- Email: [email protected]
Enhanced strength and corrosion resistance of Mg-Gd-Y-Zr alloy with ultrafine grains
Materials Letters 213 (2018) 274–277
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced strength and corrosion resistance of Mg-Gd-Y-Zr alloy with ultrafine grains Yingchun Wan a, Shiyuan Xu a, Chuming Liu a, Yonghao Gao a,⇑, Shunong Jiang b, Zhiyong Chen a a b
School of Materials Science and Engineering, Central South University, Changsha 410083, China School of Civil Engineering, Central South University, Changsha 410075, China
a r t i c l e
i n f o
Article history: Received 19 September 2017 Received in revised form 8 November 2017 Accepted 20 November 2017 Available online 22 November 2017 Keywords: Mg-Gd-Y-Zr alloy Microstructure Strength Corrosion
a b s t r a c t Simultaneous improvements in strength and corrosion resistance of Mg-Gd-Y-Zr alloy were realized through forging at room temperature with formation of ultrafine grained (UFG) structure. Compared with its coarse grained (CG) counterpart, the UFG sample exhibits an enhancement of 211 MPa (103%) in yield strength and a reduction of 0.644 105 A/cm2 (25%) in corrosion current density. Effect of microstructure on the mechanical property was quantitatively investigated, revealing a good coincidence of the experimental result with the calculated one based on Bailey-Hirsch equation. The improvement in corrosion resistance is attributed to the formation of sub-grain boundaries. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Mg-Gd-Y-Zr alloy is considered a promising candidate as implant material owing to the acceptable cytotoxicity, close elastic modulus to natural bone and high mechanical properties [1,2]. However, loss of mechanical integrity due to rapid corrosion rate may cause premature failure, hindering Mg alloy’s clinical applications [3,4]. Further improving the strength and corrosion resistance will broaden the application of Mg-Gd-Y-Zr alloy in medical use. Developing ultrafine grained (UFG) structure via cold deformation at room temperature (RT) was proved to be an effective method in mechanical performance enhancing [5,6]. After RT-rolled, Mg14Gd-0.5Zr alloy was enhanced by 115–305 MPa in yield strength [7] and that of Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr alloy was improved by 95–401 MPa [8]. The micro-hardness of Mg-4.7Y4.6Gd-0.3Zr alloy was enhanced by 400–1400 MPa after RT-highpressure-torsion [6]. However, the corrosion behavior of the cold deformed Mg-Gd-Y-Zr alloy has rarely been investigated. For UFG Mg alloys obtained through RT deformation, contrary tendencies in corrosion resistance were usually observed. Enhanced corrosion resistance was witnessed in AZ31 alloy after cryogenic burnishing processing [9] or sliding friction treatment [10], while similar corrosion behaviors were observed for UFG WE43 alloy [11] after equal channel angular pressed or multiaxial deformed. Till now,
⇑ Corresponding author. E-mail address: [email protected] (Y. Gao). https://doi.org/10.1016/j.matlet.2017.11.096 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
how cold deformation affect the corrosion resistance of Mg alloy is not clearly understood. In the present study, the pre-extruded Mg-Gd-Y-Zr alloy was forged at RT with UFG structure generated. The microstructure, its effects on mechanical and corrosion properties, and the underlying mechanisms are investigated. 2. Material and methods The pre-extruded Mg-8Gd-3Y-0.4Zr (wt%) alloy was forged by 20% at RT. Microstructure was observed on a FEI Tecnai G20 Transmission electron microscope (TEM) at 200 kV with samples prepared through a twin jet electro polishing method at 35 °C under a potential of 40 V in a solution containing 5% HClO4, 35% C2H5OH and 60% CH3OH. The micro-strain was measured on a Bruker D8-Ray diffractometer at 0.5°/min with a power of 3 kW. The mechanical property was tested on an Instron 3369 tester following the ASTM method B557M-94 under a strain rate of 103/s using specimens of 8 mm in diameter. The fracture morphology was observed on a Sirion 200 field-emission scanning electron microscope (SEM) with an accelerating voltage of 15 kV. Electrochemical measurements were performed in a 0.9% NaCl solution at 37 °C using an IM6ex electrochemical workstation with samples (three replicates for each) of 8 8 mm2 in exposed area. The polarization curves were obtained with a scanning rate of 0.1 mV/s in a range of ±0.2 V vs open circuit potential (OCP). The electrochemical impedance spectroscope (EIS) was carried out at OCP from 100 kHz to 0.01 Hz.
