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Effects of tensile and compressive deformation on corrosion behaviour of a Mg–Zn alloy
Accepted Manuscript Effects of tensile and compressive deformation on corrosion behaviour of a MgZn alloy Yang Zheng, Yan Li, Jihua Chen, Zhengyang Zou PII: DOI: Reference:
S0010-938X(14)00500-9 http://dx.doi.org/10.1016/j.corsci.2014.10.043 CS 6073
To appear in:
Corrosion Science
Received Date: Accepted Date:
18 March 2014 29 October 2014
Please cite this article as: Y. Zheng, Y. Li, J. Chen, Z. Zou, Effects of tensile and compressive deformation on corrosion behaviour of a Mg-Zn alloy, Corrosion Science (2014), doi: http://dx.doi.org/10.1016/j.corsci. 2014.10.043
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Effects of tensile and compressive deformation on corrosion behaviour of a Mg-Zn alloy Yang Zhenga,b, Yan Lia,c,*, Jihua Chend, Zhengyang Zoud a.
School of Materials Science and Engineering, Beihang University, Beijing 100191, China.
b.
Key Laboratory of Aerospace Advanced Materials and Performance (Beihang University), Ministry of Education, Beijing 100191, China.
c.
Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology (Beihang University), Beijing, 100191, China.
d.
School of Materials Science and Engineering, Hunan University, Changsha, Hunan, 410082, China.
Abstract The corrosion behaviour of a Mg-2.65wt.%Zn alloy after tensile and compressive deformation has been investigated using dynamic polarization and immersion tests. The microstructure has been characterized using X-ray diffraction spectroscopy and transmission electron microscopy. Tensile and compressive deformation introduces high density dislocations and deformation twins to the alloy and accelerates the corrosion rate. This is ascribed to the enhanced stress corrosion associated with dislocations and deformation twins. In particular, we find that tensile deformation triggers a different intergranular corrosion mechanism in contrast to compressive deformation. The relationship between the corrosion behaviour and microstructure has been discussed.
Keywords: A. Magnesium B. TEM B. Polarization B. Weight loss C. Effects of strain
_________________________ * Corresponding Author: Yan Li, E-mail: [email protected], Tel: +86 10 82315989
1. Introduction
Stainless steels, titanium alloys and Co-Cr alloys are traditional biomedical alloys which exhibit excellent mechanical properties, such as high strength, good ductility, favorable processing ability and outstanding corrosion resistance. These alloys are usually used for orthopedic and cardiovascular implants, which can survive long-time service with negligible degradation in the human body. In contrast, biodegradable materials are able to decompose gradually in human body fluids and their corrosion products can be excreted through metabolism. Patients may thus avoid a secondary removal surgery and the subsequent physiological pain as well as economic burden [1-3]. Mg and its alloys attract substantial attention in the biodegradable material community because of their advantageous mechanical properties, such as low density, high specific strength, low Young’s modulus and excellent biocompatibility [4-8]. Mg-Zn alloys have been extensively investigated as promising candidates for orthopedic implants [9-13]. The mechanical properties of MgZn alloys can be improved by the addition of Zn. The yield strength of Mg-Zn alloys has been increased from 60.62MPa for Mg-1Zn alloy to 75.60MPa for Mg-5Zn alloy through grain refinement, solid solution and second phase [9]. Moreover, the corrosion resistance of MgZn alloys is also drastically influenced by the addition of Zn. The average corrosion rate of Mg-Zn alloys in simulated body fluids (SBF) decreased from 0.53mm·y-1 to 0.26mm·y-1 with increasing Zn content ranging from 1wt.% to 5wt.% attributed to the formation of passivation film on the surface [9]. Besides, Mg-1wt.%Zn [12] and Mg-6wt.%Zn [13] alloys showed a much less hydrogen evolution rate in Hank’s solution, and a lower in vitro degradation rate in SBF, respectively, compared with the pure Mg [12]. However, the precipitation of excessive MgxZny second phase in Mg-5Zn alloy impaired its corrosion resistance due to galvanic corrosion [10]. In general, Mg-Zn alloys exhibit very good biocompatibility in both in vitro and in vivo experiments. It has been demonstrated that the Mg-6wt.%Zn alloy had no cytotoxicity to L-929 cells. Moreover, new bone tissues started to form around the implant site in rabbit 6 weeks after implantation and no bad reaction of important organs was found [13]. Furthermore, some ternary alloys, such as Mg-Zn-X (X= Ca, Mn, Si and Y), have been developed to enhance the mechanical strength and corrosion resistance [14-16]. Biomedical alloys for implantation, in general, are subjected to a certain degree of plastic deformation in clinical operations, which inevitably induces microstructural variations. Some
investigations indicated that the structural defects, such as dislocations and deformation twins, could significantly affect the corrosion behaviour of Mg and its alloys [17-20]. The corrosion resistance of AM50 and AZ91D alloys with tensile strain up to 4.39% were studied and the results showed that the anodic current density of both alloys in sodium borate buffer solution firstly increased with increasing tensile strain due to accumulation of dislocations and it subsequently decreased because of dynamic recovery [18]. The corrosion resistance of AZ91 alloy was deteriorated after the extrusion due to increased population of dislocations, twins and grain boundaries. Moreover, the corrosion rates of anodic and cathodic processes were both accelerated by the rearrangement of precipitates [20]. The influence of hot rolling on the corrosion behaviour of various binary Mg-X alloys (X=Zr, Si, Gd, Ca, Al, Mn, Sn, Sr, Nd, La and Ce) were studied by immersion tests in 3.5% NaCl solution saturated with Mg(OH)2. A reduction in corrosion rate could be found for all the binary magnesium alloys (except Mg-Zr and Mg-Si) because of the grain refinement and less second phase particles. Whereas the Mg-Zr and Mg-Si alloys exhibited higher corrosion rate which can be attributed to a greater sensitivity to the precipitation of Fe-rich particles [21]. Despite the aforementioned studies, there are few systemic and comparative investigations on the corrosion behaviour of Mg-Zn alloy under tensile and compressive deformation to date. In the present work, we compare the effects of tensile and compressive plastic deformation on the microstructure, mechanical properties and in vitro corrosion behaviour of the Mg-2.65Zn alloy.
