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Developing composite of ZE41 magnesium alloy- calcium by friction stir processing for biodegradable implant applications
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 270–277
www.materialstoday.com/proceedings
ICAMME-2018
Developing composite of ZE41 magnesium alloycalcium by friction stir processing for biodegradable implant applications Ch. Vasua, K. J. A. Naga Durgaa, I. Srinivasa, Shaik Dariyavalia, B. Venkateswarlub, B. Ratna Sunila,* aDepartment
of Mechanical Engineering, Rajiv Gandhi University of Knowledge Technologies (AP-IIIT), Nuzvid 521202, India bDepartment of Metallurgical and Materials Engineering, Rajiv Gandhi University of Knowledge Technologies (AP-IIIT), Nuzvid 521202, India
Abstract In the present work, ZE41Mg alloy - Ca composite was produced by friction stir processing (FSP) targeted for biodegradable implant applications. Microstructural studies revealed grain refinement in the composite as 7 µm from a starting size of 110 µm. Due to grain refinement, increased hardness was observed in the composite. Degradation behavior was analyzed by immersion method conducted in Ringers physiological solution for 168 h. From the immersion studies, it was a clear observation that the degradation rate of composite has been decreased with increase in the immersion time compare with unprocessed ZE41. pH measurements and also surface phase and morphology analysis done by X-Ray diffractometry (XRD) and scanning electron microscopy (SEM) respectively, revealed that the corrosion behavior was better for the composite compared with the unprocessed ZE41. The promising behavior of the composite can be attributed to the grain refinement, texture effect and the presence of Ca which decreased the formation of magnesium hydroxide and promoted better corrosion resistance. Hence from the preliminary results, it can be understood that ZE41-Ca composites can be successfully produced by FSP targeted for degradable implant applications with controlled degradation rate without deteriorating the mechanical properties. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engi- neering, ICAMME-2018. Keywords: ZE41; FSP; composite; biomaterial; weight loss; degradation; hardness.
1. Introduction Recently, magnesium (Mg) and its alloys have attracted the interest of materials engineers as promising candidates for degradable implant applications [1]. Biocompatibility, non-toxicity and mechanical properties close to natural human bone are the important properties make Mg as promising candidate for degradable load bearing implant applications [2]. However, higher degradation rate in the physiological environment is the important
∗
Corresponding author. Tel.: +91-9677119819, 7799111300 ; fax: +91 08656 235150. E-mail address: [email protected], [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018.
Ch. Vasu et al. / Materials Today: Proceedings 18 (2019) 270–277
271
limitation with Mg as a biomaterial which is being addressed by developing new alloys and composites, surface coatings, and microstructural modification [3]. Developing Mg based composites is an attractive alternate to introduce desired properties in developing Mg based biomaterials [3]. Liquid state methods are usual processing routes adopted to develop metal matrix composites. However, handling Mg in liquid state is difficult and also oxidation of Mg is additional issue needs keen attention in developing Mg based composites by liquid state methods. Friction stir processing (FSP), a solid state processing rout developed based on the basic principles of friction stir welding; in which a non consumable rotating rod is plunged and moved in traverse direction to cause severe plastic deformation and to modify the surface microstructure of metals without melting the substrate [4]. The mechanism behind the grain refinement during FSP can be seen elsewhere [5]. The stirring action of the FSP tool can be utilized to introduce secondary phase powder into a metallic substrate to develop metal matrix composites. In our earlier study, hydroxyapatite (HA), a well known bioceramic material was introduced into Mg and AZ31 Mg alloys and excellent bio-properties were observed [6, 7]. Sun et al., [8] produced composites of Mg–3Zn– 0.5Zr/HA (0. 0.5, 1 and 1.5 Wt.%) and decreased corrosion was reported. Similarly, Zhao et al., [9] also developed Mg-6%Zn-5%HA and increased mechanical and corrosion properties were observed. Several other authors have also reported using HA as dispersing phase to develop Mg based composites for biomedical applications [3, 10]. In developing Mg based composites for biomedical applications, it has been observed from the literature that the calcium (Ca), that is a constituting element in HA can be an effective secondary phase to introduce into Mg substrate [11-16]. On the other hand, ZE41 (Zinc 4% and rare earths 1%) is a new Mg alloy widely used in aerospace and aviation applications. Zinc is non toxic and rare earths control the degradation of Mg. Hence, in the present work, ZE41Mg alloy – Ca composites were produced by FSP and the effect of modified microstructure and added Ca on the hardness and degradation properties was assessed. 