Y. Wan et al. / Materials Letters 213 (2018) 274–277
3. Results and discussion direction and X-ray diffracTEM micrographs obtained in ½1120 tion patterns are presented in Fig. 1. As shown in Fig. 1a, UFG structure was obtained with sub-grained regions (indicated by the white solid arrow) taking >90% of the area and deformed regions (indicated by the black solid arrow) taking less than 10%. The sub-grains were revealed with curved boundaries of dislocation arrays as indicated by the red lines in Fig. 1b. From a measurement of about 300 sub-grains, the average size was determined to be about 320 nm with the distribution illustrated in Fig. 1c. In addition, very few grains with clean and sharp boundaries were observed (indicated by the black arrows in Fig. 1b, suggesting the occurrence of DRX in a localized region. The localized DRX is speculated to be induced by a super high stress (strain) concentration and a resultant localized temperature elevation, which could be obtained in the RT deformation of Mg alloy [12]. Fig. 1d presents the X-ray diffraction patterns, based on which the micro-strain (he2i1/2) was estimated to be 0.2116 by the equation (he2i1/2) = b/4tanh, where b stands for the broadening of the peaks. For the alloy with UFG structure obtained from sever plastic deformation, the dislocation density (q) can be estimated by the equation:
pffiffiffi
q ¼ 2 3h2 i1=2 =D b
ð1Þ
[13–15] D is the average sub-grain size and b is the Burgers Vector with a value of 3.21 1010 m. Indeed, the q of the UFG sample here was determined to be 0.3834 1016 m2. Fig. 2 presents the tensile mechanical properties of the alloy. After forging, tremendous increment of 211 MPa (103%) in yield
275
strength was obtained as well as reduced elongation, as shown in Fig. 2a. In order to further understand the mechanical property, the fracture surface morphology was characterized. As shown in Fig. 2b, the CG sample presented a mixed fracture surface with both dimples and cleavage planes. The dimples are the result of micro-void coalescence controlled by dislocation activity, indicative of ductile fracture, while the cleavage planes are associate with brittle failure. Compared with the CG sample, the UFG sample exhibited obviously different fracture feature with dimples of much reduced depth as shown in Fig. 2c. The shallow dimples imply declined ductility after forging, which coincides with the elongation tendency as shown in Fig. 2a. From the SEM morphology in higher magnification shown in Fig. 2d, some of the dimple surfaces were not as smooth as that in the CG sample but of convex contour, resembling ‘‘Cones” of 200–500 nm as indicated by the white solid arrows. It is notable that the dimensions of the cones here are comparable with that of the sub-grains observed in Fig. 1c. Thus, it can be deduced that the crack propagated along the sub-grain boundaries, at least some of them. ‘‘Cones” or ‘‘Cups” on the fracture surface were considered to be associate with the grain boundary sliding or rotation during failure of the material with very fine grains [16]. Therefore, it can be concluded that the grain-boundary-mediate or sub-grain-boundary-mediate mechanism participate in the deformation during tension of the UFG sample. The pronounced increment in strength is obviously a direct result of the UFG structure with sub-grains constituted by dislocation arrays. The strengthening effect induced by dislocations (Drq) usually follows Bailey-Hirsch equation:
Drq ¼ MaGbq1=2
ð2Þ
Fig. 1. TEM microstructure and X-ray diffraction (a) TEM microstructure (b) magnification of the region in the white rectangle in (a) (c) sub-grain sizes distribution (d) X-ray diffraction patterns.