2. Experimental and methods 2.1 Sample preparation The alloy with the nominal composition of Mg-2.65wt. %Zn, corresponding to the Mg-1at. %Zn, was prepared in an electric resistance furnace with a mild steel crucible protected by the RJ-2 flux (37.1% KCl - 43.2% MgCl2 - 6.8% BaCl2 - 4.8% CaF2 - 6.6% [NaCl+CaCl2] - 1.3% water-insoluble - 0.2% moisture). Commercially pure Mg (99.9wt.%) and Zn (99.9wt.%) were used. The melt was purged at 720oC for 10 min and held for 15 min at 760oC for homogenization before the molten alloy was cast into a steel mould. Samples for mechanical and corrosion tests were cut by electro-discharge machining technique. Uniaxial tensile or compressive deformation was conducted using a universal material testing machine (MTS-880). The gauge length of the samples for tension was 30mm×5mm×3mm and the strain rate was
2.78×10-4·s-1. The compressive deformation was conducted on cylindrical rods of Φ10×20mm at a strain rate of 4.17×10-4·s-1. The samples with applied tensile (T) strain of 2%, 6% and 10% and applied compressive (C) strain of 8%, 12% and 16% were named as T2, T6, T10, C8, C12 and C16, respectively. The specimen without deformation was named as D0 for comparison. The Vickers micro-hardness was measured at a load of 50gf for 15s using an FM800 micro-hardness tester and each specimen was measured 15 times to get an average value. 2.2 Microstructure characterization X-ray diffractometer (XRD, Rigaku D/Max 2500) using Cu Kα radiation at a scanning rate of 2o·min-1 was employed to identify phase constitution. Optical microscopy specimens were successively ground using SiC abrasive papers, polished by Al2O3 alcohol solution and then etched in 4% HNO3 alcohol solution for 1 min. The metallographical microstructure was then observed using an Olympus BX51 optical microscope (OM). Transmission electron microscopy (TEM) was conducted using a JEOL-2100F microscope operated at 200kV. TEM specimens were manually ground, polished and milled using a Gatan plasma ion polishing system with a working voltage of 5kV. 2.3 Corrosion tests The electrochemical tests were carried out in Hank’s solution (8.00g·L-1 NaCl, 0.14g·L-1 CaCl2, 0.40g·L-1 KCl, 0.10g·L-1 MgCl2·6H2O, 0.10g·L-1 MgSO4·7H2O, 0.35g·L-1 NaHCO3, 0.12g·L-1 Na2HPO4·12H2O, 0.06g·L-1 KH2PO4 and 1.00g·L-1 Glucose [22]) with a pH value of 7.4 at 37oC by a traditional three-electrode system using the electrochemical workstation (CHI660C). The working electrode, the counter electrode and the reference electrode were the specimen, a platinum electrode and a saturated calomel electrode (SCE), respectively. The exposed area of the working electrode to the solution was 0.25cm2 for the tensioned specimens and 0.785cm2 for the compressed specimens. The open circuit potential (OCP) was recorded and potentiodynamic polarization measurements were performed at a scanning rate of 1mV·s-1 until the OCP values at five different time intervals remained steady for the purpose of stabilization of surface/solution interface. The corrosion current density was extrapolated from the cathodic branch of each polarization curve [23]. The cathodic polarization slope (βc) was determined from the fitted linear region [24]. The degradation behaviour of the Mg-2.65Zn alloy was measured by immersion test according to ASTM-G31-72 [25] in a neutral 0.9% NaCl solution at 37oC. The tensioned plates and compressed rods were immersed in 25ml and 65ml 0.9% NaCl solutions, respectively. After 24 hours and 72 hours
immersion periods, the samples were removed from the solution, rinsed with distilled water, and dried at ambient temperature. The changes of surface morphology after different immersion times were characterized by scanning electron microscopy (SEM, CS3400) and the composition of corrosion products were analyzed by the attached energy disperse X-ray spectrometer (EDX, Oxford). After the complete removal of the corrosion products by 200g·L-1 CrO3 solution, the weight loss of samples was measured by a scale with an accuracy of 10-4 g.
