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Effects of Zn content on microstructure, mechanical and degradation behaviors of Mg-xZn-0.2Ca-0.1Mn alloys
Journal Pre-proof Effects of Zn content on Microstructure, Mechanical and Degradation Behaviors of Mg-xZn-0.2Ca-0.1Mn Alloys
Liangyu Wei, Jingyuan Li, Yuan Zhang, Huiying Lai PII:
S0254-0584(19)31255-6
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122441
Reference:
MAC 122441
To appear in:
Materials Chemistry and Physics
Received Date:
01 September 2019
Accepted Date:
08 November 2019
Please cite this article as: Liangyu Wei, Jingyuan Li, Yuan Zhang, Huiying Lai, Effects of Zn content on Microstructure, Mechanical and Degradation Behaviors of Mg-xZn-0.2Ca-0.1Mn Alloys, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122441
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Journal Pre-proof Effects of Zn content on Microstructure, Mechanical and Degradation Behaviors of Mg-xZn-0.2Ca-0.1Mn Alloys Liangyu Wei1, Jingyuan Li1*, Yuan Zhang1,2*, Huiying Lai1 1
School of Materials Science and Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China 2 School of Metallurgy and Energy, North China University of Science and Technology (NCST), Tangshan, 063210, China
Abstract： In this study, the microstructure characterization, mechanical properties and degradation behaviors of the Mg-xZn-0.1Mn-0.2Ca alloys with various Zn contents were analyzed via the polarization test, electrochemical impedance spectroscopy and the static immersion measurements. Meantime, the surface film layer features and the phase constitutions of the degradation production were characterized via the confocal laser scanning microscopy (3D/CLSM) and scanning electron microscopy. The results indicated that the more secondary phases appeared and the grain sizes were remarkably refined with the further addition of Zn, such as the Ca2Mg6Zn3, Mg2Ca and Mg4Zn7. The mechanical and corrosion resistance of the Mg-xZn-0.1Mn-0.2Ca alloys both initially increased and then decreased as Zn contents increased. Besides, the special anodic phase has been detected in the interior of the matrix, where the Mg2Ca phase with the lower potentials can protect Mg matrix from dissolving in the simulated body fluids (SBF). However, the Ca2Mg6Zn3 phase with the high potentials can accelerate dissolution rate of the Mg matrix. The best mechanical
properties
and
corrosion
resistance
were
obtained
in
the
Mg-1Zn-0.1Mn-0.2Ca alloys, with the corrosion rate of 6.09 mm/y, exhibiting the uniform degradation morphology. Key words: Mg-Zn-Mn-Ca, Microstructure, Mechanical Properties, Degradation 1. Introduction Due to the excellent mechanical properties and functions, implant materials play an important role in the cardiovascular diseases and bone fracture
[1-5].
Currently, the
metals materials, such as the stainless steel, titanium alloys, cobalt chromium alloys, were widely used as the implant materials 1Corresponding
[6-8].
Unfortunately, their modulus was
Author: E-mail address: [email protected] (J.Y. Li) and [email protected] (Y. Zhang).
Journal Pre-proof higher than that of the human bones, leading to the stress shielding effects
[9-10].
Moreover, due to the non-biodegradability of these inert alloys, the second operation must be performed to remove them [9]. With regarding to the Polymer-based materials, they can be gradually degraded in the human body with the immersion time extended. However, the low mechanical strength limited its application [11-13]. To overcome and solve these drawbacks, many researches were focused on the corrosion rate and mechanical properties of the degradable Mg-based alloys [2-4, 14-19]. The density of the Mg-based alloys was close to that of human bones (about 1.7 g/cm3) and far lower than that of stainless steels, meeting the requirements of ideal orthopedic implants. Besides, Mg-based alloys were different from other inert metals in that it can be gradually degraded in physiological environment, and its degradation products can be absorbed by the body during fracture healing [1-4]. Furthermore, the Mg element acting as a basic element of human body was also participated in the metabolic functions and has a better biological safety. However, the fast corrosion rate of Mg-based alloys in the human physiological environment not only causes the loss of mechanical integrity, but also causes the local alkalinity poisoning
[20].
As a
result, developing new corrosion resistant biomedical magnesium alloys is the focus research area of biomedical magnesium alloys. Previous studies
[2-4, 21]
have been indicated that Mg-based alloys containing Al
and rare earth elements (REs) were harmful to the human body, due to its potential toxic. Thus, the selection criteria of the alloying elements must be cautious and scientific. The essential elements of the Zn, Mn, Ca and Sr with various contents have been added into the Mg alloys to develop suitable implant materials
[22-31].
Previous
literatures have showed that the Mn element can refine the grain size, remove the heavy metal elements and eliminate the harmful intermetallic compounds
[32-33].
Alloying element of Ca can improve the metallurgical quality of magnesium alloys as well as refine the grain-size [34-38]. The effect of Sr addition also has a beneficial effect on the microstructure, mechanical properties and biological corrosion behaviors of Mg-2Zn alloys in Kokubo’s solution
[39].
Moreover, the studies further indicated that
the Zn element was involved into the reproductive, genetic, immune, endocrine and other physiological processes, which can also significantly improve the service performance of the Mg matrix alloys. Similarly, T.T. Sasaki
[40]
found that the age
hardening reponse of Mg-Sn was enhanced by addition of Zn. The increase in
Journal Pre-proof hardness is mainly attributed to the homogeneous distribution, the refinement of Mg2Sn precipitates and the increase in the number of precipitates lying on non-basal planes. W.D. Cui [41] stated that the fine lamellar long period stacking ordered (LPSO) structure formed inside of the Mg-9Gd-3Y-0.5Zr alloys and its quantity increases with raising Zn content. Peak-aged Mg-9Gd-3Y-0.5Zn-0.5Zr alloy exhibited a desirable mechanical property. The in vivo corrosion and the biocompatibility of Mg-Zn-Ca alloy were also analyzed by X.N. Gu[42]. The results showed that the extruded Mg-Zn-Ca alloy provided sufficient biocompatibility for orthopedic application, though the in vivo corrosion rate should be further reduced for clinical use. Mentioned above, the three-dimensional (3D) degradation morphology, special Volta potentials distribution and degradation mechanism of the Mg-Zn-Mn-Ca in the simulated body fluids have not been systematically investigated. As a result, the Mg-0.1Mn-0.2Ca alloys with various Zn addition were explored the degradation mechanism, which was characterized using the static immersion and electrochemical measurements. 2. Experimental and Methods 2.1. Materials preparation and materials characterization The studied alloys with the nominal composition of the Mg-xZn-0.2Ca-0.1Mn (x = 0.25, 0.5, 1.0 and 2.0 wt.%) were fabricated using the electromagnetic induction melting method, with the raw materials of high-purity Mg ingots (99.94 wt.%), high-purity Zn ingots (99.99 wt.%), Mg-20Ca (99.89 wt.%) and Mg-5Mn (99.81%). All raw materials were received from Global Jinxin International Technology Co., Ltd, Beijing, China. Prior to the melting process, it is necessary to wash the crucible with pure Mg ingots to prevent the incorporation of the impurities. When the raw materials are all loaded into the crucible, the lid is closed. Subsequently, the vacuum pumping system was performed and the argon was simultaneously charged until the positive pressure condition is reached. Raw materials were melted at approximately 750°C and the alloys at this temperature were maintained for approximately 30 min. Afterwards, the melts were poured into a cylinder-shaped graphite crucible, which was preheated at 250℃. The actual chemical compositions were analyzed using the ICP-AES (Varian715-ES), which was listed in Table 1. The ingots were homogenized at 420 ℃ for 18 h and then water quenched. For the microstructure observations, the specimens were grounded with the abrasive
Journal Pre-proof papers from #2000 to #5000 in order to obtain a mirror-like surface. Subsequently, the samples were etched using the mixed picric acid solution (10 mL acetic acid + 5 g picric acid + 100 mL ethyl alcohol + 10 mL deionized water). The microstructure was characterized using the optical microscopy (Leica, DM2500-M) and scanning electron microscopy together with energy dispersive spectroscopy (Zeiss Auriga). Moreover, the phase composition and the surface potentials were proven using the X-ray diffraction (Smart-Lab), TEM (Tecnai G2) and the atomic force microscopy (AFM, MFP 3D Infinity). 2.2. Static soaking test Samples for immersion tests were cut into cylinders with the dimensions of φ10 mm × 5 mm. The soaking tests were conducted in the simulated body fluids (SBF) at the 37.5°C using a thermostatic water bath kettle. The chemical compositions comparison between SBF and human blood plasma was summarized in Table 2. Prior to the immersion test, the specimens were polished with the abrasive paper from 2000 to 5000 grits. Subsequently, specimens were washed in the acetone and ethanol solutions, respectively. Meantime, the initial mass of each specimen was the m0, which was measured using the electronic analytical balance. According to the ASTM-G31-72, the ratio of SBF solution to the samples area was the 30 mL/cm2. The specimens were suspended in the SBF with the initial pH value of 7.42 for 10 days, and the solution was refreshed every 24 h. After immersion, the surface degradation products were removed through the boiling solution (200 g/L Cr2O3+ 10 g/L AgNO3). Following, the samples were cleaned with the ultrasonic acetone and ethanol, and then dried with a hair dryer. Therewith, the final mass of each sample was recorded in m1. The three-dimensional degradation topography of each sample was characterized using the confocal laser scanning microscopy (CLSM). The average corrosion rates (Pi, mm/year) of experimental samples were obtained by the weight loss (∆m = m0 – m1, g), using the following equation [43]: Pi
3650m . Mg At
(1)
Where, the ∆m is the weight loss of each experimental sample, g; ρMg is the density of Mg matrix alloys, g/cm3; A is the sample area exposed to the SBF solution, cm3; t is the immersion time, d. Another way to determine the corrosion resistance of
Journal Pre-proof specimens is the pH test. The samples and solutions prepared in this test are the same as those prepared in the immersion test. During the immersion periods, the pH value was measured every 8 h. 2.3 Electrochemical measurements The potentio-dynamic polarization tests of the Mg-bsed alloys were carried out in the SBF using the VersaSTAT 3 electrochemical workstation. Where, the platinum electrode was the counter electrode, the saturated calomel electrode and the samples were the reference electrode and working electrode, respectively. The samples were sealed with epoxy resin and left 1cm2 area exposed to the SBF. Prior to the electrochemical measurements, the sealed samples were polished with the 5000 grits abrasive-paper. The open circuit potential value was recorded until it is stable. Then, the polarization curves were measured with the scanning rate of 1 mV/s. 2.4. Mechanical properties According to ASTM-E8-04 standard[44], the tensile samples were cut into standard dog bone shape with a tensile speed of 1 mm/min (CMT5105, MTS, China). Each group was repeated three times. 3. Results and Discussion 3.1. Microstructure The optical micrographs of Mg-xZn-0.1Mn-0.2Ca alloys were depicted in Fig.1. It can be showed that Zn was an effective grain refiner which can enhance the mechanical strength and corrosion resistance of Mg alloys [40-42]. The average grain sizes of Mg-xZn-0.1Mn-0.2Ca alloys were refined from 458 μm to 272 μm as the Zn content increased from 0.2 to 2%, which was measure by the linear intercept method. Previous studies have been indicated that the both grain sizes and volume fraction of phases synergistically change the mechanical properties of Mg matrix alloys
[31].