276
Y. Wan et al. / Materials Letters 213 (2018) 274–277
Fig. 2. Mechanical properties of the alloy (a) stress-strain curves (b–d) fracture surface morphology of CG sample (b) and UFG sample (c and d).
Fig. 3. Corrosion property of the alloy (a) polarization curves (b) Nyquist plots.
where a is a constant with a value of 0.24 [17], M is the Taylor Factor with a value of 2.1 [18], G is the shear modulus. The value of Dr is calculated to be 236 MPa based on the value of q obtained in Fig. 1d. It is known that broadening of the X-ray diffraction peaks can be induced by both the micro-strain and the grain refinement into a scale of lower than 100 nm. In the present study, owing to the very few quantity of NC grains, only the broadening effect induced by micro-strain was taken into consideration. Thus, the value of micro-strain determined by the X-ray diffraction and the resultant dislocation density were slightly higher. What is more, as revealed in Fig. 1, very few regions free of ultrafine grains (deformed regions) still existed in the sample, which was also ignored when determining the average sub-grain size. These two factors resulted in the higher value through calculation compared with that derived from the test, as Drq is positively associated with the dislocation density and negatively with the sub-grain size. Considering these two factors, the forecasting result based on the
Bailey-Hirsch equation shows good coincidence with the experimental one. Corrosion property of the alloy was investigated and presented in Fig. 3. As shown in the polarization curves in Fig. 3(a), the corrosion potential (Ecorr) moved positively and the corrosion current intensity (Icorr) decreased from 2.5826 105 A/cm2 to 1.9386 105 A/cm2 after forging, suggesting an enhanced corrosion resistance. Fig. 3b exhibits the Nyquist plots drawn from the EIS spectra, which also confirmed the enhanced corrosion resistance of the UFG sample. Notable enhancement in corrosion resistance was also considered as a result of the UFG structure obtained via cold forging. The extremely refined grains are usually considered to benefit decreasing the corrosion rate by improving the stability of the protective oxide (MgO) film formed on the substrate [19,20]. When the oxide film forms, the free volume mismatch will arise and introduce cracks or ruptures [10]. It is believed that increased grain
Y. Wan et al. / Materials Letters 213 (2018) 274–277
boundary density could provide a route to relieve the stress through enhanced atomic flow along grain boundaries in substrate [10,21]. Thereby, the oxide layer could adhere better to the substrate, resulting in an enhanced corrosion resistance. 4. Conclusions Mg-Gd-Y-Zr alloy exhibits tremendous improvements in strength (103%) and corrosion resistance (25%) after RT forged. The improvement in yield strength is attributed to the accumulation of dislocation and formation of sub-grains, showing good coincidence with the forecast based on Bailey-Hirsch equation. The enhanced corrosion resistance is also considered a result of formation of quantity of sub-grain boundaries. Acknowledgement This work was supported by the National Basic Research Program of China (grant number 2013CB632200); and the National Basic Research Program of China (grant number 51574291). References [1] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, H. Windhagen, Biomaterials 26 (2005) 3557–3563.