3. Results and discussion The XRD pattern, optical image and TEM micrograph of the Mg-2.65Zn alloy without deformation (D0) are shown in Fig.1. Mg-2.65Zn alloy is composed of α-Mg phase, which is consistent with the literature [11]. According to the binary phase diagram of Mg-Zn alloy, the solubility of Zn in α-Mg matrix is 6.2 wt.% at the eutectic temperature (340oC) and reduces to below 2 wt.% at room temperature [26]. Additional Zn causes the formation of Zn-rich second phase in Mg-Zn alloy, for instance, Mg-1Zn alloy is essentially a single-phase alloy but MgxZny second phase typically appears in Mg-5Zn and Mg-7Zn alloys [9]. It is noted that no MgxZny second phase was observed in our Mg-2.65Zn alloy. It is likely that the cooling rate was high enough during casting to prevent the precipitation of Mg-Zn second phase. The absence of MgxZny second phase was believed to favor the corrosion resistance of Mg-Zn alloys [10, 11]. As seen from Fig. 1(b), the alloy contains typical equiaxed grains with an average grain size of 150 μm. What is more, no second phase was observed from TEM, as shown in Fig. 1(c), which is in good consistency with the XRD results. Meanwhile, dislocations were observed in the α-Mg matrix. Similar features have been reported in cast AZ31 alloy, which is ascribed to the fast cooling rate during casting [27]. Mechanical properties of Mg-2.65Zn alloy in this study and other Mg and Mg-Zn alloys are listed in Table 1. The tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation (EL) of the Mg-2.65Zn alloy are 45MPa, 145MPa and 12%, respectively. The tensile properties of the Mg-2.65Zn alloy are comparable to other as-cast Mg-Zn alloys, such as Mg-1Zn alloy with TYS of 20~25MPa and UTS of 102~130MPa, and Mg-4Zn alloy with TYS of 58MPa and UTS of 217MPa [28, 29]. On the other hand, it should be noted that the tensile data of the referred Mg and Mg-Zn alloys with the same nominal composition varies significantly. This may be due to the composition and grain size differences caused by the variation of casting methods. In addition, our Mg-2.65Zn alloy can withstand a compressive strain
of more than 20% without visible cracks, and the corresponding compressive yield strength (CYS) is up to 44MPa. The optical and TEM micrographs of the T10 and C16 samples are shown in Fig. 2. As seen from Fig. 2(a) and (b), the dislocation networks with very high dislocation density appear in T10. However, deformation twins (as indicated by red arrows), were massively observed in C16, as shown in Fig. 2(c). As shown in Fig. 2(d), deformation twins with an average thickness of 300 to 500nm are distributed in parallel in C16. The corresponding twinning system is { 1011 }< 1012 >, which has been readily observed in Mg and its alloys [30-32]. These results are consistent well with previous investigations on the deformation mechanism of Mg and Mg alloys during monotonic tension or compression, which suggest that dislocation slip is the dominant deformation mechanism during tension, while deformation twinning starts to carry more plastic deformation during compression [32-34]. Fig. 3 presents the potentiodynamic polarization curves of the Mg-2.65Zn alloy samples with and without deformation in Hank’s solution at 37oC. It is seen that a current plateau lies between -1.53V (SCE) and -1.41V (SCE) for the experimental samples, which means the corrosion potential (Ecorr) fluctuates between -1.53V (SCE) and -1.41V (SCE). Furthermore, the cathodic branches of the deformed samples shift to higher current density direction, indicating an increase in the corrosion current density (icorr) of the deformed samples. Table 2 summarizes the electrochemical parameters deduced from the potentiodynamic polarization curves shown in Fig. 3. The Ecorr and icorr values of D0 are -1.48V (SCE) and 2.57×10-5 A·cm-2, respectively, which are comparable to those of Mg-1Zn and Mg-5Zn [9, 28]. The Ecorr values of samples with different tensile or compressive strain exhibits an erratic variation trend. The C16 and T10 show larger Ecorr values than that of D0, which can be attributed to the surface passivation [9]. The similar phenomenon has been found in the corrosion potential vs. elongation curve of the AM50 alloy in 0.05M sodium tetraborate solution (pH=9.7) [35]. Furthermore, the icorr value of the samples increases with the increasing tensile or compressive strain, indicating that both compressive and tensile deformation accelerated the corrosion rate. In addition, the highest icorr value is found in C16. This is because that the deformation twins in the C16 sample (see Fig. 2(c) and (d)) deteriorated the corrosion resistance and similar situation has been reported in the AZ31B alloy [36]. In order to clarify the influence of plastic deformation on the corrosion rate of the Mg-2.65Zn alloy,
the weight loss was measured using 0.9% NaCl solution at 37oC during 24h and 72h immersion tests, as shown in Fig. 4. For the samples in 24h immersion tests, the maximum weight loss is 20.9 mg·cm-2 for T10 and 22.5 mg·cm-2 for C16, which are 2.2 and 2.3 times larger than that of D0 (9.7 mg·cm-2). The weight loss increases monotonously with increasing tensile or compressive strain. Similar trend can be found in the weight loss vs. strain curve in 72h immersion test. These results are in good consistency with the potentiodynamic polarization experiments (see Fig. 3). Therefore, it is concluded that the corrosion rate of the Mg-2.65Zn alloy was significantly accelerated by tensile or compressive deformation. The morphologies of the D0, T10 and C16 samples after 24 hours immersion in 0.9% NaCl solution at 37oC are shown in Fig. 5. As seen from Fig. 5(a) and (b), the corroded degree of D0 is relatively low as compared to its strained counterparts. EDX results of Zone D and E (see Fig. 5(b)) are listed in Table 3. Elements of O, Mg and Zn were detected on the corroded surface. Zone D (featured with high content of O and Zn) shows a corrosion pit, while Zone E (featured with the low content of O and Zn) is covered by the corrosion products. The detection of the O element is due to the surface oxidation of the Mg-2.65Zn alloy. As seen from Fig. 5(c), corrosion starts as the localized erosion and then pits spread all over the surface in the T10 sample. After the immersion test, the corroded surfaces of all the samples are covered by the dense corrosion products. The pits in the deformed samples are larger and deeper than those in the D0 sample. Moreover, the pits on the surface of C16 link with each other, as shown in Fig. 5(d). This indicates that C16 underwent the most severe corrosion, which is in accordance with the potentiodynamic polarization results (see Fig. 3). The corrosion behaviour of