2. Experimental details ZE41 Mg alloy sheets of size 100×50×5 mm3 were cut from a cast billet purchased from Exclusive Magnesium, Hyderabad. The chemical composition of ZE41 Mg alloy is shown in Table 1. Friction stir processing was carried out using an automated universal milling machine (Bharat Fritz Werner Ltd., India). FSP tool was made of H13 tool steel consisting of a shoulder diameter of 15 mm with a tapered pin having 5 mm to an end diameter of 2 mm with a length of 3 mm. In order to produce as composite, a groove of 1 mm width and 2 mm depth was produced on the work piece as schematically shown in Fig. 1. Then the groove has been filled with Ca and the groove was closed with a pin-less FSP tool to avoid the escape of the powder particles during the process. FSP was carried out at 1400 rpm and 25 mm/min tool travel speed to produce the composite and the sample was named as ZE41-Ca. Table 1. Chemical composition of ZE41 Mg alloy used in the present work. Element (Wt. %) Ce Fe Mn Zn ZE41 Mg alloy
0.56
0.003
0.01
3.90
Zr
Si
TR*
Mg
0.56
0.005
1.10
Balance
Fig 1. a) Schematic representation showing composite fabrication by FSP (Source: Ratna Sunil et al., [7])
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Microstructural observations were carried out at the cross section of FSPed regions. The specimens were metallographically polished by using different grades of emery papers followed by diamond paste polishing. The polished specimens were cleaned ultrasonically and dried. Chemical etching was done using picric acid reagent, cleaned in ethanol and dried. Optical microscope (Leica, Germany) observations were carried out at the different regions. Microhardness (Omnitech, India) measurements were obtained across the stir zone and in the base material by applying 100 g load for 15 sec. For immersion studies, Ringer’s solution (7.8 g NaCl, 0.37 g KCl, 0.17 g CaCl2 for 1000 ml of water) was prepared with 7.3 pH (pH was maintained by using appropriate amount of diluted HCl). Each sample ( of size 10 mm × 10 mm × 4 mm) was immersed in 50 ml of Ringers solution and kept in a constant temperature water bath maintained at 37°C for up to 168 h and the weight loss was measured for all the samples at different intervals of time by removing the corrosion product by immersing them in boiling chromate solution (180 g CrO3/1000 ml of water). From the weight loss measurements before and after the immersion tests, corrosion rate (CR) of the sample was measured as per the equation given below [17]. CR (mils/year) = 534 ΔW/ ρAt
(1)
Where ΔW is weight loss in mg (weight of the sample before immersion – weight of the sample after immersion), ρ is the density in g/cc, A is the surface area of the samples before immersion (in2) and t is the immersion time in hours (h). The CR is then represented considering 1 mils/year = 0.0254 mm/year. The specimens after immersion study were characterized by X-ray diffraction method (XRD, D8 Advanced, Bruker, USA) with Cu-Kα radiation (λ = 1.54 Å) between 20 and 80 deg with a scanning rate of 1step/s and step size of 0.1°. Scanning electron microscope (FE-SEM. Carl ZEISS, Germany, operated at 30 kV) images were obtained at the surface of the immersed samples. 3. Results and discussions Fig 2 shows microstructural observations before and after FSP. The composite was successfully developed without any defect at the surface and the cross section. Fig 2 (a) shows photograph of ZE41-Ca. From the optical microscope images the starting grain size of ZE41 was measured as 110 µm. Fig 2 (c) shows different interfaces i.e nugget zone, thermo mechanically affected zone (TMAZ), and base material. As shown in Fig 2 (d) and (e) obtained at the cross section, grain refinement as well as agglomeration of Ca is evident. The grain refinement was observed up to 7 µm in the composite. Usually, in ZE41 Mg alloy, solid solution of Mg and Zn known as α-Mg and intermetallic (secondary) phase of Mg and Zn (MgZn) are the two distinct regions usually observed as shown in Fig 2 (a). The MgZn intermetallic phase is distributed at the grain boundaries as a network like structure [18] as indicated with arrows in Fig 2 (a). Interestingly, this continuous phase at the grain boundaries has been observed as completely affected and a discontinuous particle like distribution was noticed after FSP as shown in Fig 2 (e). This is similar to what earlier reported while processing AZ91 Mg alloy by FSP [19]. From the microstructral studies, grain size reduction and decreased amount of secondary phase are the two important factors observed as resulted due to FSP in the composite which certainly play a role on influencing the mechanical and corrosion behavior.