In
Fig. 1(a) and (b), the distribution of the secondary phases was not obvious, but the presence of a small amount of precipitation at the internal grains was detected. As the Zn content reaches or exceeds 1wt. %, the grain boundaries become more obvious and intensive, as shown in Fig. 1c and Fig. 1d. The results indicate that the addition of Zn facilitated the grains refinement and the formation of secondary phases.
Journal Pre-proof The SEM morphology and EDS analysis of the Mg-xZn-0.1Mn-0.2Ca alloys were depicted in Fig. 2. It can be seen in Fig. 2 that the higher Zn content resulted in more intensive distribution of white secondary phases. These phases were distributed in the inner grains in the form of dot shape, and these phases were stripe and Y-shape along the grain boundaries. As the Zn contents increased from 0.2 to 1 wt.%, the Ca fraction (marked in yellow in the embedded tables) of the secondary phases obviously increased from 3.75 to 8.64 wt.%. Oppositely, the Zn contents in the secondary phase increased slightly from 20.36 to 23.54 wt.%, but the change was not obvious. However, with the zinc contents further addition (exceeding 1 wt.% and 2 wt.%,), the increase trend of calcium content in the Mg matrix slows down, while the zinc content significantly increases from 23.54 wt.% to 28.32 wt.%. As shown in Fig.3, the secondary phases in Mg-xZn-0.1Mn-0.2Ca alloys were identified as α-Mg, Mg2Ca and Ca2Mg6Zn3 phases. Combined with EDS and XRD results, it can be inferred that the addition of zinc promotes the formation of Mg2Ca and Ca2Mg6Zn3 compound/phases. As the zinc content reaches 1 wt.%, the amount of Mg2Ca increased significantly. When the zinc content exceeds 1 wt.% and 2 wt.%, the content of other second phases, such as Ca2Mg6Zn3, increases even more. The microstructure observation of Mg-2Zn-0.1Mn-0.2Ca was further characterized using the TEM. TEM bright field (BF) image and the corresponding selected area electron diffraction (SAED) patterns of the Mg-2Zn-0.1Mn-0.2Ca alloy were illustrated in Fig. 4. The results indicated that the precipitates were in the shape of irregular lamella, with an approximate size of 2-3 μm. The SAED patterns analysis confirm that the secondary phase was Mg4Zn7 intermetallic phase of monoclinic crystal system, with lattice parameters a = 8.499 Å, b= 6.303 Å, c = 6.753 Å, α = 117°, β = 47° and γ = 74°, respectively. 3.2. Mechanical properties The mechanical properties of test samples were depicted in Fig. 5. Sun et al. [28] reported that the minor addition of Zn into the matrix was beneficial to enhance the mechanical strength. In the present study, the strength and ductility of the Mg-xZn-0.1Mn-0.2Ca alloys both exhibited an initial increasing trend as the Zn content increased from 0.2 to 1 wt.%. The YS, UTS and elongation increased from 44.27 ± 2.12 MPa, 115.17 ± 8.71 MPa and 7.76 ± 0.08 % to 67.64 ± 6.75 MPa, 181.90 ± 9.87 MPa and 9.20 ± 1.2 %, respectively. However, as the Zn was further
Journal Pre-proof added and up to 2 wt.%, the YS, UTS and elongation were decreased to 0.71 %, 1.66% and 22.56%, respectively. As a result, the specimen with the addition of 1% Zn presented the best mechanical properties. 3.3. Experimental tests in SBF 3.3.1 Experiments of pH variation The pH value fluctuation tests in Fig. 6 were conducted in the SBF to explore the corrosion resistance of Mg matrix alloys. The results concluded that the pH value of the Mg-0.2Zn-0.1Mn-0.2Ca increased rapidly to 8.62 after 36 h immersion test, while the pH of specimens with 0.5, 1 and 2% Zn addition was 8.43, 8.35 and 8.58, respectively. During this process, the Mg matrix alloys contacts and reacts with the SBF, resulting in the formation of corrosion product layer on the surface of Mg-xZn-0.1Mn-0.2Ca alloys. As the contact areas between Mg matrix and SBF gradually decreases, less transformation of Mg atoms into Mg2+ and H2O into OHoccurred. The dissolution rate of the Mg matrix decreased as the protection of corrosion product layer increased. The increment speed of pH values in the later 36-144 hours was also lowered. Lower pH values represented that the less Mg matrix was corroded and less H2O was transformed into OH-. Therefore, the best corrosion resistance was obtained in Mg-1Zn-0.1Mn-0.2Ca alloy. 3.3.2 Soaking test Corrosion rates of the Mg-xZn-0.1Mn-0.2Ca alloys were displayed in Fig. 7, which were calculated by the weight loss and Tafel curves. The results illustrated that the experimental alloys with 0.2 and 2 wt.% Zn addition exhibited a higher corrosion rate than the other two alloys with 0.5 and 1 wt.% Zn addition. In the weight loss test, the corrosion rate of the Mg-xZn-0.1Mn-0.2Ca alloys first decreased from 6.52 to 5.33 mm/year with Zn content increased from 0.25 to 1 wt.%. And then, the corrosion rate increased from 5.33 to 7.56 mm/year with further Zn added up to 2 wt.%. In the Tafel test, the corrosion rate has the same variety trend compared to the weight-loss test. Among all of the specimens, the Mg-2Zn-0.1Mn-0.2Ca alloy has the maximum corrosion rate of 7.56 and 8.65 mm/year, which was calculated by weight loss and Tafel curves, respectively. Based on these results, it indicated that the Mg-1Zn-0.1Mn-0.2Ca alloy exhibited the best corrosion resistance, with the corrosion
Journal Pre-proof rate of 5.33 and 6.09 mm/year, respectively. The result was in accordance with the pH value change test. The surface degradation morphology of the Mg-xZn-0.1Mn-0.2Ca alloys after 10 days’ immersion was depicted in Fig. 8. It can be seen that some bulk degradation products were formed on the Mg-0.2Zn-0.1Mn-0.2Ca alloy. The cracks existed on the corrosion surface of all the immersed specimens were similar, but the damage level was remarkably different. The results demonstrated that the Mg-xZn-0.1Mn-0.2Ca alloys containing 1 wt.% Zn possessed the denser degradation film layer than those containing 0.2, 0.5, and 2 wt.% Zn. The magnified picture in Fig. 8b revealed that the degradation morphology of the Mg-0.5Zn-0.1Mn-0.2Ca alloy was porous. The distribution of portion cracks on the Mg-2Zn-0.1Mn-0.2Ca alloy was shown in Fig. 8(d), which was remarked using the orange dotted lines. The hydrogen release during soaking test damages the compactness of the corrosion product layer and results in the formation of porous defects on the surface. As a result, these observations were consistent with the corrosion rate. After removal of the degradation products, the surface morphology of the sample was analyzed/characterized using the laser scanning confocal microscopy (CLSM). Fig. 9 reveals the profile curve of the surface morphology. It showed that the samples with different Zn content have the similar corrosion features and pitting corrosion. Apparent degradation micro-holes can be detected in Fig. 9(a, d), revealing that the Mg-xZn-0.1Mn-0.2Ca alloys with 0.2 and 2 wt.% Zn additions were more probable to generate the localized corrosion. It can be also seen that the Mg-1Zn-0.1Mn-0.2Ca alloy exhibited the flattest surface morphology, with the tiny corrosion pits. Due to the protection of Mg2Ca phase distributed in Mg matrix, the surface morphology of Mg-1Zn-0.1Mn-0.2Ca specimen was relatively uniform. 3.3.3 Electrochemical measurements The potentio-dynamic polarization measurements of the Mg-xZn-0.1Mn-0.2Ca in the simulated body fluids were presented in Fig. 10. The cathodic branch of the polarization curve is concerned with the reaction of H2O into the OH- and H2, while the anodic branch represents the Mg matrix dissolution behavior. The results illuminated that the Mg-xZn-0.1Mn-0.2Ca alloys with various Zn contents exhibit a similar corrosion tendency. The detailed electrochemical parameters were listed in Table 3. As the Zn addition increased, the corrosion potentials of the experimental
Journal Pre-proof alloys towards to the positive direction, implying the Zn was beneficial to improve the corrosion resistance. To sum up, the corrosion potentials of the Mg-xZn-0.1Mn-0.2Ca alloys in the simulated body fluids were ordered as follows: Mg-1Zn-0.1Mn-0.2Ca > Mg-0.5Zn-0.1Mn-0.2Ca > Mg-0.2Zn-0.1Mn-0.2Ca > Mg-2Zn-0.1Mn-0.2Ca. The corrosion rate (Pi) was obtained through the corrosion current density (Icorr), using the following equation [45]: Pi = 22.85·Icorr.
(2)
According to the equation, specimens with the lower corrosion current density has the better corrosion property. Therein, the Mg-1Zn-0.1Mn-0.2Ca alloy has the minimum corrosion current density (266.573 μA·cm−2) than that of the other three alloys. The corrosion rates calculated by corrosion current densities were 7.37, 6.09, 8.65 mm/year. The polarization resistance (RP, 1.09 kΩ·cm2) of the specimens indicated that the Mg-1Zn-0.1Mn-0.2Ca was most difficult to react with the electrolyte solutions. 4. Discussion As depicted in Fig. 1, the average grain sizes of Mg-xZn-Mn-Ca alloys decreased, which was due to the refining effect of Zn element. During the solidification process, Zn atoms could gradually diffuse to the solid/liquid interface front location, resulting in the formation of undercooling areas, which restricted the grain growth. Moreover, the increase of Zn addition led to the formation of more secondary phases. The similar results have been demonstrated on the previous literatures, such as the Mg2Ca, Ca2Mg6Zn3, Ca2Mg5Zn13, Mg12Zn13, MgZn2, et al
[3, 18-25, 30-36].