the Mg-2.65Zn alloy is a normal process related to the reactions between Mg, H2O and Cl-. The protective Mg(OH)2 layer first formed spontaneously on the surface of the Mg-2.65Zn alloy in humid environment. Encountering Cl- in the 0.9% NaCl solution, they became soluble MgCl2, resulting in the corrosion pits on the surface (see Fig. 5) and weight loss (see Fig. 4). We have shown evidence that the corrosion resistance of the Mg-2.65Zn alloy is weakened by either tensile or compressive deformation, indicated by the increased icorr value and the corresponding weight loss in 0.9% NaCl solution immersion test, as seen in Table 2 and Fig. 4. This is obviously attributed to the formation of structural defects, such as dislocations and twins, which are harmful to corrosion resistance of Mg alloys [27, 35, 36]. The acceleration of anodic metal dissolution is caused by a local reduction of equilibrium potential in the vicinity of dislocations [35]. As mentioned above, the tensile deformation of the Mg-2.65Zn alloy induced high-density dislocations (see Fig. 2(b)) and the compressive deformation
is associated with the formation of deformation twins (see Fig. 2(c) and 2(d)). Dislocations tangling and twins interaction may give rise to a higher residual stress in the material and lead to work hardening. We thus performed hardness tests to quantify the magnitude and build up a relationship between the hardness and the corrosion resistance. The Vickers-hardness as well as icorr of the Mg-2.65Zn alloy as a function of tensile/compressive strain curves is shown in Fig. 6. The micro-hardness of the D0 sample is 49.7 HV, and it increases with the increasing tensile or compressive strain. Furthermore, it should be noted that the icorr value shows a well positive correlation with micro-hardness. Therefore, it can be concluded that the tensile or compressive deformation induces accumulations of structural defects, e.g. dislocations and deformation twins, increases residual stress level and leads to the enhancement of stress corrosion. Besides the stress corrosion enhanced by deformation, it should be noted that the intergranular corrosion behaviour is different between the tensioned and compressed samples. As mentioned in section 2, samples are quickly etched by 4% HNO3 alcohol solution for optical observation, so the morphologies shown in Fig. 1(b), 2(a) and 2(c) display the results of instant corrosion of the D0, T10 and C16 samples, respectively. As seen from Fig. 1(b), the uniform corrosion occurs on the D0 sample and some corrosion pits are randomly distributed on the surface. As for the T10 sample with 10% tensile strain, serious intergranular corrosion is detected since the large pits occur at the grain boundaries, which is attributed to the damage of grain boundary structures caused by large tensile deformation. No obvious intergranular corrosion is observed on the C16 sample, although the corresponding deformation strain and micro-hardness value are much larger than those of T10 sample (see Fig. 6). It is likely that the compressive deformation contributes to the compaction of the alloy and thus suppresses the reaction between the corrosive medium and the Mg-Zn matrix at the grain boundaries.
4. Conclusion
Corrosion behaviour of the Mg-2.65Zn alloy under different tensile and compressive deformation amounts were investigated in this study. The results showed that the Mg-2.65Zn alloy was composed of a single α-Mg phase. The Mg-2.65Zn alloy deformed in tension was featured with the high-density dislocations, and that experienced compressive deformation was primarily characterized by deformation twins. Both of the corrosion current density and the weight loss were increased by the tensile and compressive deformation. Microstructural defects such as dislocations and deformation twins increased
residual stress level, led to the enhancement of stress corrosion, and thus reduced the corrosion resistance. In addition, the tensile deformation brings about the distinct intergranular corrosion behaviour compared with the compressive deformation.
Acknowledgment This work is supported by the National Natural Science Foundation of China (NSFC, No. 51431002) and the National Research Foundation for the Doctoral Program of Higher Education of China (20120161110040).
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Figure captions: Fig. 1 The XRD pattern (a), the optical image (b), the TEM micrograph (c) and the SAED pattern (d) of the Mg-2.65Zn alloy. Fig. 2 The optical micrograph (a) and the TEM micrograph (b) of the T10 sample. The SAED pattern of B showing the beam direction along [0001]Mg, the optical micrograph (c) and the TEM micrograph (d) of the C16 sample. The SAED pattern of C is viewed along [ 1123 ] zone axis. The twinning system is
{ 1011 }< 1012 >.
Fig. 3 Potentiodynamic polarization curves of Mg-2.65Zn samples with different plastic deformation in Hank’s solution at 37oC. Fig. 4 Weight loss of the Mg-2.65Zn alloy with different tensile and compressive deformation after immersion tests in 0.9% NaCl solution for 24h and 72h. Fig. 5 The morphologies of the corroded surfaces of the Mg-2.65Zn alloy: D0 (a) and (b), T10 (c) and C16 (d) after 24 hours immersion in 0.9% NaCl solution at 37oC. Fig. 6 The relationship between Vickers-hardness as well as icorr and tensile/compressive strain of the Mg-2.65Zn alloys.
Table 1 Mechanical properties of as-cast Mg and Mg-Zn alloys. Materials
Tensile yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation (%)
Mg-2.65Zn (wt.%)
45
145
12
Mg [28]
20
85
13
Mg [9]
30
100
7.4
Mg-1Zn (wt.%) [28]
25
130
18
Mg-1Zn (wt.%) [29]
20
102
7
Mg-1Zn (wt.%) [9]
61
188
14
Mg-4Zn (wt.%) [29]
58
217
16
Mg-5Zn (wt.%) [9]
76
195
8.5
Mg-7Zn (wt.%) [9]
67
136
6
Table 2 The corrosion potential (Ecorr), corrosion current density (icorr) and cathodic polarization slope (βc) of the Mg-2.65Zn samples derived from the polarization curves in Fig. 3. Sample ID
D0
Ecorr (V SCE)
-1.48
-2
T2
T6
-1.41 -5
-1.50 -5
icorr (A·cm )
2.57×10
3.89×10
5.75×10
βc (V·decade )
-0.221
-0.249
-0.201
-1
T10
C8
-1.45 -5
8.91×10
C12
-1.47 -5
-0.199
7.94×10
C16
-1.53 -5
-0.217
7.36×10
-1.44 -5
3.55×10-4
-0.230
-0.253
Table 3
EDS results of the framed zone D and E in Fig. 5(b) after the Mg-2.65Zn alloy immersed in 0.9% NaCl solution at 37oC for 24 hours. zone
Element (Atomic percentage %) OK
Mg K
Zn K
D
68.31
28.93
2.76
E
42.26
56.96
0.78
Highlights
1.
Mg-Zn experienced tensile deformation is featured with high-density dislocations.
2.
Mg-Zn after compressive deformation is characterized by deformation twins.
3.
Corrosion current density is increased by tensile or compressive deformation.
4.
Weight loss increases with increasing tensile or compressive deformation strain.