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Fig 2. a) Photograph showing FSPed ZE41-calcium composite and optical microscope images of b) ZE41 Mg alloy, c) interface of different zones after FSP, d) agglomerated calcium and e) fine grains and discontinued phase of MgZn intermetallic phase.
Fig 3 shows the XRD patterns of the samples (ZE41 and ZE41-Ca composite). All the peaks were identified and indexed. From the XRD analysis, no peak corresponding to Ca was identified. As the added Ca was lower than the detecting range, no corresponding peaks were identified in XRD of the composite. The peak intensity of (002) was observed as increased and the peak intensities for (010) and (011) were observed as decreased after FSP compared with ZE41.
Fig 3. XRD patterns of ZE41 and ZE41-Ca.
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The change in the intensities of XRD peaks indicates the development of preferred orientation after FSP which is known as texture effect. (002) peak represents the basal plane in Mg system (hcp crystal structure). It is evident from the XRD analysis that the FSP lead to develop composite with texture effect. During FSP, heat is generated and plastic deformation is initiated around the FSP tool pin. Then the material at the surface shears along the surface of the FSP tool pin and results in shear texture components of (0002) planes [20]. Fig 4 shows the hardness measurements obtained across the samples. For the measurements, increased hardness from 60.98± 5.3 Hv to 73.7± 9.9 Hv was noticed for the composite which can be attributed to the grain refinement, texture effect and the presence of the Ca as a dispersing phase. The secondary phase (MgZn) that present in ZE41 Mg alloy is brittle and hard. As the amount of this secondary phase was observed as decreased and the distribution was also completely changed from network like structure to particle like structure, the hardness should have been decreased. However, the contribution of grain boundary strengthening due to smaller grains, texture effect and the addition of Ca to increase the hardness is higher compared with the effect of decreased MgZn phase and therefore, the hardness of the composite is found to be increased .
Fig 4. Micro hardness distribution of the samples
Fig 5. Photographs of the samples after immersing in physiological solutions for different intervals of time: a) ZE41after 24 h, b) ZE41-Ca after 24 h, c) ZE-41 after 168 h and d) ZE41-Ca after 168 h
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Fig 6. SEM images of the samples after 168 h of immersion: a) ZE41 and b) ZE41-Ca
Fig 5 shows the typical photographs of the samples after immersion study. From the visual observations, the samples have found to be degraded. ZE41 samples were degraded rapidly compared with ZE41-Ca composite samples as the immersion time increased to 168 h. Appearance of large pits on unprocessed samples compared with the composites indicates higher degradation due to pitting [6, 7]. Fig 6 shows the surface morphologies of the immersed samples after 168 h of immersion. The surfaces of both the samples were found with formation of corrosion products. From the XRD analysis (Fig 7) these corrosion products were identified as magnesium hydroxide (Mg(OH)2. After 24 h of immersion the corresponding XRD patterns (Fig 7 (a)) of the samples show the presence of Mg(OH)2 on both the sample. When Mg is immersed in any aqueous solutions, (Mg(OH)2 is formed due to the corrosion reaction [1]. As observed in the XRD patterns of the samples after 168 h of immersion as shown in Fig 7 (b), the peaks corresponding to (Mg(OH)2 were higher in intensity for ZE41 compared with ZE41-Ca composite. This indicates that ZE41 has undergone higher degradation and formed more amount of (Mg(OH)2 where as the composite has relatively lower level of degradation. Table 2 shows the corrosion rates of the samples calculated from the weight loss measurements.