As regards to the Mn,
the solid solubility of Mn in Mg matrix was 2.2 wt.%. Besides, when the content of Mn was only 0.1 wt.%, Mn will not react with Mg. The main role of Mn was to reduce the content of impurities (Fe, Ni), so as to reduce the corrosion effect of impurities. With respect to the Ca, abundant Ca contents will react with Mg and lead to the formation of the secondary phases. When the Zn contents is in the range of 0.2-1 wt.%, Mg2Ca presented a more obviously increase tendency than other phases containing the Zn. Oppositely, when further Zn exceeding 1 but no more than 2 wt.%, the content of phases containing Zn increased more evidently than that of Mg2Ca. The tensile tests showed that the YS, UTS and EL of the Mg-xZn-0.1Mn-0.2Ca alloys increased firstly and then decreased subsequently as the Zn contents increased. The microstructure results show that the mechanical properties of the Mg-based alloys were mainly related with the grain refinement and the formation of secondary phase.
Journal Pre-proof Grain refinement was beneficial to improve the strength and ductility of Mg matrix alloys, and thus to improve the comprehensive properties of Mg matrix alloys. The mechanical properties of Mg-xZn-0.5Ca-0.5RE alloys with various Zn content was analyzed by M. Golrang [46]. The results declared that the higher Zn content limits the usability and hot working temperature range of the alloys, due to the hot shortness and intergranular brittle fracture. M. Razzaghi et al
[47]
found that the simultaneous
addition of 2% Al and 0.5% Zn is an effective way for refining of as-cast AZ20. It can enhance the mechanical properties and alter the fracture mechanism. Furthermore, the age-hardening of Mg-2.0Gd-1.2Y-0.2Zr with various Zn addition was analyzed by T. Honma
[48].
Although the addition of Zn degrades the age-hardening response, it
causes the discontinuous precipitation of a 14H-type long-period stacking phase at grain boundaries as well as within grains in the over-aged condition, which enhances the maximum tensile elongation. In this study, the addition of Zn into the Mg-xZn-0.1Mn-0.2Ca alloys refined the grains as well as prompted the formation of secondary phases, such as the Mg2Ca, Ca2Mg6Zn3 and Mg4Zn7. Due to the refinement effect of with the Zn addition, the plastic deformation caused by external force disperses in the grain refinement, resulting in the reduction of stress concentration. Grain refinement can also remarkably increase the grain boundaries densities and inhibit the dislocation movement, thus delaying the formation and expansion of the micro-cracks[49]. Therefore, the finer grains were favorable for the occurrence of grain boundary glide, which improved the ductility. In this study, the phases composition of the Mg-xZn-0.1Mn-0.2Ca alloys were confirmed containing the Mg2Ca, Ca2Mg6Zn3 and Mg4Zn7. Zhang et al.
[25]
reported
that the phases composition of Mg-Zn-Mn-Ca alloys mainly included Ca2Mg6Zn3 and Mg2Ca, which precipitated alternately along the grain boundaries and at the inner grains. The volume fraction of the phases increased as the Zn content increased. In previous study, Rad et al.
[18]
displayed that the electrochemical Volta potential of
phases were ranked as followed: Ca2Mg6Zn3 > α-Mg > Mg2Ca. The surface Volta potentials of the Mg-1Zn-0.1Mn-0.2Ca were characterized using the AFM, as shown in Fig.11. The high-altitude and high-potential parts indicated by the cyan dotted frame are Ca2Mg6Zn3, and the high-altitude and low-potential parts are Mg2Ca. The results further confirmed the similar results. The Volta potentials in Fig. 11(b) were lower than the Mg matrix and other secondary phases, which were further confirmed to be Mg2Ca. However, the phases with lower potential in the electrolyte solutions
Journal Pre-proof could act as anode, while the other phase with higher potential would act as the cathode. As a result, the α-Mg and Mg2Ca phase dissolved in preference to the Ca2Mg6Zn3 during the immersion period. It has been stated
[18, 25]
that after a period of immersion in the SBF, Mg2Ca
phases dissolved partly or completely, while Ca2Mg6Zn3 phase still existed along the grain boundaries or in the inner grains. Subsequently, the Ca2Mg6Zn3 phases would fall off as the Mg matrix around Ca2Mg6Zn3 phases was corroded. It can be reasonably deduced from these results that these phases on the experimental alloys dissolved in the following order: Mg2Ca > α-Mg > Ca2Mg6Zn3. Fig. 12 reveals the mechanism by which Mg2Ca protects the Mg matrix from corrosion, while the Ca2Mg6Zn3 accelerates the corrosion process. When the experimental specimens were immersed into the SBF, the solution firstly reacted with the surface oxidation layer, resulting in the hydroxides layer, such as Mg(OH)2, Zn(OH)2 and Ca(OH)2. Besides, the Mg matrix rapidly translated into the Mg2+ ions, while the phases that exposed to the SBF formed the micro-galvanic together with Mg matrix. In the micro-galvanic, the phase with a lower potential would act as anode, while the one with a higher potential would act as cathode. The normal potential of Mg2Ca phase is more negative than that of α-Mg. Conversely, the normal potential of Ca2Mg6Zn3 phase is more positive than that of α-Mg. Therefore, different galvanic corrosion reactions occurred between the α-Mg/Mg2Ca and α-Mg/Ca2Mg6Zn3. The main reactions between α-Mg and Mg2Ca were as follows: Anodic (Mg2Ca) reaction: Mg2Ca → 2Mg2+ + Ca 2- + 2e-
(3)
Cathodic (α-Mg) reaction: 2H2O (aq) + 2e- → H2 (g) + 2OH- (aq)
(4)
In the Ca2Mg6Zn3 phase, Ca2Mg6Zn3 phase performed as a cathode and Mg matrix as an anode. The corrosion reactions between α-Mg and Ca2Mg6Zn3 were as follows: Anodic (α-Mg) reaction: Mg → Mg2+ + 2e-
(5)
Cathodic (Ca2Mg6Zn3) reaction: 2H2O (aq) + 2e- → H2 (g) + 2OH- (aq)
(6)
Anodic dissolution occurred in preference to that of the cathode. The Mg2Ca phase dissolved firstly, following with the dissolution of α-Mg and then the shedding of Ca2Mg6Zn3 phase. Therefore, Mg2Ca phase protected the Mg matrix from corrosion by sacrificing itself. Oppositely, Ca2Mg6Zn3 phase accelerated the corrosion rate by sacrificing the Mg matrix. It can be speculated that the addition of Zn played a
Journal Pre-proof beneficial role in the formation of Mg2Ca phase (exceeding 0.2 wt.% but no more than 1 wt.%). The increment of Mg2Ca phase content prevented the Mg matrix from faster degradation. As a result, as the Zn content raised from 0.2 to 1wt.%, the corrosion rate of the experimental alloys calculated by weight loss dropped obviously from 6.52 to 5.33 mm/year, respectively. However, once the content of Zn exceeded 1 and up to 2 wt.%, other species secondary phases containing Zn, such as Ca2Mg6Zn3, increased more remarkably than Mg2Ca, and thus accelerated the dissolution rate of the Mg matrix. The corrosion rate of Mg-2Zn-0.1Mn-0.2Ca alloy measured by weight loss reached 7.56 mm/year. During the immersion period, the micro-galvanic corrosion occurred at the interface area between the Mg matrix and Ca2Mg6Zn3 secondary phase, leading to the formation of micro-cracks between them. As the corrosion process prolonged, Mg matrix around the Ca2Mg6Zn3 phase dissolved unceasingly, resulting in the extension of the cracks. The Ca2Mg6Zn3 phase eventually fell from the experimental samples when the circumambient Mg matrix was completely corroded, leaving the distinct corrosion pits on the sample surface. Furthermore, the existence of corrosion pits increased the contact area of Mg matrix and the SBF. Different from the Mg matrix around the Ca2Mg6Zn3 phase, Mg matrix around the Mg2Ca phase was protected from pit corrosion, presenting a more uniform corrosion characteristic. Degradation products were more inclined to deposit on the Mg matrix around Mg2Ca phase and form the denser protective layer comparing with that around Ca2Mg6Zn3 phase. The denser and intact degradation layer protected the experimental specimens from the rapid dissolution. This was the reason why the corrosion rate of the experimental alloys first decreased as Zn content increased from 0.2 to 1 wt.%, then increased later when Zn content continued increased to 2 wt.%. 5. Conclusions (1) With the Zn further addition, the grain sizes of the Mg-xZn-0.1Mn-0.2Ca alloys were remarkably refined, and the more e secondary phases were appeared, such as the Ca2Mg6Zn3, Mg2Ca and Mg4Zn7. (2) The mechanical and the corrosion resistance of the Mg-xZn-0.1Mn-0.2Ca both initially increased and then decreased as Zn contents increased. The Mg-1Zn-0.1Mn-0.2Ca alloy delivered the best mechanical performance of 67.64 ± 6.75 MPa for YS, 181.90 ± 9.87 MPa for UTS and 9.20 ± 1.2 % for elongation at