5.
Plastic deformation increases residual stress level and enhances stress corrosion.
S0010-938X(14)00500-9 http://dx.doi.org/10.1016/j.corsci.2014.10.043 CS 6073
To appear in:
Corrosion Science
Received Date: Accepted Date:
18 March 2014 29 October 2014
Please cite this article as: Y. Zheng, Y. Li, J. Chen, Z. Zou, Effects of tensile and compressive deformation on corrosion behaviour of a Mg-Zn alloy, Corrosion Science (2014), doi: http://dx.doi.org/10.1016/j.corsci. 2014.10.043
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Effects of tensile and compressive deformation on corrosion behaviour of a Mg-Zn alloy Yang Zhenga,b, Yan Lia,c,*, Jihua Chend, Zhengyang Zoud a.
School of Materials Science and Engineering, Beihang University, Beijing 100191, China.
b.
Key Laboratory of Aerospace Advanced Materials and Performance (Beihang University), Ministry of Education, Beijing 100191, China.
c.
Beijing Key Laboratory for Advanced Functional Materials and Thin Film Technology (Beihang University), Beijing, 100191, China.
d.
School of Materials Science and Engineering, Hunan University, Changsha, Hunan, 410082, China.
Abstract The corrosion behaviour of a Mg-2.65wt.%Zn alloy after tensile and compressive deformation has been investigated using dynamic polarization and immersion tests. The microstructure has been characterized using X-ray diffraction spectroscopy and transmission electron microscopy. Tensile and compressive deformation introduces high density dislocations and deformation twins to the alloy and accelerates the corrosion rate. This is ascribed to the enhanced stress corrosion associated with dislocations and deformation twins. In particular, we find that tensile deformation triggers a different intergranular corrosion mechanism in contrast to compressive deformation. The relationship between the corrosion behaviour and microstructure has been discussed.
Keywords: A. Magnesium B. TEM B. Polarization B. Weight loss C. Effects of strain
_________________________ * Corresponding Author: Yan Li, E-mail: [email protected], Tel: +86 10 82315989
1. Introduction
Stainless steels, titanium alloys and Co-Cr alloys are traditional biomedical alloys which exhibit excellent mechanical properties, such as high strength, good ductility, favorable processing ability and outstanding corrosion resistance. These alloys are usually used for orthopedic and cardiovascular implants, which can survive long-time service with negligible degradation in the human body. In contrast, biodegradable materials are able to decompose gradually in human body fluids and their corrosion products can be excreted through metabolism. Patients may thus avoid a secondary removal surgery and the subsequent physiological pain as well as economic burden [1-3]. Mg and its alloys attract substantial attention in the biodegradable material community because of their advantageous mechanical properties, such as low density, high specific strength, low Young’s modulus and excellent biocompatibility [4-8]. Mg-Zn alloys have been extensively investigated as promising candidates for orthopedic implants [9-13]. The mechanical properties of MgZn alloys can be improved by the addition of Zn. The yield strength of Mg-Zn alloys has been increased from 60.62MPa for Mg-1Zn alloy to 75.60MPa for Mg-5Zn alloy through grain refinement, solid solution and second phase [9]. Moreover, the corrosion resistance of MgZn alloys is also drastically influenced by the addition of Zn. The average corrosion rate of Mg-Zn alloys in simulated body fluids (SBF) decreased from 0.53mm·y-1 to 0.26mm·y-1 with increasing Zn content ranging from 1wt.% to 5wt.% attributed to the formation of passivation film on the surface [9]. Besides, Mg-1wt.%Zn [12] and Mg-6wt.%Zn [13] alloys showed a much less hydrogen evolution rate in Hank’s solution, and a lower in vitro degradation rate in SBF, respectively, compared with the pure Mg [12]. However, the precipitation of excessive MgxZny second phase in Mg-5Zn alloy impaired its corrosion resistance due to galvanic corrosion [10]. In general, Mg-Zn alloys exhibit very good biocompatibility in both in vitro and in vivo experiments. It has been demonstrated that the Mg-6wt.%Zn alloy had no cytotoxicity to L-929 cells. Moreover, new bone tissues started to form around the implant site in rabbit 6 weeks after implantation and no bad reaction of important organs was found [13]. Furthermore, some ternary alloys, such as Mg-Zn-X (X= Ca, Mn, Si and Y), have been developed to enhance the mechanical strength and corrosion resistance [14-16]. Biomedical alloys for implantation, in general, are subjected to a certain degree of plastic deformation in clinical operations, which inevitably induces microstructural variations. Some
investigations indicated that the structural defects, such as dislocations and deformation twins, could significantly affect the corrosion behaviour of Mg and its alloys [17-20]. The corrosion resistance of AM50 and AZ91D alloys with tensile strain up to 4.39% were studied and the results showed that the anodic current density of both alloys in sodium borate buffer solution firstly increased with increasing tensile strain due to accumulation of dislocations and it subsequently decreased because of dynamic recovery [18]. The corrosion resistance of AZ91 alloy was deteriorated after the extrusion due to increased population of dislocations, twins and grain boundaries. Moreover, the corrosion rates of anodic and cathodic processes were both accelerated by the rearrangement of precipitates [20]. The influence of hot rolling on the corrosion behaviour of various binary Mg-X alloys (X=Zr, Si, Gd, Ca, Al, Mn, Sn, Sr, Nd, La and Ce) were studied by immersion tests in 3.5% NaCl solution saturated with Mg(OH)2. A reduction in corrosion rate could be found for all the binary magnesium alloys (except Mg-Zr and Mg-Si) because of the grain refinement and less second phase particles. Whereas the Mg-Zr and Mg-Si alloys exhibited higher corrosion rate which can be attributed to a greater sensitivity to the precipitation of Fe-rich particles [21]. Despite the aforementioned studies, there are few systemic and comparative investigations on the corrosion behaviour of Mg-Zn alloy under tensile and compressive deformation to date. In the present work, we compare the effects of tensile and compressive plastic deformation on the microstructure, mechanical properties and in vitro corrosion behaviour of the Mg-2.65Zn alloy.