Fig 7. XRD patterns of samples after immersion studies: a) 24 h and b) 168 h.
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Fig 8. pH change of the solutions containing ZE41 and ZE41-Ca samples (measurements were recorded for every 12 h)
Table 2. Corrosion rate of the sample obtained from the immersion studies Corrosion rate (CR) mm/y Sample/immersion time
24 h
168 h
ZE41
4.8
5.0
ZE41-Ca composite
4.4
1.8
pH measurements obtained for every 12 h of interval time as shown in Fig 8 also indicates higher pH for the solution that contained ZE41 for 12 h compared with the solution contained ZE41-Ca composite. The increase in pH is due to the increase of the concentration of OH- ions into the solution. When Mg degrades by forming a corrosion product (Mg(OH)2 in any aqueous solution, the stability of (Mg(OH)2 layer depends on the concentration of the Clions in the solution. In the presence of Cl- ions, (Mg(OH)2 is unstable and magnesium chloride crystals (MgCl2) are formed by releasing OH- ions [3, 21]. These MgCl2 crystals are easily dissolved in the aqueous solution and the Mg substrate beneath these crystals is further exposed to the corroding environment. Therefore, degradation rate is increased and the release of OH- ions into the solution is also increased. Hence, higher pH is an indication to the higher amount of degradation as observed in Fig 8. However, after 168 h of immersion, the solution was reached to a saturation level and the pH values of both the samples were observed as almost close to each other. Hence from the results, it can be concluded that ZE41-Ca composite can be successfully developed by FSP with improved mechanical and corrosion properties for degradable implant applications. 4. Conclusions ZE41 Mg alloy – calcium (ZE41-Ca) composite was successfully fabricated by friction stir processing (FSP) at 1400 rpm and 25 mm/min tool travel speeds. Grain refinement from 110 µm to 7 µm was observed in the composite. From the XRD studies, texture effect was observed as induced in the composite. Increased hardness was observed for the composite due to grain refinement, texture and the presence of Ca. From the immersion studies carried out in Ringers solution, better corrosion resistance was observed for the composite which can be attributed to the grain size effect and the presence of Ca. pH measurements also indicate the rapid degradation during the initial 12 h of immersion for ZE41 compared with ZE41-Ca sample. Hence, from the results, it can be understood that the composites of ZE41-Ca can be produced within the solid state by FSP with enhanced mechanical and degradation properties targeted for degradable implant applications.
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References [1] F. Witte, N. Hort , C. Vogt, S. Cohen, K. U. Kainer, R. Willumeit and F. Feyerabend, Curr. Opin. Solid. St. Mater. Sci. 12 (2008) 63–72. [2] R. Zeng and W. Dietzel, Adv. Eng. Mater. 10(8) (2008) B3–B14. [3] Kirkland NT, Birbilis N. Magnesium Biomaterials Design, Testing, and Best Practice. Springer Science and Business Media, New York, USA, 2014. [4] R. S. Mishra, P. Sarathi De, Nilesh Kumar, Friction Stir Welding and Processing, second ed. Springer International Publishing, Switzerland, 2014. [5] R. S. Mishra, Z.Y. Ma, Mater. Sci. Eng. R, 50 (2005) 1–78. [6] B. Ratna Sunil, T.S.Sampath Kumar, Uday Chakkingal, V. Nandakumar and Mukesh Doble, Mater. Sci. Eng. C, 39 (2014) 315–324. [7] B. Ratna Sunil, T. S. Sampath Kumar, Uday Chakkingal, V. Nandakumar, Mukesh Doble, J. Mater. Sci.: Mater. Med., 25(4) (2014) 975– 988. [8] J. Sun, M. Chen, G. Cao, Y. Bi, D. Liu, J. Wei, J. Compos. Mater., 48 (2013) 825–834. [9] J. Zhao, Z. Yu, K. Yu, L. Chen, Adv. Mater. Res., 239–242 (2011) 1287–1291. [10] B. RatnaSunil,C.Ganapathy,T.S.SampathKumarn, UdayChakkingal, J. Mech. Behav. Biomed. Mater. 