277
[2] C.H. Ye, Y.F. Zheng, S.Q. Wang, T.F. Xi, Y.D. Li, Appl. Surf. Sci. 258 (2012) 3420– 3427. [3] I. Popov, D. Starosvetsky, D. Shechtman, J. Mater. Sci. 35 (2000) 1–8. [4] G. Song, Corros. Sci. 49 (2007) 1696–1701. [5] R. Alizadeh, R. Mahmudi, A.H.W. Ngan, Y. Huang, T.G. Langdon, Mater. Sci. Eng. A 651 (2016) 786–794. [6] S.V. Dobatkin, L.L. Rokhlin, E.A. Lukyanova, M.Y. Murashkin, T.V. Dobatkina, N. Y. Tabachkova, Mater. Sci. Eng. A 667 (2016) 217–223. [7] R.G. Li, J.F. Nie, G.J. Huang, Y.C. Xin, Q. Liu, Scr. Mater. 64 (2011) 950–953. [8] Z. Yu, Y. Huang, X. Qiu, G. Wang, F. Meng, N. Hort, J. Meng, Mater. Sci. Eng. A 622 (2015) 121–130. [9] Z. Pu, S. Yang, G.L. Song, O.W.D. Jr, D.A. Puleo, I.S. Jawahir, Scr. Mater. 65 (2011) 520–523. [10] W.T. Huo, W. Zhang, J.W. Lu, Y.S. Zhang, J. Alloys Compd. 720 (2017) 324–331. [11] S.V. Dobatkin, E.A. Lukyanova, N.S. Martynenko, N.Y. Anisimova, M.V. Kiselevskiy, M.V. Gorshenkov, N.Y. Yurchenko, G.I. Raab, V.S. Yusupov, N. Birbilis, IOP Conf. 194 (2017) 012004. [12] H. Sun, Y.-N. Shi, M.-X. Zhang, K. Lu, Acta Mater. 55 (2007) 975–982. [13] G. Williamson, R. Smallman III, Philos. Magn. 1 (1956) 34–46. [14] R. Smallman, K. Westmacott, Philos. Magn. 2 (1957) 669–683. [15] Y.H. Zhao, X.Z. Liao, Z. Jin, R.Z. Valiev, Y.T. Zhu, Acta Mater. 52 (2004) 4589– 4599. [16] H. Li, F. Ebrahimi, Adv. Mater. 17 (2005) 1969–1972. [17] R. Neuhaus, C. Schwink, Philos. Magn. A 65 (1992) 1463–1484. [18] C.H. Cáceres, P. Lukácˇ, Philos. Magn. 88 (2008) 977–989. [19] H.S. Kim, G.H. Kim, H. Kim, W.J. Kim, Corros. Sci. 74 (2013) 139–148. [20] D. Orlov, K.D. Ralston, N. Birbilis, Y. Estrin, Acta Mater. 59 (2011) 6176–6186. [21] H.S. Kim, W.J. Kim, Corros. Sci. 75 (2013) 228–238.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Enhanced strength and corrosion resistance of Mg-Gd-Y-Zr alloy with ultrafine grains Yingchun Wan a, Shiyuan Xu a, Chuming Liu a, Yonghao Gao a,⇑, Shunong Jiang b, Zhiyong Chen a a b
School of Materials Science and Engineering, Central South University, Changsha 410083, China School of Civil Engineering, Central South University, Changsha 410075, China
a r t i c l e
i n f o
Article history: Received 19 September 2017 Received in revised form 8 November 2017 Accepted 20 November 2017 Available online 22 November 2017 Keywords: Mg-Gd-Y-Zr alloy Microstructure Strength Corrosion
a b s t r a c t Simultaneous improvements in strength and corrosion resistance of Mg-Gd-Y-Zr alloy were realized through forging at room temperature with formation of ultrafine grained (UFG) structure. Compared with its coarse grained (CG) counterpart, the UFG sample exhibits an enhancement of 211 MPa (103%) in yield strength and a reduction of 0.644 105 A/cm2 (25%) in corrosion current density. Effect of microstructure on the mechanical property was quantitatively investigated, revealing a good coincidence of the experimental result with the calculated one based on Bailey-Hirsch equation. The improvement in corrosion resistance is attributed to the formation of sub-grain boundaries. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Mg-Gd-Y-Zr alloy is considered a promising candidate as implant material owing to the acceptable cytotoxicity, close elastic modulus to natural bone and high mechanical properties [1,2]. However, loss of mechanical integrity due to rapid corrosion rate may cause premature failure, hindering Mg alloy’s clinical applications [3,4]. Further improving the strength and corrosion resistance will broaden the application of Mg-Gd-Y-Zr alloy in medical use. Developing ultrafine grained (UFG) structure via cold deformation at room temperature (RT) was proved to be an effective method in mechanical performance enhancing [5,6]. After RT-rolled, Mg14Gd-0.5Zr alloy was enhanced by 115–305 MPa in yield strength [7] and that of Mg-11Gd-4.5Y-1Nd-1.5Zn-0.5Zr alloy was improved by 95–401 MPa [8]. The micro-hardness of Mg-4.7Y4.6Gd-0.3Zr alloy was enhanced by 400–1400 MPa after RT-highpressure-torsion [6]. However, the corrosion behavior of the cold deformed Mg-Gd-Y-Zr alloy has rarely been investigated. For UFG Mg alloys obtained through RT deformation, contrary tendencies in corrosion resistance were usually observed. Enhanced corrosion resistance was witnessed in AZ31 alloy after cryogenic burnishing processing [9] or sliding friction treatment [10], while similar corrosion behaviors were observed for UFG WE43 alloy [11] after equal channel angular pressed or multiaxial deformed. Till now,