2. Experimental and methods 2.1 Sample preparation The alloy with the nominal composition of Mg-2.65wt. %Zn, corresponding to the Mg-1at. %Zn, was prepared in an electric resistance furnace with a mild steel crucible protected by the RJ-2 flux (37.1% KCl - 43.2% MgCl2 - 6.8% BaCl2 - 4.8% CaF2 - 6.6% [NaCl+CaCl2] - 1.3% water-insoluble - 0.2% moisture). Commercially pure Mg (99.9wt.%) and Zn (99.9wt.%) were used. The melt was purged at 720oC for 10 min and held for 15 min at 760oC for homogenization before the molten alloy was cast into a steel mould. Samples for mechanical and corrosion tests were cut by electro-discharge machining technique. Uniaxial tensile or compressive deformation was conducted using a universal material testing machine (MTS-880). The gauge length of the samples for tension was 30mm×5mm×3mm and the strain rate was
2.78×10-4·s-1. The compressive deformation was conducted on cylindrical rods of Φ10×20mm at a strain rate of 4.17×10-4·s-1. The samples with applied tensile (T) strain of 2%, 6% and 10% and applied compressive (C) strain of 8%, 12% and 16% were named as T2, T6, T10, C8, C12 and C16, respectively. The specimen without deformation was named as D0 for comparison. The Vickers micro-hardness was measured at a load of 50gf for 15s using an FM800 micro-hardness tester and each specimen was measured 15 times to get an average value. 2.2 Microstructure characterization X-ray diffractometer (XRD, Rigaku D/Max 2500) using Cu Kα radiation at a scanning rate of 2o·min-1 was employed to identify phase constitution. Optical microscopy specimens were successively ground using SiC abrasive papers, polished by Al2O3 alcohol solution and then etched in 4% HNO3 alcohol solution for 1 min. The metallographical microstructure was then observed using an Olympus BX51 optical microscope (OM). Transmission electron microscopy (TEM) was conducted using a JEOL-2100F microscope operated at 200kV. TEM specimens were manually ground, polished and milled using a Gatan plasma ion polishing system with a working voltage of 5kV. 2.3 Corrosion tests The electrochemical tests were carried out in Hank’s solution (8.00g·L-1 NaCl, 0.14g·L-1 CaCl2, 0.40g·L-1 KCl, 0.10g·L-1 MgCl2·6H2O, 0.10g·L-1 MgSO4·7H2O, 0.35g·L-1 NaHCO3, 0.12g·L-1 Na2HPO4·12H2O, 0.06g·L-1 KH2PO4 and 1.00g·L-1 Glucose [22]) with a pH value of 7.4 at 37oC by a traditional three-electrode system using the electrochemical workstation (CHI660C). The working electrode, the counter electrode and the reference electrode were the specimen, a platinum electrode and a saturated calomel electrode (SCE), respectively. The exposed area of the working electrode to the solution was 0.25cm2 for the tensioned specimens and 0.785cm2 for the compressed specimens. The open circuit potential (OCP) was recorded and potentiodynamic polarization measurements were performed at a scanning rate of 1mV·s-1 until the OCP values at five different time intervals remained steady for the purpose of stabilization of surface/solution interface. The corrosion current density was extrapolated from the cathodic branch of each polarization curve [23]. The cathodic polarization slope (βc) was determined from the fitted linear region [24]. The degradation behaviour of the Mg-2.65Zn alloy was measured by immersion test according to ASTM-G31-72 [25] in a neutral 0.9% NaCl solution at 37oC. The tensioned plates and compressed rods were immersed in 25ml and 65ml 0.9% NaCl solutions, respectively. After 24 hours and 72 hours
immersion periods, the samples were removed from the solution, rinsed with distilled water, and dried at ambient temperature. The changes of surface morphology after different immersion times were characterized by scanning electron microscopy (SEM, CS3400) and the composition of corrosion products were analyzed by the attached energy disperse X-ray spectrometer (EDX, Oxford). After the complete removal of the corrosion products by 200g·L-1 CrO3 solution, the weight loss of samples was measured by a scale with an accuracy of 10-4 g.