Journal Pre-proof break. Immersion and electrochemical test both indicated the Mg-1Zn-0.1Mn-0.2Ca alloy possessed the best the corrosion resistance, with the corrosion rate of 5.33 and 6.09 mm/year, respectively. (3) During the degradation process, the Mg2Ca phase had a lower Volta potential than the Mg matrix, which acted as the anodic and consequently protected the Mg matrix from pit corrosion, exhibiting the uniform degradation morphology. Inversely, the Volta potentials of the other Zn-containing phases/intermetallics were higher than the matrix, including the Ca2Mg6Zn3, which can act as the cathode and accelerate the degradation rate of Mg matrix. Acknowledgment This research was supported by the financial support of the National Key Research and Development Program of China (2016YFB 0700303). Reference [1] M.S. Song, R.C. Zeng, Y.F. Ding, et al. Recent advances in biodegradation controls over Mg alloys for bone fracture management: A review[J]. J. Mater. Sci. Technol, 2019, 35(4): 535-544. [2] R. Hou, J. Victoria-Hernandez, P. Jiang, et al. In vitro evaluation of the ZX11 magnesium alloy as potential bone plate: Degradability and mechanical integrity[J]. Acta Biomater, 2019, 97: 608-622. [3] H. Ibrahim, A. Dehghanghadikolaei, R. Advincula, et al. Ceramic coating for delayed
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instrumentation[J]. Thin Solid Films, 2019, 687: 137456. [4] J. Liu, Y. Lin, D. Bian, et al. In vitro and in vivo studies of Mg-30Sc alloys with different phase structure for potential usage within bone[J]. Acta Biomater, 2019. [5] A. Santos-Coquillat, M. Esteban-Lucia, E. Martinez-Campos, et al. PEO coatings design for Mg-Ca alloy for cardiovascular stent and bone regeneration applications[J]. Mat. Sci. Eng. C, 2019, 105: 110026. [6] A. Mahajan, S.S. Sidhu. In vitro corrosion and hemocompatibility evaluation of electrical discharge treated cobalt–chromium implant[J]. J. Mater. Res, 2019, 34(8): 1363-1370. [7] G. Song, S. Song. A possible biodegradable magnesium implant material. Advanced Engineering Materials, 2007,9(4): 298-302
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Journal Pre-proof nano-coating on AZ91E Mg alloy staple in gastrectomy surgery[J]. Mat. Sci. Eng. C, 2019, 103: 109780. [21]S.L. Jin, Z.J. Hu, B.Y. Man, et al. Application of phosphate-containing materials affects bioavailability of rare earth elements and bacterial community in soils[J]. Sci China Technol Sc, 2019, 62(9): 1616-1627. [22]Y. Zheng, Y. Li., J. Chen, Z. Zou. Effects of tensile and compressive deformation on corrosion behavior of a Mg-Zn alloy[J]. Corros Sci, 2015, 90: 445-450. [23]Y. Lu, A.R. Bradshaw, Y.L. Chiu, I. P Jones. Effects of secondary phase and grain size on the corrosion of biodegradable Mg-Zn-Ca alloys[J]. Mat. Sci. Eng. C, 48(2015): 480-486. [24]P.R. Cha, H.S. Han, G.F. Yang. Biodegradability engineering of biodegradable Mg alloys: tailoring the electrochemical properties and microstructure of constituent phases[J]. Sci. Rep-UK, 2013, 3(4): 2367. [25]E. Zhang, L. Yang. Microstructure, mechanical properties and bio-corrosion properties of Mg-Zn-Mn-Ca alloy for biomedical application[J]. Mat. Sci. Eng. A, 2008, 497(1-2): 111-118. [26]E. Zhang, L. Yang, J. Xu, H. Chen. Microstructure, mechanical properties and bio-corrosion properties of Mg-Si (-Ca, Zn) alloy for biomedical application. Acta. Biomater, 6(2010): 1756-1762. [27]S. Zhang, X. Zhang, C. Zhao, et al. Research on an Mg-Zn alloy as a degradable biomaterial[J]. Acta Biomater, 2010, 6(2): 626-640. [28]Y. Sun, B. Zhang, Y. Wang, et al. Preparation and characterization of a new biomedical Mg-Zn-Ca alloy[J]. Mater & Design, 2012, 34: 58-64. [29]D. Vojtěch, J. Kubásek, J. Šerák, et al. Mechanical and corrosion properties of newly developed biodegradable Zn-based alloys for bone fixation[J]. Acta Biomater, 2011, 7(9): 3515-3522. [30]H. Du, Z. Wei, X. Liu, et al. Effects of Zn on the microstructure, mechanical property and bio-corrosion property of Mg-3Ca alloys for biomedical application[J]. Mater. Chem. Phys, 2011, 125(3): 568-575. [31]F. Zhang, M. Aibin, S. Dan, et al. Improving in-vitro biocorrosion resistance of Mg-Zn-Mn-Ca alloy in Hank's solution through addition of cerium[J]. J. Rare Earth, 2015, 33(1): 93-101. [32]S.A. Khan, Y. Miyashita, Y. Mutoh, et al. Influence of Mn content on mechanical properties and fatigue behavior of extruded Mg alloys[J]. Mater. Scie. Eng: A, 2006, 420(1-2): 315-321.
Journal Pre-proof [33]Cho D H, Lee B W, Park J Y, et al. Effect of Mn addition on corrosion properties of biodegradable Mg-4Zn-0.5Ca-xMn alloys[J]. J. Alloy. Compd, 2017, 695: 1166-1174. [34]Z. Li, X. Gu, S. Lou, et al. The development of binary Mg-Ca alloys for use as biodegradable materials within bone[J]. Biomaterials, 2008, 29(10): 1329-1344. [35]J.W. Seong, W.J. Kim. Development of biodegradable Mg-Ca alloy sheets with enhanced strength and corrosion properties through the refinement and uniform dispersion of the Mg2Ca phase by high-ratio differential speed rolling[J]. Acta Biomater, 11 (2015): 531-542. [36]R.C. Zeng, W.C. Qi, H.Z. Cui, et al. In vitro corrosion of as-extruded Mg-Ca alloys—the influence of Ca concentration[J]. Corros Sci, 2015, 96: 23-31. [37]Y.S. Jeong, W.J. Kim. Enhancement of mechanical properties and corrosion resistance of Mg-Ca alloys through microstructural refinement by indirect extrusion[J]. Corros Sci, 2014, 82: 392-403. [38]W.C. Kim, J.G. Kim, J.Y. Lee, et al. Influence of Ca on the corrosion properties of magnesium for biomaterials[J]. Mater Lett, 2008, 62(25): 4146-4148. [39]H.Y. Lai, J.Y. Li, et al. Effects of Sr on the microstructure, mechanical properties and corrosion behavior of Mg-2Zn-xSr alloys. J. Mater. Sci-Mater. M, 2018, 29(6): 87. [40]T.T. Sasaki, K. Oh-Ishi, T. Ohkubo, et al. Enhanced age hardening response by the addition of Zn in Mg-Sn alloys[J]. Scripta. Mater, 2006, 55(3): 251-254. [41]W. Cui, L. Xiao, W. Liu, et al. Effect of Zn addition on microstructure and mechanical properties of Mg-9Gd-3Y-0.5Zr alloy[J]. J. Mater. Res, 2018, 33(6): 733-744. [42]X.N. Gu, Y.F. Zheng. A review on magnesium alloys as biodegradable materials[J]. Front. Mater Sci, 2010, 4(2): 111-115. [43]ASTM G31-72. Standard Practice for Laboratory Immersion Corrosion Testing of 369 Metals; ASTM International: West Conshohocken, PA, USA, 2004. [44]ASTM E8-04, Standard Test Methods for Tension Testing of Metallic Materials, ASM, International, 2007. [45]H.R. Bakhsheshi-Rad, M.R. Abdul-Kadir, M.H. Idris, Farahany S. Relationship between the corrosion behavior and the thermal characteristics and microstructure of Mg-0.5Ca-xZn alloys. Corros Sci, 64(2012): 184-197. [46]M. Golrang, M. Mobasheri, H. Mirzadeh, et al. Effect of Zn addition on the microstructure and mechanical properties of Mg-0.5Ca-0.5RE magnesium alloy[J]. J. Alloy. Compd, 2020, 815: 152380.
Journal Pre-proof [47]Razzaghi M, Mirzadeh H, Emamy M. Unraveling the effects of Zn addition and hot extrusion process on the microstructure and mechanical properties of as-cast Mg–2Al magnesium alloy[J]. Vacuum, 2019, 167: 214-222. [48]Honma T, Ohkubo T, Kamado S, et al. Effect of Zn additions on the age-hardening of Mg-2.0 Gd-1.2 Y-0.2 Zr alloys[J]. Acta. Mater, 2007, 55(12): 4137-4150. [49]J. Wang, W. Jiang, Y. Ma, et al. Substantial corrosion resistance improvement in heat-treated Mg–Gd–Zn alloys with a long period stacking ordered structure[J]. Mater. Chem. Phys, 2018, 203: 352-361.
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Fig. 1. Optical micrographs of the Mg-xZn-0.1Mn-0.2Ca alloys: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 2. SEM surface microstructures of the Mg-xZn-0.1Mn-0.2Ca alloys and EDS results for the marked positions: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 3. X-ray diffraction patterns of the experimental alloys, wherein Mg, Mg2Ca and Ca2Mg6Zn3 are detected.
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Fig. 4. TEM bright field micrograph and the corresponding SAED pattern of the Mg-2Zn-0.1Mn-0.2Ca alloy.
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Fig. 5. Yield strength, ultimate tensile strength and elongation at break of the Mg-xZn-0.1Mn-0.2Ca alloys measured by tensile tests at ambient temperature.
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Fig. 6. Variation of pH tested in the SBF during 144h immersion tests of the Mg-xZn-0.1Mn-0.2Ca alloys.
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Fig. 7. Corrosion rates of the Mg-xZn-0.1Mn-0.2Ca alloys calculated by the weight loss and Tafel curves.
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Fig. 8. Surface morphologies of the Mg-xZn-0.1Mn-0.2Ca alloys observed by SEM after 10 d of immersion in the SBF: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 9. 3D corrosion appearances of the Mg-xZn-0.1Mn-0.2Ca alloys after removing the corrosion products: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 10. Potentiodynamic polarization curves of the Mg-xZn-0.1Mn-0.2Ca alloys in the SBF.
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Fig. 11. Three-dimensional surface distribution of (a) altitude and (b) potential, plane distribution of (c) altitude and (d) potential on the same region of Mg-1Zn-0.1Mn-0.2Ca alloy measured by AFM; (e) Topography and potential profile of line as indicated in (a), (b), (c) and (d).
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Fig. 12. Schematic for the corrosion process of Mg-Zn-Mn-Ca alloys in SBF: (a) the SBF reacted with the surface oxidation layer; (b) the SBF attacked the Mg matrix, the Mg2Ca phase dissolved while the Ca2Mg6Zn3 phase remained; (c) the corrosion process proceeded and the Ca2Mg6Zn3 phase shed from the Mg matrix.