⇑ Corresponding author. E-mail address: [email protected] (Y. Gao). https://doi.org/10.1016/j.matlet.2017.11.096 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
how cold deformation affect the corrosion resistance of Mg alloy is not clearly understood. In the present study, the pre-extruded Mg-Gd-Y-Zr alloy was forged at RT with UFG structure generated. The microstructure, its effects on mechanical and corrosion properties, and the underlying mechanisms are investigated. 2. Material and methods The pre-extruded Mg-8Gd-3Y-0.4Zr (wt%) alloy was forged by 20% at RT. Microstructure was observed on a FEI Tecnai G20 Transmission electron microscope (TEM) at 200 kV with samples prepared through a twin jet electro polishing method at 35 °C under a potential of 40 V in a solution containing 5% HClO4, 35% C2H5OH and 60% CH3OH. The micro-strain was measured on a Bruker D8-Ray diffractometer at 0.5°/min with a power of 3 kW. The mechanical property was tested on an Instron 3369 tester following the ASTM method B557M-94 under a strain rate of 103/s using specimens of 8 mm in diameter. The fracture morphology was observed on a Sirion 200 field-emission scanning electron microscope (SEM) with an accelerating voltage of 15 kV. Electrochemical measurements were performed in a 0.9% NaCl solution at 37 °C using an IM6ex electrochemical workstation with samples (three replicates for each) of 8 8 mm2 in exposed area. The polarization curves were obtained with a scanning rate of 0.1 mV/s in a range of ±0.2 V vs open circuit potential (OCP). The electrochemical impedance spectroscope (EIS) was carried out at OCP from 100 kHz to 0.01 Hz.
Y. Wan et al. / Materials Letters 213 (2018) 274–277
3. Results and discussion direction and X-ray diffracTEM micrographs obtained in ½1120 tion patterns are presented in Fig. 1. As shown in Fig. 1a, UFG structure was obtained with sub-grained regions (indicated by the white solid arrow) taking >90% of the area and deformed regions (indicated by the black solid arrow) taking less than 10%. The sub-grains were revealed with curved boundaries of dislocation arrays as indicated by the red lines in Fig. 1b. From a measurement of about 300 sub-grains, the average size was determined to be about 320 nm with the distribution illustrated in Fig. 1c. In addition, very few grains with clean and sharp boundaries were observed (indicated by the black arrows in Fig. 1b, suggesting the occurrence of DRX in a localized region. The localized DRX is speculated to be induced by a super high stress (strain) concentration and a resultant localized temperature elevation, which could be obtained in the RT deformation of Mg alloy [12]. Fig. 1d presents the X-ray diffraction patterns, based on which the micro-strain (he2i1/2) was estimated to be 0.2116 by the equation (he2i1/2) = b/4tanh, where b stands for the broadening of the peaks. For the alloy with UFG structure obtained from sever plastic deformation, the dislocation density (q) can be estimated by the equation:
pffiffiffi
q ¼ 2 3h2 i1=2 =D b
ð1Þ
[13–15] D is the average sub-grain size and b is the Burgers Vector with a value of 3.21 1010 m. Indeed, the q of the UFG sample here was determined to be 0.3834 1016 m2. Fig. 2 presents the tensile mechanical properties of the alloy. After forging, tremendous increment of 211 MPa (103%) in yield
275
strength was obtained as well as reduced elongation, as shown in Fig. 2a. In order to further understand the mechanical property, the fracture surface morphology was characterized. As shown in Fig. 2b, the CG sample presented a mixed fracture surface with both dimples and cleavage planes. The dimples are the result of micro-void coalescence controlled by dislocation activity, indicative of ductile fracture, while the cleavage planes are associate with brittle failure. Compared with the CG sample, the UFG sample exhibited obviously different fracture feature with dimples of much reduced depth as shown in Fig. 2c. The shallow dimples imply declined ductility after forging, which coincides with the elongation tendency as shown in Fig. 2a. From the SEM morphology in higher magnification shown in Fig. 2d, some of the dimple surfaces were not as smooth as that in the CG sample but of convex contour, resembling ‘‘Cones” of 200–500 nm as indicated by the white solid arrows. It is notable that the