40 (2014) 178-189 [11] Z. Li, X. Gu, S. Lou, Y. Zheng, Biomaterials, 29 (2008) 1329. [12] S.G. Steinemann, Corrosion of Surgical Implants —in Vivo and in Vitro Tests, John Wiley and Sons, USA, 1980. [13] Y. Okazaki, S. Rao, T. Tateishi, Y. Ito, Mater. Sci. Eng. A 243, (1998) 250. [14] H. Wang, Y. Estrin, Z. Zúberová, Mater. Lett., 62 (2008) 2476–2479. [15] Krause, N. von der Höh, D. Bormann, C. Krause, F.W. Bach, H. Windhagen, A. Meyer-Lindenberg, J. Mater. Sci. Mater. Med., 45 (2010), 624. [16] F. Witte, Acta Biomater. 6, 2010, 1680. [17] ASTM Standard NACE TM0169/ G31 – 12a. Standard practice for laboratory immersion corrosion testing of metals. West Conshohocken, PA: ASTM International, 2012. DOI: 10.1520/G0031-12A. [18] M. M. Avedesian, H. Baker, ASM Specialty Handbook, Magnesium and Magnesium Alloys, ASM International, USA, 1999. [19] G. Surya Kiran, K.H. Krishna, S. Sameer, M. Bhargavi, B. S. Kumar, G. Mohana Rao, Y. Naidubabu, R. Dumpala R. Ratna Sunil, Trans. Nonferrous. Met. Soc. China, 27 (2017) 804-811. [20] S. Park, Y. Sato and H. Kokawa, Basal plane texture and flow pattern in friction stir weld of a magnesium alloy, Metall. Mater. Trans A 34 (2003) 987–994. [21] Y. Wang, M. Wei, J. Gao, J. Hu, Y. Zhang, Mater. Lett., 62 (2008) 2181–2184.
ScienceDirect Materials Today: Proceedings 18 (2019) 270–277
www.materialstoday.com/proceedings
ICAMME-2018
Developing composite of ZE41 magnesium alloycalcium by friction stir processing for biodegradable implant applications Ch. Vasua, K. J. A. Naga Durgaa, I. Srinivasa, Shaik Dariyavalia, B. Venkateswarlub, B. Ratna Sunila,* aDepartment
of Mechanical Engineering, Rajiv Gandhi University of Knowledge Technologies (AP-IIIT), Nuzvid 521202, India bDepartment of Metallurgical and Materials Engineering, Rajiv Gandhi University of Knowledge Technologies (AP-IIIT), Nuzvid 521202, India
Abstract In the present work, ZE41Mg alloy - Ca composite was produced by friction stir processing (FSP) targeted for biodegradable implant applications. Microstructural studies revealed grain refinement in the composite as 7 µm from a starting size of 110 µm. Due to grain refinement, increased hardness was observed in the composite. Degradation behavior was analyzed by immersion method conducted in Ringers physiological solution for 168 h. From the immersion studies, it was a clear observation that the degradation rate of composite has been decreased with increase in the immersion time compare with unprocessed ZE41. pH measurements and also surface phase and morphology analysis done by X-Ray diffractometry (XRD) and scanning electron microscopy (SEM) respectively, revealed that the corrosion behavior was better for the composite compared with the unprocessed ZE41. The promising behavior of the composite can be attributed to the grain refinement, texture effect and the presence of Ca which decreased the formation of magnesium hydroxide and promoted better corrosion resistance. Hence from the preliminary results, it can be understood that ZE41-Ca composites can be successfully produced by FSP targeted for degradable implant applications with controlled degradation rate without deteriorating the mechanical properties. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engi- neering, ICAMME-2018. Keywords: ZE41; FSP; composite; biomaterial; weight loss; degradation; hardness.
1. Introduction Recently, magnesium (Mg) and its alloys have attracted the interest of materials engineers as promising candidates for degradable implant applications [1]. Biocompatibility, non-toxicity and mechanical properties close to natural human bone are the important properties make Mg as promising candidate for degradable load bearing implant applications [2]. However, higher degradation rate in the physiological environment is the important
∗
Corresponding author. Tel.: +91-9677119819, 7799111300 ; fax: +91 08656 235150. E-mail address: [email protected], [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018.