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Table 1 Chemical composition of the Mg-xZn-0.2Ca-0.1Mn alloys (wt.%)
Alloy
Mg
Zn
Ca
Mn
Al
Cu
Fe
Ni
Mg-0.2Zn-0.2Ca-0.1Mn
Bal.
0.24
0.20
0.066
0.065
＜0.001
0.025
＜0.001
Mg-0.5Zn-0.2Ca-0.1Mn
Bal.
0.51
0.22
0.070
0.058
＜0.001
0.033
＜0.001
Mg-1Zn-0.2Ca-0.1Mn
Bal.
1.00
0.21
0.071
0.072
＜0.001
0.018
＜0.001
Mg-2Zn-0.2Ca-0.1Mn
Bal.
1.95
0.22
0.068
0.061
＜0.001
0.026
＜0.001
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Table 2 Chemical composition of the Kokubo simulated body fluid (SBF) and human blood plasma.
Ion concentration (mmol/L) Solution Na+
K+
Ca2+
Mg2+
HCO3-
Cl-
HPO42-
SO42-
Human plasma
142.0
5.0
2.5
1.5
27.0
103.0
1.0
0.5
Kokubo SBF
142.0
5.0
2.5
1.5
4.2
147.8
1.0
0.5
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Table 3 Electrochemical parameters of the Mg-xZn-0.1Mn-0.2Ca alloys in SBF obtained from polarization tests.
Mg-0.2Zn-0.1 Mn-0.2Ca Mg-0.5Zn-0.1 Mn-0.2Ca Mg-1Zn-0.1M n-0.2Ca Mg-2Zn-0.1M n-0.2Ca
Pi
Ecorr
Icorr
βc
βa
RP
(VSCE)
(μA·cm-2)
(mV/decade)
(mV/decade)
(kΩ·cm2)
-1.816
322.595
-139.347
110.358
0.71
7.37
-1.809
285.797
-120.85
100.548
0.91
6.53
-1.804
266.573
-117.121
99.684
1.09
6.09
-1.824
378.391
-162.305
127.22
0.68
8.65
(mm/yea r)
Journal Pre-proof Declaration of Interest Statement
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Journal Pre-proof Highlights
1
The novel degradable Mg-xZn-Mn-Ca alloys were designed and fabricated.
2
The secondary phase features were studied using the XRD, SEM, TEM, AFM.
3
Degradation mechanism was studied via the electrochemical and immersion tests.
4
Special anode phase was detected using the AFM/SKPFM.
Liangyu Wei, Jingyuan Li, Yuan Zhang, Huiying Lai PII:
S0254-0584(19)31255-6
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122441
Reference:
MAC 122441
To appear in:
Materials Chemistry and Physics
Received Date:
01 September 2019
Accepted Date:
08 November 2019
Please cite this article as: Liangyu Wei, Jingyuan Li, Yuan Zhang, Huiying Lai, Effects of Zn content on Microstructure, Mechanical and Degradation Behaviors of Mg-xZn-0.2Ca-0.1Mn Alloys, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122441
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Journal Pre-proof Effects of Zn content on Microstructure, Mechanical and Degradation Behaviors of Mg-xZn-0.2Ca-0.1Mn Alloys Liangyu Wei1, Jingyuan Li1*, Yuan Zhang1,2*, Huiying Lai1 1
School of Materials Science and Engineering, University of Science and Technology Beijing (USTB), Beijing 100083, China 2 School of Metallurgy and Energy, North China University of Science and Technology (NCST), Tangshan, 063210, China
Abstract： In this study, the microstructure characterization, mechanical properties and degradation behaviors of the Mg-xZn-0.1Mn-0.2Ca alloys with various Zn contents were analyzed via the polarization test, electrochemical impedance spectroscopy and the static immersion measurements. Meantime, the surface film layer features and the phase constitutions of the degradation production were characterized via the confocal laser scanning microscopy (3D/CLSM) and scanning electron microscopy. The results indicated that the more secondary phases appeared and the grain sizes were remarkably refined with the further addition of Zn, such as the Ca2Mg6Zn3, Mg2Ca and Mg4Zn7. The mechanical and corrosion resistance of the Mg-xZn-0.1Mn-0.2Ca alloys both initially increased and then decreased as Zn contents increased. Besides, the special anodic phase has been detected in the interior of the matrix, where the Mg2Ca phase with the lower potentials can protect Mg matrix from dissolving in the simulated body fluids (SBF). However, the Ca2Mg6Zn3 phase with the high potentials can accelerate dissolution rate of the Mg matrix. The best mechanical
properties
and
corrosion
resistance
were
obtained
in
the
Mg-1Zn-0.1Mn-0.2Ca alloys, with the corrosion rate of 6.09 mm/y, exhibiting the uniform degradation morphology. Key words: Mg-Zn-Mn-Ca, Microstructure, Mechanical Properties, Degradation 1. Introduction Due to the excellent mechanical properties and functions, implant materials play an important role in the cardiovascular diseases and bone fracture
[1-5].
Currently, the
metals materials, such as the stainless steel, titanium alloys, cobalt chromium alloys, were widely used as the implant materials 1Corresponding
[6-8].
Unfortunately, their modulus was
Author: E-mail address: [email protected] (J.Y. Li) and [email protected] (Y. Zhang).
Journal Pre-proof higher than that of the human bones, leading to the stress shielding effects
[9-10].
Moreover, due to the non-biodegradability of these inert alloys, the second operation must be performed to remove them [9]. With regarding to the Polymer-based materials, they can be gradually degraded in the human body with the immersion time extended. However, the low mechanical strength limited its application [11-13]. To overcome and solve these drawbacks, many researches were focused on the corrosion rate and mechanical properties of the degradable Mg-based alloys [2-4, 14-19]. The density of the Mg-based alloys was close to that of human bones (about 1.7 g/cm3) and far lower than that of stainless steels, meeting the requirements of ideal orthopedic implants. Besides, Mg-based alloys were different from other inert metals in that it can be gradually degraded in physiological environment, and its degradation products can be absorbed by the body during fracture healing [1-4]. Furthermore, the Mg element acting as a basic element of human body was also participated in the metabolic functions and has a better biological safety. However, the fast corrosion rate of Mg-based alloys in the human physiological environment not only causes the loss of mechanical integrity, but also causes the local alkalinity poisoning
[20].
As a
result, developing new corrosion resistant biomedical magnesium alloys is the focus research area of biomedical magnesium alloys. Previous studies
[2-4, 21]
have been indicated that Mg-based alloys containing Al
and rare earth elements (REs) were harmful to the human body, due to its potential toxic. Thus, the selection criteria of the alloying elements must be cautious and scientific. The essential elements of the Zn, Mn, Ca and Sr with various contents have been added into the Mg alloys to develop suitable implant materials
[22-31].
Previous
literatures have showed that the Mn element can refine the grain size, remove the heavy metal elements and eliminate the harmful intermetallic compounds
[32-33].
Alloying element of Ca can improve the metallurgical quality of magnesium alloys as well as refine the grain-size [34-38]. The effect of Sr addition also has a beneficial effect on the microstructure, mechanical properties and biological corrosion behaviors of Mg-2Zn alloys in Kokubo’s solution
[39].
Moreover, the studies further indicated that
the Zn element was involved into the reproductive, genetic, immune, endocrine and other physiological processes, which can also significantly improve the service performance of the Mg matrix alloys. Similarly, T.T. Sasaki
[40]
found that the age
hardening reponse of Mg-Sn was enhanced by addition of Zn. The increase in
Journal Pre-proof hardness is mainly attributed to the homogeneous distribution, the refinement of Mg2Sn precipitates and the increase in the number of precipitates lying on non-basal planes. W.D. Cui [41] stated that the fine lamellar long period stacking ordered (LPSO) structure formed inside of the Mg-9Gd-3Y-0.5Zr alloys and its quantity increases with raising Zn content. Peak-aged Mg-9Gd-3Y-0.5Zn-0.5Zr alloy exhibited a desirable mechanical property. The in vivo corrosion and the biocompatibility of Mg-Zn-Ca alloy were also analyzed by X.N. Gu[42]. The results showed that the extruded Mg-Zn-Ca alloy provided sufficient biocompatibility for orthopedic application, though the in vivo corrosion rate should be further reduced for clinical use. Mentioned above, the three-dimensional (3D) degradation morphology, special Volta potentials distribution and degradation mechanism of the Mg-Zn-Mn-Ca in the simulated body fluids have not been systematically investigated. As a result, the Mg-0.1Mn-0.2Ca alloys with various Zn addition were explored the degradation mechanism, which was characterized using the static immersion and electrochemical measurements. 2. Experimental and Methods 2.1. Materials preparation and materials characterization The studied alloys with the nominal composition of the Mg-xZn-0.2Ca-0.1Mn (x = 0.25, 0.5, 1.0 and 2.0 wt.%) were fabricated using the electromagnetic induction melting method, with the raw materials of high-purity Mg ingots (99.94 wt.%), high-purity Zn ingots (99.99 wt.%), Mg-20Ca (99.89 wt.%) and Mg-5Mn (99.81%). All raw materials were received from Global Jinxin International Technology Co., Ltd, Beijing, China. Prior to the melting process, it is necessary to wash the crucible with pure Mg ingots to prevent the incorporation of the impurities. When the raw materials are all loaded into the crucible, the lid is closed. Subsequently, the vacuum pumping system was performed and the argon was simultaneously charged until the positive pressure condition is reached. Raw materials were melted at approximately 750°C and the alloys at this temperature were maintained for approximately 30 min. Afterwards, the melts were poured into a cylinder-shaped graphite crucible, which was preheated at 250℃. The actual chemical compositions were analyzed using the ICP-AES (Varian715-ES), which was listed in Table 1. The ingots were homogenized at 420 ℃ for 18 h and then water quenched. For the microstructure observations, the specimens were grounded with the abrasive
Journal Pre-proof papers from #2000 to #5000 in order to obtain a mirror-like surface. Subsequently, the samples were etched using the mixed picric acid solution (10 mL acetic acid + 5 g picric acid + 100 mL ethyl alcohol + 10 mL deionized water). The microstructure was characterized using the optical microscopy (Leica, DM2500-M) and scanning electron microscopy together with energy dispersive spectroscopy (Zeiss Auriga). Moreover, the phase composition and the surface potentials were proven using the X-ray diffraction (Smart-Lab), TEM (Tecnai G2) and the atomic force microscopy (AFM, MFP 3D Infinity). 2.2. Static soaking test Samples for immersion tests were cut into cylinders with the dimensions of φ10 mm × 5 mm. The soaking tests were conducted in the simulated body fluids (SBF) at the 37.5°C using a thermostatic water bath kettle. The chemical compositions comparison between SBF and human blood plasma was summarized in Table 2. Prior to the immersion test, the specimens were polished with the abrasive paper from 2000 to 5000 grits. Subsequently, specimens were washed in the acetone and ethanol solutions, respectively. Meantime, the initial mass of each specimen was the m0, which was measured using the electronic analytical balance. According to the ASTM-G31-72, the ratio of SBF solution to the samples area was the 30 mL/cm2. The specimens were suspended in the SBF with the initial pH value of 7.42 for 10 days, and the solution was refreshed every 24 h. After immersion, the surface degradation products were removed through the boiling solution (200 g/L Cr2O3+ 10 g/L AgNO3). Following, the samples were cleaned with the ultrasonic acetone and ethanol, and then dried with a hair dryer. Therewith, the final mass of each sample was recorded in m1. The three-dimensional degradation topography of each sample was characterized using the confocal laser scanning microscopy (CLSM). The average corrosion rates (Pi, mm/year) of experimental samples were obtained by the weight loss (∆m = m0 – m1, g), using the following equation [43]: Pi