dimensions of the cones here are comparable with that of the sub-grains observed in Fig. 1c. Thus, it can be deduced that the crack propagated along the sub-grain boundaries, at least some of them. ‘‘Cones” or ‘‘Cups” on the fracture surface were considered to be associate with the grain boundary sliding or rotation during failure of the material with very fine grains [16]. Therefore, it can be concluded that the grain-boundary-mediate or sub-grain-boundary-mediate mechanism participate in the deformation during tension of the UFG sample. The pronounced increment in strength is obviously a direct result of the UFG structure with sub-grains constituted by dislocation arrays. The strengthening effect induced by dislocations (Drq) usually follows Bailey-Hirsch equation:
Drq ¼ MaGbq1=2
ð2Þ
Fig. 1. TEM microstructure and X-ray diffraction (a) TEM microstructure (b) magnification of the region in the white rectangle in (a) (c) sub-grain sizes distribution (d) X-ray diffraction patterns.
276
Y. Wan et al. / Materials Letters 213 (2018) 274–277
Fig. 2. Mechanical properties of the alloy (a) stress-strain curves (b–d) fracture surface morphology of CG sample (b) and UFG sample (c and d).
Fig. 3. Corrosion property of the alloy (a) polarization curves (b) Nyquist plots.
where a is a constant with a value of 0.24 [17], M is the Taylor Factor with a value of 2.1 [18], G is the shear modulus. The value of Dr is calculated to be 236 MPa based on the value of q obtained in Fig. 1d. It is known that broadening of the X-ray diffraction peaks can be induced by both the micro-strain and the grain refinement into a scale of lower than 100 nm. In the present study, owing to the very few quantity of NC grains, only the broadening effect induced by micro-strain was taken into consideration. Thus, the value of micro-strain determined by the X-ray diffraction and the resultant dislocation density were slightly higher. What is more, as revealed in Fig. 1, very few regions free of ultrafine grains (deformed regions) still existed in the sample, which was also ignored when determining the average sub-grain size. These two factors resulted in the higher value through calculation compared with that derived from the test, as Drq is positively associated with the dislocation density and negatively with the sub-grain size. Considering these two factors, the forecasting result based on the
Bailey-Hirsch equation shows good coincidence with the experimental one. Corrosion property of the alloy was investigated and presented in Fig. 3. As shown in the polarization curves in Fig. 3(a), the corrosion potential (Ecorr) moved positively and the corrosion current intensity (Icorr) decreased from 2.5826 105 A/cm2 to 1.9386 105 A/cm2 after forging, suggesting an enhanced corrosion resistance. Fig. 3b exhibits the Nyquist plots drawn from the EIS spectra, which also confirmed the enhanced corrosion resistance of the UFG sample. Notable enhancement in corrosion resistance was also considered as a result of the UFG structure obtained via cold forging. The extremely refined grains are usually considered to benefit decreasing the corrosion rate by improving the stability of the protective oxide (MgO) film formed on the substrate [19,20]. When the oxide film forms, the free volume mismatch will arise and introduce cracks or ruptures [10]. It is believed that increased grain
Y. Wan et al. / Materials Letters 213 (2018) 274–277
boundary density could provide a route to relieve the stress through enhanced atomic flow along grain boundaries in substrate [10,21]. Thereby, the oxide layer could adhere better to the substrate, resulting in an enhanced corrosion resistance. 4. Conclusions Mg-Gd-Y-Zr alloy exhibits tremendous improvements in strength (103%) and corrosion resistance (25%) after RT forged. The improvement in yield strength is attributed to the accumulation of dislocation and formation of sub-grains, showing good coincidence with the forecast based on Bailey-Hirsch equation. The enhanced corrosion resistance is also considered a result of formation of quantity of sub-grain boundaries. Acknowledgement This work was supported by the National Basic Research Program of China (grant number 2013CB632200); and the National Basic Research Program of China (grant number 51574291). References [1] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, H. Windhagen, Biomaterials 26 (2005) 3557–3563.