Ch. Vasu et al. / Materials Today: Proceedings 18 (2019) 270–277
271
limitation with Mg as a biomaterial which is being addressed by developing new alloys and composites, surface coatings, and microstructural modification [3]. Developing Mg based composites is an attractive alternate to introduce desired properties in developing Mg based biomaterials [3]. Liquid state methods are usual processing routes adopted to develop metal matrix composites. However, handling Mg in liquid state is difficult and also oxidation of Mg is additional issue needs keen attention in developing Mg based composites by liquid state methods. Friction stir processing (FSP), a solid state processing rout developed based on the basic principles of friction stir welding; in which a non consumable rotating rod is plunged and moved in traverse direction to cause severe plastic deformation and to modify the surface microstructure of metals without melting the substrate [4]. The mechanism behind the grain refinement during FSP can be seen elsewhere [5]. The stirring action of the FSP tool can be utilized to introduce secondary phase powder into a metallic substrate to develop metal matrix composites. In our earlier study, hydroxyapatite (HA), a well known bioceramic material was introduced into Mg and AZ31 Mg alloys and excellent bio-properties were observed [6, 7]. Sun et al., [8] produced composites of Mg–3Zn– 0.5Zr/HA (0. 0.5, 1 and 1.5 Wt.%) and decreased corrosion was reported. Similarly, Zhao et al., [9] also developed Mg-6%Zn-5%HA and increased mechanical and corrosion properties were observed. Several other authors have also reported using HA as dispersing phase to develop Mg based composites for biomedical applications [3, 10]. In developing Mg based composites for biomedical applications, it has been observed from the literature that the calcium (Ca), that is a constituting element in HA can be an effective secondary phase to introduce into Mg substrate [11-16]. On the other hand, ZE41 (Zinc 4% and rare earths 1%) is a new Mg alloy widely used in aerospace and aviation applications. Zinc is non toxic and rare earths control the degradation of Mg. Hence, in the present work, ZE41Mg alloy – Ca composites were produced by FSP and the effect of modified microstructure and added Ca on the hardness and degradation properties was assessed. 2. Experimental details ZE41 Mg alloy sheets of size 100×50×5 mm3 were cut from a cast billet purchased from Exclusive Magnesium, Hyderabad. The chemical composition of ZE41 Mg alloy is shown in Table 1. Friction stir processing was carried out using an automated universal milling machine (Bharat Fritz Werner Ltd., India). FSP tool was made of H13 tool steel consisting of a shoulder diameter of 15 mm with a tapered pin having 5 mm to an end diameter of 2 mm with a length of 3 mm. In order to produce as composite, a groove of 1 mm width and 2 mm depth was produced on the work piece as schematically shown in Fig. 1. Then the groove has been filled with Ca and the groove was closed with a pin-less FSP tool to avoid the escape of the powder particles during the process. FSP was carried out at 1400 rpm and 25 mm/min tool travel speed to produce the composite and the sample was named as ZE41-Ca. Table 1. Chemical composition of ZE41 Mg alloy used in the present work. Element (Wt. %) Ce Fe Mn Zn ZE41 Mg alloy
0.56
0.003
0.01
3.90
Zr
Si
TR*
Mg
0.56
0.005
1.10
Balance
Fig 1. a) Schematic representation showing composite fabrication by FSP (Source: Ratna Sunil et al., [7])
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Microstructural observations were carried out at the cross section of FSPed regions. The specimens were metallographically polished by using different grades of emery papers followed by diamond paste polishing. The polished specimens were cleaned ultrasonically and dried. Chemical etching was done using picric acid reagent, cleaned in ethanol and dried. Optical microscope (Leica, Germany) observations were carried out at the different regions. Microhardness (Omnitech, India) measurements were obtained across the stir zone and in the base material by applying 100 g load for 15 sec. For immersion studies, Ringer’s solution (7.8 g NaCl, 0.37 g KCl, 0.17 g CaCl2 for 1000 ml of water) was prepared with 7.3 pH (pH was maintained by using appropriate amount of diluted HCl). Each sample ( of size 10 mm × 10 mm × 4 mm) was immersed in 50 ml of Ringers solution and kept in a constant temperature water bath maintained at 37°C for up to 168 h and the weight loss was measured for all the samples at different intervals of time by removing the corrosion product by immersing them in boiling chromate solution (180 g CrO3/1000 ml of water). From the weight loss measurements before and after the immersion tests, corrosion rate (CR) of the sample was measured as per the equation given below [17]. CR (mils/year) = 534 ΔW/ ρAt