3650m . Mg At
(1)
Where, the ∆m is the weight loss of each experimental sample, g; ρMg is the density of Mg matrix alloys, g/cm3; A is the sample area exposed to the SBF solution, cm3; t is the immersion time, d. Another way to determine the corrosion resistance of
Journal Pre-proof specimens is the pH test. The samples and solutions prepared in this test are the same as those prepared in the immersion test. During the immersion periods, the pH value was measured every 8 h. 2.3 Electrochemical measurements The potentio-dynamic polarization tests of the Mg-bsed alloys were carried out in the SBF using the VersaSTAT 3 electrochemical workstation. Where, the platinum electrode was the counter electrode, the saturated calomel electrode and the samples were the reference electrode and working electrode, respectively. The samples were sealed with epoxy resin and left 1cm2 area exposed to the SBF. Prior to the electrochemical measurements, the sealed samples were polished with the 5000 grits abrasive-paper. The open circuit potential value was recorded until it is stable. Then, the polarization curves were measured with the scanning rate of 1 mV/s. 2.4. Mechanical properties According to ASTM-E8-04 standard[44], the tensile samples were cut into standard dog bone shape with a tensile speed of 1 mm/min (CMT5105, MTS, China). Each group was repeated three times. 3. Results and Discussion 3.1. Microstructure The optical micrographs of Mg-xZn-0.1Mn-0.2Ca alloys were depicted in Fig.1. It can be showed that Zn was an effective grain refiner which can enhance the mechanical strength and corrosion resistance of Mg alloys [40-42]. The average grain sizes of Mg-xZn-0.1Mn-0.2Ca alloys were refined from 458 μm to 272 μm as the Zn content increased from 0.2 to 2%, which was measure by the linear intercept method. Previous studies have been indicated that the both grain sizes and volume fraction of phases synergistically change the mechanical properties of Mg matrix alloys
[31].
In
Fig. 1(a) and (b), the distribution of the secondary phases was not obvious, but the presence of a small amount of precipitation at the internal grains was detected. As the Zn content reaches or exceeds 1wt. %, the grain boundaries become more obvious and intensive, as shown in Fig. 1c and Fig. 1d. The results indicate that the addition of Zn facilitated the grains refinement and the formation of secondary phases.
Journal Pre-proof The SEM morphology and EDS analysis of the Mg-xZn-0.1Mn-0.2Ca alloys were depicted in Fig. 2. It can be seen in Fig. 2 that the higher Zn content resulted in more intensive distribution of white secondary phases. These phases were distributed in the inner grains in the form of dot shape, and these phases were stripe and Y-shape along the grain boundaries. As the Zn contents increased from 0.2 to 1 wt.%, the Ca fraction (marked in yellow in the embedded tables) of the secondary phases obviously increased from 3.75 to 8.64 wt.%. Oppositely, the Zn contents in the secondary phase increased slightly from 20.36 to 23.54 wt.%, but the change was not obvious. However, with the zinc contents further addition (exceeding 1 wt.% and 2 wt.%,), the increase trend of calcium content in the Mg matrix slows down, while the zinc content significantly increases from 23.54 wt.% to 28.32 wt.%. As shown in Fig.3, the secondary phases in Mg-xZn-0.1Mn-0.2Ca alloys were identified as α-Mg, Mg2Ca and Ca2Mg6Zn3 phases. Combined with EDS and XRD results, it can be inferred that the addition of zinc promotes the formation of Mg2Ca and Ca2Mg6Zn3 compound/phases. As the zinc content reaches 1 wt.%, the amount of Mg2Ca increased significantly. When the zinc content exceeds 1 wt.% and 2 wt.%, the content of other second phases, such as Ca2Mg6Zn3, increases even more. The microstructure observation of Mg-2Zn-0.1Mn-0.2Ca was further characterized using the TEM. TEM bright field (BF) image and the corresponding selected area electron diffraction (SAED) patterns of the Mg-2Zn-0.1Mn-0.2Ca alloy were illustrated in Fig. 4. The results indicated that the precipitates were in the shape of irregular lamella, with an approximate size of 2-3 μm. The SAED patterns analysis confirm that the secondary phase was Mg4Zn7 intermetallic phase of monoclinic crystal system, with lattice parameters a = 8.499 Å, b= 6.303 Å, c = 6.753 Å, α = 117°, β = 47° and γ = 74°, respectively. 3.2. Mechanical properties The mechanical properties of test samples were depicted in Fig. 5. Sun et al. [28] reported that the minor addition of Zn into the matrix was beneficial to enhance the mechanical strength. In the present study, the strength and ductility of the Mg-xZn-0.1Mn-0.2Ca alloys both exhibited an initial increasing trend as the Zn content increased from 0.2 to 1 wt.%. The YS, UTS and elongation increased from 44.27 ± 2.12 MPa, 115.17 ± 8.71 MPa and 7.76 ± 0.08 % to 67.64 ± 6.75 MPa, 181.90 ± 9.87 MPa and 9.20 ± 1.2 %, respectively. However, as the Zn was further
Journal Pre-proof added and up to 2 wt.%, the YS, UTS and elongation were decreased to 0.71 %, 1.66% and 22.56%, respectively. As a result, the specimen with the addition of 1% Zn presented the best mechanical properties. 3.3. Experimental tests in SBF 3.3.1 Experiments of pH variation The pH value fluctuation tests in Fig. 6 were conducted in the SBF to explore the corrosion resistance of Mg matrix alloys. The results concluded that the pH value of the Mg-0.2Zn-0.1Mn-0.2Ca increased rapidly to 8.62 after 36 h immersion test, while the pH of specimens with 0.5, 1 and 2% Zn addition was 8.43, 8.35 and 8.58, respectively. During this process, the Mg matrix alloys contacts and reacts with the SBF, resulting in the formation of corrosion product layer on the surface of Mg-xZn-0.1Mn-0.2Ca alloys. As the contact areas between Mg matrix and SBF gradually decreases, less transformation of Mg atoms into Mg2+ and H2O into OHoccurred. The dissolution rate of the Mg matrix decreased as the protection of corrosion product layer increased. The increment speed of pH values in the later 36-144 hours was also lowered. Lower pH values represented that the less Mg matrix was corroded and less H2O was transformed into OH-. Therefore, the best corrosion resistance was obtained in Mg-1Zn-0.1Mn-0.2Ca alloy. 3.3.2 Soaking test Corrosion rates of the Mg-xZn-0.1Mn-0.2Ca alloys were displayed in Fig. 7, which were calculated by the weight loss and Tafel curves. The results illustrated that the experimental alloys with 0.2 and 2 wt.% Zn addition exhibited a higher corrosion rate than the other two alloys with 0.5 and 1 wt.% Zn addition. In the weight loss test, the corrosion rate of the Mg-xZn-0.1Mn-0.2Ca alloys first decreased from 6.52 to 5.33 mm/year with Zn content increased from 0.25 to 1 wt.%. And then, the corrosion rate increased from 5.33 to 7.56 mm/year with further Zn added up to 2 wt.%. In the Tafel test, the corrosion rate has the same variety trend compared to the weight-loss test. Among all of the specimens, the Mg-2Zn-0.1Mn-0.2Ca alloy has the maximum corrosion rate of 7.56 and 8.65 mm/year, which was calculated by weight loss and Tafel curves, respectively. Based on these results, it indicated that the Mg-1Zn-0.1Mn-0.2Ca alloy exhibited the best corrosion resistance, with the corrosion
Journal Pre-proof rate of 5.33 and 6.09 mm/year, respectively. The result was in accordance with the pH value change test. The surface degradation morphology of the Mg-xZn-0.1Mn-0.2Ca alloys after 10 days’ immersion was depicted in Fig. 8. It can be seen that some bulk degradation products were formed on the Mg-0.2Zn-0.1Mn-0.2Ca alloy. The cracks existed on the corrosion surface of all the immersed specimens were similar, but the damage level was remarkably different. The results demonstrated that the Mg-xZn-0.1Mn-0.2Ca alloys containing 1 wt.% Zn possessed the denser degradation film layer than those containing 0.2, 0.5, and 2 wt.% Zn. The magnified picture in Fig. 8b revealed that the degradation morphology of the Mg-0.5Zn-0.1Mn-0.2Ca alloy was porous. The distribution of portion cracks on the Mg-2Zn-0.1Mn-0.2Ca alloy was shown in Fig. 8(d), which was remarked using the orange dotted lines. The hydrogen release during soaking test damages the compactness of the corrosion product layer and results in the formation of porous defects on the surface. As a result, these observations were consistent with the corrosion rate. After removal of the degradation products, the surface morphology of the sample was analyzed/characterized using the laser scanning confocal microscopy (CLSM). Fig. 9 reveals the profile curve of the surface morphology. It showed that the samples with different Zn content have the similar corrosion features and pitting corrosion. Apparent degradation micro-holes can be detected in Fig. 9(a, d), revealing that the Mg-xZn-0.1Mn-0.2Ca alloys with 0.2 and 2 wt.% Zn additions were more probable to generate the localized corrosion. It can be also seen that the Mg-1Zn-0.1Mn-0.2Ca alloy exhibited the flattest surface morphology, with the tiny corrosion pits. Due to the protection of Mg2Ca phase distributed in Mg matrix, the surface morphology of Mg-1Zn-0.1Mn-0.2Ca specimen was relatively uniform. 3.3.3 Electrochemical measurements The potentio-dynamic polarization measurements of the Mg-xZn-0.1Mn-0.2Ca in the simulated body fluids were presented in Fig. 10. The cathodic branch of the polarization curve is concerned with the reaction of H2O into the OH- and H2, while the anodic branch represents the Mg matrix dissolution behavior. The results illuminated that the Mg-xZn-0.1Mn-0.2Ca alloys with various Zn contents exhibit a similar corrosion tendency. The detailed electrochemical parameters were listed in Table 3. As the Zn addition increased, the corrosion potentials of the experimental
Journal Pre-proof alloys towards to the positive direction, implying the Zn was beneficial to improve the corrosion resistance. To sum up, the corrosion potentials of the Mg-xZn-0.1Mn-0.2Ca alloys in the simulated body fluids were ordered as follows: Mg-1Zn-0.1Mn-0.2Ca > Mg-0.5Zn-0.1Mn-0.2Ca > Mg-0.2Zn-0.1Mn-0.2Ca > Mg-2Zn-0.1Mn-0.2Ca. The corrosion rate (Pi) was obtained through the corrosion current density (Icorr), using the following equation [45]: Pi = 22.85·Icorr.