3. Results and discussion The XRD pattern, optical image and TEM micrograph of the Mg-2.65Zn alloy without deformation (D0) are shown in Fig.1. Mg-2.65Zn alloy is composed of α-Mg phase, which is consistent with the literature [11]. According to the binary phase diagram of Mg-Zn alloy, the solubility of Zn in α-Mg matrix is 6.2 wt.% at the eutectic temperature (340oC) and reduces to below 2 wt.% at room temperature [26]. Additional Zn causes the formation of Zn-rich second phase in Mg-Zn alloy, for instance, Mg-1Zn alloy is essentially a single-phase alloy but MgxZny second phase typically appears in Mg-5Zn and Mg-7Zn alloys [9]. It is noted that no MgxZny second phase was observed in our Mg-2.65Zn alloy. It is likely that the cooling rate was high enough during casting to prevent the precipitation of Mg-Zn second phase. The absence of MgxZny second phase was believed to favor the corrosion resistance of Mg-Zn alloys [10, 11]. As seen from Fig. 1(b), the alloy contains typical equiaxed grains with an average grain size of 150 μm. What is more, no second phase was observed from TEM, as shown in Fig. 1(c), which is in good consistency with the XRD results. Meanwhile, dislocations were observed in the α-Mg matrix. Similar features have been reported in cast AZ31 alloy, which is ascribed to the fast cooling rate during casting [27]. Mechanical properties of Mg-2.65Zn alloy in this study and other Mg and Mg-Zn alloys are listed in Table 1. The tensile yield strength (TYS), ultimate tensile strength (UTS) and elongation (EL) of the Mg-2.65Zn alloy are 45MPa, 145MPa and 12%, respectively. The tensile properties of the Mg-2.65Zn alloy are comparable to other as-cast Mg-Zn alloys, such as Mg-1Zn alloy with TYS of 20~25MPa and UTS of 102~130MPa, and Mg-4Zn alloy with TYS of 58MPa and UTS of 217MPa [28, 29]. On the other hand, it should be noted that the tensile data of the referred Mg and Mg-Zn alloys with the same nominal composition varies significantly. This may be due to the composition and grain size differences caused by the variation of casting methods. In addition, our Mg-2.65Zn alloy can withstand a compressive strain
of more than 20% without visible cracks, and the corresponding compressive yield strength (CYS) is up to 44MPa. The optical and TEM micrographs of the T10 and C16 samples are shown in Fig. 2. As seen from Fig. 2(a) and (b), the dislocation networks with very high dislocation density appear in T10. However, deformation twins (as indicated by red arrows), were massively observed in C16, as shown in Fig. 2(c). As shown in Fig. 2(d), deformation twins with an average thickness of 300 to 500nm are distributed in parallel in C16. The corresponding twinning system is { 1011 }< 1012 >, which has been readily observed in Mg and its alloys [30-32]. These results are consistent well with previous investigations on the deformation mechanism of Mg and Mg alloys during monotonic tension or compression, which suggest that dislocation slip is the dominant deformation mechanism during tension, while deformation twinning starts to carry more plastic deformation during compression [32-34]. Fig. 3 presents the potentiodynamic polarization curves of the Mg-2.65Zn alloy samples with and without deformation in Hank’s solution at 37oC. It is seen that a current plateau lies between -1.53V (SCE) and -1.41V (SCE) for the experimental samples, which means the corrosion potential (Ecorr) fluctuates between -1.53V (SCE) and -1.41V (SCE). Furthermore, the cathodic branches of the deformed samples shift to higher current density direction, indicating an increase in the corrosion current density (icorr) of the deformed samples. Table 2 summarizes the electrochemical parameters deduced from the potentiodynamic polarization curves shown in Fig. 3. The Ecorr and icorr values of D0 are -1.48V (SCE) and 2.57×10-5 A·cm-2, respectively, which are comparable to those of Mg-1Zn and Mg-5Zn [9, 28]. The Ecorr values of samples with different tensile or compressive strain exhibits an erratic variation trend. The C16 and T10 show larger Ecorr values than that of D0, which can be attributed to the surface passivation [9]. The similar phenomenon has been found in the corrosion potential vs. elongation curve of the AM50 alloy in 0.05M sodium tetraborate solution (pH=9.7) [35]. Furthermore, the icorr value of the samples increases with the increasing tensile or compressive strain, indicating that both compressive and tensile deformation accelerated the corrosion rate. In addition, the highest icorr value is found in C16. This is because that the deformation twins in the C16 sample (see Fig. 2(c) and (d)) deteriorated the corrosion resistance and similar situation has been reported in the AZ31B alloy [36]. In order to clarify the influence of plastic deformation on the corrosion rate of the Mg-2.65Zn alloy,
the weight loss was measured using 0.9% NaCl solution at 37oC during 24h and 72h immersion tests, as shown in Fig. 4. For the samples in 24h immersion tests, the maximum weight loss is 20.9 mg·cm-2 for T10 and 22.5 mg·cm-2 for C16, which are 2.2 and 2.3 times larger than that of D0 (9.7 mg·cm-2). The weight loss increases monotonously with increasing tensile or compressive strain. Similar trend can be found in the weight loss vs. strain curve in 72h immersion test. These results are in good consistency with the potentiodynamic polarization experiments (see Fig. 3). Therefore, it is concluded that the corrosion rate of the Mg-2.65Zn alloy was significantly accelerated by tensile or compressive deformation. The morphologies of the D0, T10 and C16 samples after 24 hours immersion in 0.9% NaCl solution at 37oC are shown in Fig. 5. As seen from Fig. 5(a) and (b), the corroded degree of D0 is relatively low as compared to its strained counterparts. EDX results of Zone D and E (see Fig. 5(b)) are listed in Table 3. Elements of O, Mg and Zn were detected on the corroded surface. Zone D (featured with high content of O and Zn) shows a corrosion pit, while Zone E (featured with the low content of O and Zn) is covered by the corrosion products. The detection of the O element is due to the surface oxidation of the Mg-2.65Zn alloy. As seen from Fig. 5(c), corrosion starts as the localized erosion and then pits spread all over the surface in the T10 sample. After the immersion test, the corroded surfaces of all the samples are covered by the dense corrosion products. The pits in the deformed samples are larger and deeper than those in the D0 sample. Moreover, the pits on the surface of C16 link with each other, as shown in Fig. 5(d). This indicates that C16 underwent the most severe corrosion, which is in accordance with the potentiodynamic polarization results (see Fig. 3). The corrosion behaviour of the Mg-2.65Zn alloy is a normal