(1)
Where ΔW is weight loss in mg (weight of the sample before immersion – weight of the sample after immersion), ρ is the density in g/cc, A is the surface area of the samples before immersion (in2) and t is the immersion time in hours (h). The CR is then represented considering 1 mils/year = 0.0254 mm/year. The specimens after immersion study were characterized by X-ray diffraction method (XRD, D8 Advanced, Bruker, USA) with Cu-Kα radiation (λ = 1.54 Å) between 20 and 80 deg with a scanning rate of 1step/s and step size of 0.1°. Scanning electron microscope (FE-SEM. Carl ZEISS, Germany, operated at 30 kV) images were obtained at the surface of the immersed samples. 3. Results and discussions Fig 2 shows microstructural observations before and after FSP. The composite was successfully developed without any defect at the surface and the cross section. Fig 2 (a) shows photograph of ZE41-Ca. From the optical microscope images the starting grain size of ZE41 was measured as 110 µm. Fig 2 (c) shows different interfaces i.e nugget zone, thermo mechanically affected zone (TMAZ), and base material. As shown in Fig 2 (d) and (e) obtained at the cross section, grain refinement as well as agglomeration of Ca is evident. The grain refinement was observed up to 7 µm in the composite. Usually, in ZE41 Mg alloy, solid solution of Mg and Zn known as α-Mg and intermetallic (secondary) phase of Mg and Zn (MgZn) are the two distinct regions usually observed as shown in Fig 2 (a). The MgZn intermetallic phase is distributed at the grain boundaries as a network like structure [18] as indicated with arrows in Fig 2 (a). Interestingly, this continuous phase at the grain boundaries has been observed as completely affected and a discontinuous particle like distribution was noticed after FSP as shown in Fig 2 (e). This is similar to what earlier reported while processing AZ91 Mg alloy by FSP [19]. From the microstructral studies, grain size reduction and decreased amount of secondary phase are the two important factors observed as resulted due to FSP in the composite which certainly play a role on influencing the mechanical and corrosion behavior.
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Fig 2. a) Photograph showing FSPed ZE41-calcium composite and optical microscope images of b) ZE41 Mg alloy, c) interface of different zones after FSP, d) agglomerated calcium and e) fine grains and discontinued phase of MgZn intermetallic phase.
Fig 3 shows the XRD patterns of the samples (ZE41 and ZE41-Ca composite). All the peaks were identified and indexed. From the XRD analysis, no peak corresponding to Ca was identified. As the added Ca was lower than the detecting range, no corresponding peaks were identified in XRD of the composite. The peak intensity of (002) was observed as increased and the peak intensities for (010) and (011) were observed as decreased after FSP compared with ZE41.
Fig 3. XRD patterns of ZE41 and ZE41-Ca.
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The change in the intensities of XRD peaks indicates the development of preferred orientation after FSP which is known as texture effect. (002) peak represents the basal plane in Mg system (hcp crystal structure). It is evident from the XRD analysis that the FSP lead to develop composite with texture effect. During FSP, heat is generated and plastic deformation is initiated around the FSP tool pin. Then the material at the surface shears along the surface of the FSP tool pin and results in shear texture components of (0002) planes [20]. Fig 4 shows the hardness measurements obtained across the samples. For the measurements, increased hardness from 60.98± 5.3 Hv to 73.7± 9.9 Hv was noticed for the composite which can be attributed to the grain refinement, texture effect and the presence of the Ca as a dispersing phase. The secondary phase (MgZn) that present in ZE41 Mg alloy is brittle and hard. As the amount of this secondary phase was observed as decreased and the distribution was also completely changed from network like structure to particle like structure, the hardness should have been decreased. However, the contribution of grain boundary strengthening due to smaller grains, texture effect and the addition of Ca to increase the hardness is higher compared with the effect of decreased MgZn phase and therefore, the hardness of the composite is found to be increased .