(2)
According to the equation, specimens with the lower corrosion current density has the better corrosion property. Therein, the Mg-1Zn-0.1Mn-0.2Ca alloy has the minimum corrosion current density (266.573 μA·cm−2) than that of the other three alloys. The corrosion rates calculated by corrosion current densities were 7.37, 6.09, 8.65 mm/year. The polarization resistance (RP, 1.09 kΩ·cm2) of the specimens indicated that the Mg-1Zn-0.1Mn-0.2Ca was most difficult to react with the electrolyte solutions. 4. Discussion As depicted in Fig. 1, the average grain sizes of Mg-xZn-Mn-Ca alloys decreased, which was due to the refining effect of Zn element. During the solidification process, Zn atoms could gradually diffuse to the solid/liquid interface front location, resulting in the formation of undercooling areas, which restricted the grain growth. Moreover, the increase of Zn addition led to the formation of more secondary phases. The similar results have been demonstrated on the previous literatures, such as the Mg2Ca, Ca2Mg6Zn3, Ca2Mg5Zn13, Mg12Zn13, MgZn2, et al
[3, 18-25, 30-36].
As regards to the Mn,
the solid solubility of Mn in Mg matrix was 2.2 wt.%. Besides, when the content of Mn was only 0.1 wt.%, Mn will not react with Mg. The main role of Mn was to reduce the content of impurities (Fe, Ni), so as to reduce the corrosion effect of impurities. With respect to the Ca, abundant Ca contents will react with Mg and lead to the formation of the secondary phases. When the Zn contents is in the range of 0.2-1 wt.%, Mg2Ca presented a more obviously increase tendency than other phases containing the Zn. Oppositely, when further Zn exceeding 1 but no more than 2 wt.%, the content of phases containing Zn increased more evidently than that of Mg2Ca. The tensile tests showed that the YS, UTS and EL of the Mg-xZn-0.1Mn-0.2Ca alloys increased firstly and then decreased subsequently as the Zn contents increased. The microstructure results show that the mechanical properties of the Mg-based alloys were mainly related with the grain refinement and the formation of secondary phase.
Journal Pre-proof Grain refinement was beneficial to improve the strength and ductility of Mg matrix alloys, and thus to improve the comprehensive properties of Mg matrix alloys. The mechanical properties of Mg-xZn-0.5Ca-0.5RE alloys with various Zn content was analyzed by M. Golrang [46]. The results declared that the higher Zn content limits the usability and hot working temperature range of the alloys, due to the hot shortness and intergranular brittle fracture. M. Razzaghi et al
[47]
found that the simultaneous
addition of 2% Al and 0.5% Zn is an effective way for refining of as-cast AZ20. It can enhance the mechanical properties and alter the fracture mechanism. Furthermore, the age-hardening of Mg-2.0Gd-1.2Y-0.2Zr with various Zn addition was analyzed by T. Honma
[48].
Although the addition of Zn degrades the age-hardening response, it
causes the discontinuous precipitation of a 14H-type long-period stacking phase at grain boundaries as well as within grains in the over-aged condition, which enhances the maximum tensile elongation. In this study, the addition of Zn into the Mg-xZn-0.1Mn-0.2Ca alloys refined the grains as well as prompted the formation of secondary phases, such as the Mg2Ca, Ca2Mg6Zn3 and Mg4Zn7. Due to the refinement effect of with the Zn addition, the plastic deformation caused by external force disperses in the grain refinement, resulting in the reduction of stress concentration. Grain refinement can also remarkably increase the grain boundaries densities and inhibit the dislocation movement, thus delaying the formation and expansion of the micro-cracks[49]. Therefore, the finer grains were favorable for the occurrence of grain boundary glide, which improved the ductility. In this study, the phases composition of the Mg-xZn-0.1Mn-0.2Ca alloys were confirmed containing the Mg2Ca, Ca2Mg6Zn3 and Mg4Zn7. Zhang et al.
[25]
reported
that the phases composition of Mg-Zn-Mn-Ca alloys mainly included Ca2Mg6Zn3 and Mg2Ca, which precipitated alternately along the grain boundaries and at the inner grains. The volume fraction of the phases increased as the Zn content increased. In previous study, Rad et al.
[18]
displayed that the electrochemical Volta potential of
phases were ranked as followed: Ca2Mg6Zn3 > α-Mg > Mg2Ca. The surface Volta potentials of the Mg-1Zn-0.1Mn-0.2Ca were characterized using the AFM, as shown in Fig.11. The high-altitude and high-potential parts indicated by the cyan dotted frame are Ca2Mg6Zn3, and the high-altitude and low-potential parts are Mg2Ca. The results further confirmed the similar results. The Volta potentials in Fig. 11(b) were lower than the Mg matrix and other secondary phases, which were further confirmed to be Mg2Ca. However, the phases with lower potential in the electrolyte solutions
Journal Pre-proof could act as anode, while the other phase with higher potential would act as the cathode. As a result, the α-Mg and Mg2Ca phase dissolved in preference to the Ca2Mg6Zn3 during the immersion period. It has been stated
[18, 25]
that after a period of immersion in the SBF, Mg2Ca
phases dissolved partly or completely, while Ca2Mg6Zn3 phase still existed along the grain boundaries or in the inner grains. Subsequently, the Ca2Mg6Zn3 phases would fall off as the Mg matrix around Ca2Mg6Zn3 phases was corroded. It can be reasonably deduced from these results that these phases on the experimental alloys dissolved in the following order: Mg2Ca > α-Mg > Ca2Mg6Zn3. Fig. 12 reveals the mechanism by which Mg2Ca protects the Mg matrix from corrosion, while the Ca2Mg6Zn3 accelerates the corrosion process. When the experimental specimens were immersed into the SBF, the solution firstly reacted with the surface oxidation layer, resulting in the hydroxides layer, such as Mg(OH)2, Zn(OH)2 and Ca(OH)2. Besides, the Mg matrix rapidly translated into the Mg2+ ions, while the phases that exposed to the SBF formed the micro-galvanic together with Mg matrix. In the micro-galvanic, the phase with a lower potential would act as anode, while the one with a higher potential would act as cathode. The normal potential of Mg2Ca phase is more negative than that of α-Mg. Conversely, the normal potential of Ca2Mg6Zn3 phase is more positive than that of α-Mg. Therefore, different galvanic corrosion reactions occurred between the α-Mg/Mg2Ca and α-Mg/Ca2Mg6Zn3. The main reactions between α-Mg and Mg2Ca were as follows: Anodic (Mg2Ca) reaction: Mg2Ca → 2Mg2+ + Ca 2- + 2e-
(3)
Cathodic (α-Mg) reaction: 2H2O (aq) + 2e- → H2 (g) + 2OH- (aq)
(4)
In the Ca2Mg6Zn3 phase, Ca2Mg6Zn3 phase performed as a cathode and Mg matrix as an anode. The corrosion reactions between α-Mg and Ca2Mg6Zn3 were as follows: Anodic (α-Mg) reaction: Mg → Mg2+ + 2e-
(5)
Cathodic (Ca2Mg6Zn3) reaction: 2H2O (aq) + 2e- → H2 (g) + 2OH- (aq)
(6)
Anodic dissolution occurred in preference to that of the cathode. The Mg2Ca phase dissolved firstly, following with the dissolution of α-Mg and then the shedding of Ca2Mg6Zn3 phase. Therefore, Mg2Ca phase protected the Mg matrix from corrosion by sacrificing itself. Oppositely, Ca2Mg6Zn3 phase accelerated the corrosion rate by sacrificing the Mg matrix. It can be speculated that the addition of Zn played a
Journal Pre-proof beneficial role in the formation of Mg2Ca phase (exceeding 0.2 wt.% but no more than 1 wt.%). The increment of Mg2Ca phase content prevented the Mg matrix from faster degradation. As a result, as the Zn content raised from 0.2 to 1wt.%, the corrosion rate of the experimental alloys calculated by weight loss dropped obviously from 6.52 to 5.33 mm/year, respectively. However, once the content of Zn exceeded 1 and up to 2 wt.%, other species secondary phases containing Zn, such as Ca2Mg6Zn3, increased more remarkably than Mg2Ca, and thus accelerated the dissolution rate of the Mg matrix. The corrosion rate of Mg-2Zn-0.1Mn-0.2Ca alloy measured by weight loss reached 7.56 mm/year. During the immersion period, the micro-galvanic corrosion occurred at the interface area between the Mg matrix and Ca2Mg6Zn3 secondary phase, leading to the formation of micro-cracks between them. As the corrosion process prolonged, Mg matrix around the Ca2Mg6Zn3 phase dissolved unceasingly, resulting in the extension of the cracks. The Ca2Mg6Zn3 phase eventually fell from the experimental samples when the circumambient Mg matrix was completely corroded, leaving the distinct corrosion pits on the sample surface. Furthermore, the existence of corrosion pits increased the contact area of Mg matrix and the SBF. Different from the Mg matrix around the Ca2Mg6Zn3 phase, Mg matrix around the Mg2Ca phase was protected from pit corrosion, presenting a more uniform corrosion characteristic. Degradation products were more inclined to deposit on the Mg matrix around Mg2Ca phase and form the denser protective layer comparing with that around Ca2Mg6Zn3 phase. The denser and intact degradation layer protected the experimental specimens from the rapid dissolution. This was the reason why the corrosion rate of the experimental alloys first decreased as Zn content increased from 0.2 to 1 wt.%, then increased later when Zn content continued increased to 2 wt.%. 5. Conclusions (1) With the Zn further addition, the grain sizes of the Mg-xZn-0.1Mn-0.2Ca alloys were remarkably refined, and the more e secondary phases were appeared, such as the Ca2Mg6Zn3, Mg2Ca and Mg4Zn7. (2) The mechanical and the corrosion resistance of the Mg-xZn-0.1Mn-0.2Ca both initially increased and then decreased as Zn contents increased. The Mg-1Zn-0.1Mn-0.2Ca alloy delivered the best mechanical performance of 67.64 ± 6.75 MPa for YS, 181.90 ± 9.87 MPa for UTS and 9.20 ± 1.2 % for elongation at