process related to the reactions between Mg, H2O and Cl-. The protective Mg(OH)2 layer first formed spontaneously on the surface of the Mg-2.65Zn alloy in humid environment. Encountering Cl- in the 0.9% NaCl solution, they became soluble MgCl2, resulting in the corrosion pits on the surface (see Fig. 5) and weight loss (see Fig. 4). We have shown evidence that the corrosion resistance of the Mg-2.65Zn alloy is weakened by either tensile or compressive deformation, indicated by the increased icorr value and the corresponding weight loss in 0.9% NaCl solution immersion test, as seen in Table 2 and Fig. 4. This is obviously attributed to the formation of structural defects, such as dislocations and twins, which are harmful to corrosion resistance of Mg alloys [27, 35, 36]. The acceleration of anodic metal dissolution is caused by a local reduction of equilibrium potential in the vicinity of dislocations [35]. As mentioned above, the tensile deformation of the Mg-2.65Zn alloy induced high-density dislocations (see Fig. 2(b)) and the compressive deformation
is associated with the formation of deformation twins (see Fig. 2(c) and 2(d)). Dislocations tangling and twins interaction may give rise to a higher residual stress in the material and lead to work hardening. We thus performed hardness tests to quantify the magnitude and build up a relationship between the hardness and the corrosion resistance. The Vickers-hardness as well as icorr of the Mg-2.65Zn alloy as a function of tensile/compressive strain curves is shown in Fig. 6. The micro-hardness of the D0 sample is 49.7 HV, and it increases with the increasing tensile or compressive strain. Furthermore, it should be noted that the icorr value shows a well positive correlation with micro-hardness. Therefore, it can be concluded that the tensile or compressive deformation induces accumulations of structural defects, e.g. dislocations and deformation twins, increases residual stress level and leads to the enhancement of stress corrosion. Besides the stress corrosion enhanced by deformation, it should be noted that the intergranular corrosion behaviour is different between the tensioned and compressed samples. As mentioned in section 2, samples are quickly etched by 4% HNO3 alcohol solution for optical observation, so the morphologies shown in Fig. 1(b), 2(a) and 2(c) display the results of instant corrosion of the D0, T10 and C16 samples, respectively. As seen from Fig. 1(b), the uniform corrosion occurs on the D0 sample and some corrosion pits are randomly distributed on the surface. As for the T10 sample with 10% tensile strain, serious intergranular corrosion is detected since the large pits occur at the grain boundaries, which is attributed to the damage of grain boundary structures caused by large tensile deformation. No obvious intergranular corrosion is observed on the C16 sample, although the corresponding deformation strain and micro-hardness value are much larger than those of T10 sample (see Fig. 6). It is likely that the compressive deformation contributes to the compaction of the alloy and thus suppresses the reaction between the corrosive medium and the Mg-Zn matrix at the grain boundaries.
4. Conclusion
Corrosion behaviour of the Mg-2.65Zn alloy under different tensile and compressive deformation amounts were investigated in this study. The results showed that the Mg-2.65Zn alloy was composed of a single α-Mg phase. The Mg-2.65Zn alloy deformed in tension was featured with the high-density dislocations, and that experienced compressive deformation was primarily characterized by deformation twins. Both of the corrosion current density and the weight loss were increased by the tensile and compressive deformation. Microstructural defects such as dislocations and deformation twins increased
residual stress level, led to the enhancement of stress corrosion, and thus reduced the corrosion resistance. In addition, the tensile deformation brings about the distinct intergranular corrosion behaviour compared with the compressive deformation.
Acknowledgment This work is supported by the National Natural Science Foundation of China (NSFC, No. 51431002) and the National Research Foundation for the Doctoral Program of Higher Education of China (20120161110040).
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Figure captions: Fig. 1 The XRD pattern (a), the optical image (b), the TEM micrograph (c) and the SAED pattern (d) of the Mg-2.65Zn alloy. Fig. 2 The optical micrograph (a) and the TEM micrograph (b) of the T10 sample. The SAED pattern of B showing the beam direction along [0001]Mg, the optical micrograph (c) and the TEM micrograph (d) of the C16 sample. The SAED pattern of C is viewed along [ 1123 ] zone axis. The twinning system is
{ 1011 }< 1012 >.
Fig. 3 Potentiodynamic polarization curves of Mg-2.65Zn samples with different plastic deformation in Hank’s solution at 37oC. Fig. 4 Weight loss of the Mg-2.65Zn alloy with different tensile and compressive deformation after immersion tests in 0.9% NaCl solution for 24h and 72h. Fig. 5 The morphologies of the corroded surfaces of the Mg-2.65Zn alloy: D0 (a) and (b), T10 (c) and C16 (d) after 24 hours immersion in 0.9% NaCl solution at 37oC. Fig. 6 The relationship between Vickers-hardness as well as icorr and tensile/compressive strain of the Mg-2.65Zn alloys.
Table 1 Mechanical properties of as-cast Mg and Mg-Zn alloys. Materials
Tensile yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation (%)
Mg-2.65Zn (wt.%)
45
145
12
Mg [28]
20
85
13
Mg [9]
30
100
7.4
Mg-1Zn (wt.%) [28]
25
130
18
Mg-1Zn (wt.%) [29]
20
102
7
Mg-1Zn (wt.%) [9]
61
188
14
Mg-4Zn (wt.%) [29]
58
217
16
Mg-5Zn (wt.%) [9]
76
195
8.5
Mg-7Zn (wt.%) [9]
67
136
6
Table 2 The corrosion potential (Ecorr), corrosion current density (icorr) and cathodic polarization slope (βc) of the Mg-2.65Zn samples derived from the polarization curves in Fig. 3. Sample ID
D0
Ecorr (V SCE)
-1.48
-2
T2
T6
-1.41 -5
-1.50 -5
icorr (A·cm )
2.57×10
3.89×10
5.75×10
βc (V·decade )
-0.221
-0.249
-0.201
-1
T10
C8
-1.45 -5
8.91×10
C12
-1.47 -5
-0.199
7.94×10
C16
-1.53 -5
-0.217
7.36×10
-1.44 -5
3.55×10-4
-0.230
-0.253
Table 3
EDS results of the framed zone D and E in Fig. 5(b) after the Mg-2.65Zn alloy immersed in 0.9% NaCl solution at 37oC for 24 hours. zone
Element (Atomic percentage %) OK
Mg K
Zn K
D
68.31
28.93
2.76
E
42.26
56.96
0.78
Highlights
1.
Mg-Zn experienced tensile deformation is featured with high-density dislocations.
2.
Mg-Zn after compressive deformation is characterized by deformation twins.
3.
Corrosion current density is increased by tensile or compressive deformation.
4.
Weight loss increases with increasing tensile or compressive deformation strain.
5.
Plastic deformation increases residual stress level and enhances stress corrosion.