Fig 4. Micro hardness distribution of the samples
Fig 5. Photographs of the samples after immersing in physiological solutions for different intervals of time: a) ZE41after 24 h, b) ZE41-Ca after 24 h, c) ZE-41 after 168 h and d) ZE41-Ca after 168 h
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Fig 6. SEM images of the samples after 168 h of immersion: a) ZE41 and b) ZE41-Ca
Fig 5 shows the typical photographs of the samples after immersion study. From the visual observations, the samples have found to be degraded. ZE41 samples were degraded rapidly compared with ZE41-Ca composite samples as the immersion time increased to 168 h. Appearance of large pits on unprocessed samples compared with the composites indicates higher degradation due to pitting [6, 7]. Fig 6 shows the surface morphologies of the immersed samples after 168 h of immersion. The surfaces of both the samples were found with formation of corrosion products. From the XRD analysis (Fig 7) these corrosion products were identified as magnesium hydroxide (Mg(OH)2. After 24 h of immersion the corresponding XRD patterns (Fig 7 (a)) of the samples show the presence of Mg(OH)2 on both the sample. When Mg is immersed in any aqueous solutions, (Mg(OH)2 is formed due to the corrosion reaction [1]. As observed in the XRD patterns of the samples after 168 h of immersion as shown in Fig 7 (b), the peaks corresponding to (Mg(OH)2 were higher in intensity for ZE41 compared with ZE41-Ca composite. This indicates that ZE41 has undergone higher degradation and formed more amount of (Mg(OH)2 where as the composite has relatively lower level of degradation. Table 2 shows the corrosion rates of the samples calculated from the weight loss measurements.
Fig 7. XRD patterns of samples after immersion studies: a) 24 h and b) 168 h.
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Fig 8. pH change of the solutions containing ZE41 and ZE41-Ca samples (measurements were recorded for every 12 h)
Table 2. Corrosion rate of the sample obtained from the immersion studies Corrosion rate (CR) mm/y Sample/immersion time
24 h
168 h
ZE41
4.8
5.0
ZE41-Ca composite
4.4
1.8
pH measurements obtained for every 12 h of interval time as shown in Fig 8 also indicates higher pH for the solution that contained ZE41 for 12 h compared with the solution contained ZE41-Ca composite. The increase in pH is due to the increase of the concentration of OH- ions into the solution. When Mg degrades by forming a corrosion product (Mg(OH)2 in any aqueous solution, the stability of (Mg(OH)2 layer depends on the concentration of the Clions in the solution. In the presence of Cl- ions, (Mg(OH)2 is unstable and magnesium chloride crystals (MgCl2) are formed by releasing OH- ions [3, 21]. These MgCl2 crystals are easily dissolved in the aqueous solution and the Mg substrate beneath these crystals is further exposed to the corroding environment. Therefore, degradation rate is increased and the release of OH- ions into the solution is also increased. Hence, higher pH is an indication to the higher amount of degradation as observed in Fig 8. However, after 168 h of immersion, the solution was reached to a saturation level and the pH values of both the samples were observed as almost close to each other. Hence from the results, it can be concluded that ZE41-Ca composite can be successfully developed by FSP with improved mechanical and corrosion properties for degradable implant applications. 4. Conclusions ZE41 Mg alloy – calcium (ZE41-Ca) composite was successfully fabricated by friction stir processing (FSP) at 1400 rpm and 25 mm/min tool travel speeds. Grain refinement from 110 µm to 7 µm was observed in the composite. From the XRD studies, texture effect was observed as induced in the composite. Increased hardness was observed for the composite due to grain refinement, texture and the presence of Ca. From the immersion studies carried out in Ringers solution, better corrosion resistance was observed for the composite which can be attributed to the grain size effect and the presence of Ca. pH measurements also indicate the rapid degradation during the initial 12 h of immersion for ZE41 compared with ZE41-Ca sample. Hence, from the results, it can be understood that the composites of ZE41-Ca can be produced within the solid state by FSP with enhanced mechanical and degradation properties targeted for degradable implant applications.
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