e574. [23] K.D. Xu, A.H. Wang, Y. Wang, X.P. Dong, X.L. Zhang, Z.W. Huang, Appl. Surf. Sci. 256 (2009) 619e626. [24] Y.H. Wei, B.S. Liu, L.F. Hou, B.S. Xu, G. Liu, J. Alloys Compd. 452 (2008) 336e342. [25] K. Lu, J. Lu, Mater. Sci. Eng. A 375e377 (2004) 38. [26] P. Schmuki, H. Bohni, J. Electrochem. Soc. 139 (1992) 1908e1913. [27] J.B. Lumsden, Z. Szklarska-Smialowska, Corrosion 34 (1978) 169.