Journal Pre-proof break. Immersion and electrochemical test both indicated the Mg-1Zn-0.1Mn-0.2Ca alloy possessed the best the corrosion resistance, with the corrosion rate of 5.33 and 6.09 mm/year, respectively. (3) During the degradation process, the Mg2Ca phase had a lower Volta potential than the Mg matrix, which acted as the anodic and consequently protected the Mg matrix from pit corrosion, exhibiting the uniform degradation morphology. Inversely, the Volta potentials of the other Zn-containing phases/intermetallics were higher than the matrix, including the Ca2Mg6Zn3, which can act as the cathode and accelerate the degradation rate of Mg matrix. Acknowledgment This research was supported by the financial support of the National Key Research and Development Program of China (2016YFB 0700303). Reference [1] M.S. Song, R.C. Zeng, Y.F. Ding, et al. Recent advances in biodegradation controls over Mg alloys for bone fracture management: A review[J]. J. Mater. Sci. Technol, 2019, 35(4): 535-544. [2] R. Hou, J. Victoria-Hernandez, P. Jiang, et al. In vitro evaluation of the ZX11 magnesium alloy as potential bone plate: Degradability and mechanical integrity[J]. Acta Biomater, 2019, 97: 608-622. [3] H. Ibrahim, A. Dehghanghadikolaei, R. Advincula, et al. Ceramic coating for delayed
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Journal Pre-proof [33]Cho D H, Lee B W, Park J Y, et al. Effect of Mn addition on corrosion properties of biodegradable Mg-4Zn-0.5Ca-xMn alloys[J]. J. Alloy. Compd, 2017, 695: 1166-1174. [34]Z. Li, X. Gu, S. Lou, et al. The development of binary Mg-Ca alloys for use as biodegradable materials within bone[J]. Biomaterials, 2008, 29(10): 1329-1344. [35]J.W. Seong, W.J. Kim. Development of biodegradable Mg-Ca alloy sheets with enhanced strength and corrosion properties through the refinement and uniform dispersion of the Mg2Ca phase by high-ratio differential speed rolling[J]. Acta Biomater, 11 (2015): 531-542. [36]R.C. Zeng, W.C. Qi, H.Z. Cui, et al. In vitro corrosion of as-extruded Mg-Ca alloys—the influence of Ca concentration[J]. Corros Sci, 2015, 96: 23-31. [37]Y.S. Jeong, W.J. Kim. Enhancement of mechanical properties and corrosion resistance of Mg-Ca alloys through microstructural refinement by indirect extrusion[J]. Corros Sci, 2014, 82: 392-403. [38]W.C. Kim, J.G. Kim, J.Y. Lee, et al. Influence of Ca on the corrosion properties of magnesium for biomaterials[J]. Mater Lett, 2008, 62(25): 4146-4148. [39]H.Y. Lai, J.Y. Li, et al. Effects of Sr on the microstructure, mechanical properties and corrosion behavior of Mg-2Zn-xSr alloys. J. Mater. Sci-Mater. M, 2018, 29(6): 87. [40]T.T. Sasaki, K. Oh-Ishi, T. Ohkubo, et al. Enhanced age hardening response by the addition of Zn in Mg-Sn alloys[J]. Scripta. Mater, 2006, 55(3): 251-254. [41]W. Cui, L. Xiao, W. Liu, et al. Effect of Zn addition on microstructure and mechanical properties of Mg-9Gd-3Y-0.5Zr alloy[J]. J. Mater. Res, 2018, 33(6): 733-744. [42]X.N. Gu, Y.F. Zheng. A review on magnesium alloys as biodegradable materials[J]. Front. Mater Sci, 2010, 4(2): 111-115. [43]ASTM G31-72. Standard Practice for Laboratory Immersion Corrosion Testing of 369 Metals; ASTM International: West Conshohocken, PA, USA, 2004. [44]ASTM E8-04, Standard Test Methods for Tension Testing of Metallic Materials, ASM, International, 2007. [45]H.R. Bakhsheshi-Rad, M.R. Abdul-Kadir, M.H. Idris, Farahany S. Relationship between the corrosion behavior and the thermal characteristics and microstructure of Mg-0.5Ca-xZn alloys. Corros Sci, 64(2012): 184-197. [46]M. Golrang, M. Mobasheri, H. Mirzadeh, et al. Effect of Zn addition on the microstructure and mechanical properties of Mg-0.5Ca-0.5RE magnesium alloy[J]. J. Alloy. Compd, 2020, 815: 152380.
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Fig. 1. Optical micrographs of the Mg-xZn-0.1Mn-0.2Ca alloys: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 2. SEM surface microstructures of the Mg-xZn-0.1Mn-0.2Ca alloys and EDS results for the marked positions: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 3. X-ray diffraction patterns of the experimental alloys, wherein Mg, Mg2Ca and Ca2Mg6Zn3 are detected.
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Fig. 4. TEM bright field micrograph and the corresponding SAED pattern of the Mg-2Zn-0.1Mn-0.2Ca alloy.
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Fig. 5. Yield strength, ultimate tensile strength and elongation at break of the Mg-xZn-0.1Mn-0.2Ca alloys measured by tensile tests at ambient temperature.
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Fig. 6. Variation of pH tested in the SBF during 144h immersion tests of the Mg-xZn-0.1Mn-0.2Ca alloys.
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Fig. 7. Corrosion rates of the Mg-xZn-0.1Mn-0.2Ca alloys calculated by the weight loss and Tafel curves.
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Fig. 8. Surface morphologies of the Mg-xZn-0.1Mn-0.2Ca alloys observed by SEM after 10 d of immersion in the SBF: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 9. 3D corrosion appearances of the Mg-xZn-0.1Mn-0.2Ca alloys after removing the corrosion products: x = (a) 0.2, (b) 0.5, (c) 1 and (d) 2.
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Fig. 10. Potentiodynamic polarization curves of the Mg-xZn-0.1Mn-0.2Ca alloys in the SBF.
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Fig. 11. Three-dimensional surface distribution of (a) altitude and (b) potential, plane distribution of (c) altitude and (d) potential on the same region of Mg-1Zn-0.1Mn-0.2Ca alloy measured by AFM; (e) Topography and potential profile of line as indicated in (a), (b), (c) and (d).
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Fig. 12. Schematic for the corrosion process of Mg-Zn-Mn-Ca alloys in SBF: (a) the SBF reacted with the surface oxidation layer; (b) the SBF attacked the Mg matrix, the Mg2Ca phase dissolved while the Ca2Mg6Zn3 phase remained; (c) the corrosion process proceeded and the Ca2Mg6Zn3 phase shed from the Mg matrix.
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Table 1 Chemical composition of the Mg-xZn-0.2Ca-0.1Mn alloys (wt.%)
Alloy
Mg
Zn
Ca
Mn
Al
Cu
Fe
Ni
Mg-0.2Zn-0.2Ca-0.1Mn
Bal.
0.24
0.20
0.066
0.065
＜0.001
0.025
＜0.001
Mg-0.5Zn-0.2Ca-0.1Mn
Bal.
0.51
0.22
0.070
0.058
＜0.001
0.033
＜0.001
Mg-1Zn-0.2Ca-0.1Mn
Bal.
1.00
0.21
0.071
0.072
＜0.001
0.018
＜0.001
Mg-2Zn-0.2Ca-0.1Mn
Bal.
1.95
0.22
0.068
0.061
＜0.001
0.026
＜0.001
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Table 2 Chemical composition of the Kokubo simulated body fluid (SBF) and human blood plasma.
Ion concentration (mmol/L) Solution Na+
K+
Ca2+
Mg2+
HCO3-
Cl-
HPO42-
SO42-
Human plasma
142.0
5.0
2.5
1.5
27.0
103.0
1.0
0.5
Kokubo SBF
142.0
5.0
2.5
1.5
4.2
147.8
1.0
0.5
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Table 3 Electrochemical parameters of the Mg-xZn-0.1Mn-0.2Ca alloys in SBF obtained from polarization tests.
Mg-0.2Zn-0.1 Mn-0.2Ca Mg-0.5Zn-0.1 Mn-0.2Ca Mg-1Zn-0.1M n-0.2Ca Mg-2Zn-0.1M n-0.2Ca
Pi
Ecorr
Icorr
βc
βa
RP
(VSCE)
(μA·cm-2)
(mV/decade)
(mV/decade)
(kΩ·cm2)
-1.816
322.595
-139.347
110.358
0.71
7.37
-1.809
285.797
-120.85
100.548
0.91
6.53
-1.804
266.573
-117.121
99.684
1.09
6.09
-1.824
378.391
-162.305
127.22
0.68
8.65
(mm/yea r)
Journal Pre-proof Declaration of Interest Statement
The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.
Journal Pre-proof Highlights
1
The novel degradable Mg-xZn-Mn-Ca alloys were designed and fabricated.
2
The secondary phase features were studied using the XRD, SEM, TEM, AFM.
3
Degradation mechanism was studied via the electrochemical and immersion tests.
4
Special anode phase was detected using the AFM/SKPFM.