277
[2] C.H. Ye, Y.F. Zheng, S.Q. Wang, T.F. Xi, Y.D. Li, Appl. Surf. Sci. 258 (2012) 3420– 3427. [3] I. Popov, D. Starosvetsky, D. Shechtman, J. Mater. Sci. 35 (2000) 1–8. [4] G. Song, Corros. Sci. 49 (2007) 1696–1701. [5] R. Alizadeh, R. Mahmudi, A.H.W. Ngan, Y. Huang, T.G. Langdon, Mater. Sci. Eng. A 651 (2016) 786–794. [6] S.V. Dobatkin, L.L. Rokhlin, E.A. Lukyanova, M.Y. Murashkin, T.V. Dobatkina, N. Y. Tabachkova, Mater. Sci. Eng. A 667 (2016) 217–223. [7] R.G. Li, J.F. Nie, G.J. Huang, Y.C. Xin, Q. Liu, Scr. Mater. 64 (2011) 950–953. [8] Z. Yu, Y. Huang, X. Qiu, G. Wang, F. Meng, N. Hort, J. Meng, Mater. Sci. Eng. A 622 (2015) 121–130. [9] Z. Pu, S. Yang, G.L. Song, O.W.D. Jr, D.A. Puleo, I.S. Jawahir, Scr. Mater. 65 (2011) 520–523. [10] W.T. Huo, W. Zhang, J.W. Lu, Y.S. Zhang, J. Alloys Compd. 720 (2017) 324–331. [11] S.V. Dobatkin, E.A. Lukyanova, N.S. Martynenko, N.Y. Anisimova, M.V. Kiselevskiy, M.V. Gorshenkov, N.Y. Yurchenko, G.I. Raab, V.S. Yusupov, N. Birbilis, IOP Conf. 194 (2017) 012004. [12] H. Sun, Y.-N. Shi, M.-X. Zhang, K. Lu, Acta Mater. 55 (2007) 975–982. [13] G. Williamson, R. Smallman III, Philos. Magn. 1 (1956) 34–46. [14] R. Smallman, K. Westmacott, Philos. Magn. 2 (1957) 669–683. [15] Y.H. Zhao, X.Z. Liao, Z. Jin, R.Z. Valiev, Y.T. Zhu, Acta Mater. 52 (2004) 4589– 4599. [16] H. Li, F. Ebrahimi, Adv. Mater. 17 (2005) 1969–1972. [17] R. Neuhaus, C. Schwink, Philos. Magn. A 65 (1992) 1463–1484. [18] C.H. Cáceres, P. Lukácˇ, Philos. Magn. 88 (2008) 977–989. [19] H.S. Kim, G.H. Kim, H. Kim, W.J. Kim, Corros. Sci. 74 (2013) 139–148. [20] D. Orlov, K.D. Ralston, N. Birbilis, Y. Estrin, Acta Mater. 59 (2011) 6176–6186. [21] H.S. Kim, W.J. Kim, Corros. Sci. 75 (2013) 228–238.