Use of Magnesium in Orthopedics

Use of Magnesium in Orthopedics41661 Metals Technology

Filip Jakub Bedka (s151842)Georgios Pitsilis (s152087)

November 24, 2016

41661 Metals Technology Use of Magnesium in Orthopedics

Abstract

This report covers the research and literature study regarding the use of magnesium in orthopedics withan emphasis put on the WE43 alloy. Manufacturing processes are described with possible surface and heattreatments. An extensive description of microsturcture is presented, as well as dislocation mechanismsin magnesium. Strengthening methods are presented: grain refinement, precipitation hardening, workhardening and solid solution hardening. Biodegradability is described. Corrosion problem and its benefitsare thoroughly deliberated, with possible use of coating or surface treatment.

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Contents

1 Introduction 1

1.1 Magnesium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Medical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Alloying elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Manufacturing 3

2.1 Magnesium Alloys Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 WE43 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3 Joining Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Creep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.5 Heat Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.6 Surface Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.7 Thermal Treatments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Microstructure 11

3.1 Magnesium and WE43 Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.3 WE43 Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.4 Ultra Fine Grain Refinement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4 Biodegradability 17

4.1 Biodegradable properties of WE43 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5 Corrosion and Surface Treatment 18

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.2 Corrosion in Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.3 Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.4 Galvanic Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.5 Stress Corrosion and Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

5.6 Pitting Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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5.7 Intergranular Cracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.8 Coating and Surface Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6 Conclusion 21

A Appendix 24

A.1 Mg-Y Phase diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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1 Introduction

1.1 Magnesium

Magnesium is a very light metal in great quantities in earth’s oceans and subsoil. The density of pureMg is 1.73 g/cm3 and the average density of its alloys varies around 1.8 g/cm3. Due to its high ratio ofstrength to density it shows a highly increasing demand over the second half of the last century, pushingaway the aluminum which used to be the dominant material for these applications, such as car enginescomponents and aircraft propulsion system (aerospace). [1] Moreover due to its high specific strengthfurther fuel consumption and components of increased reliability can be achieved. The unalloyed Mg hasthe same high damping capacity but lower strength due to the dislocation motion [2].

The magnesium has hexagonal lattice structure with atomic diameter at 3.2 A (Armstrong). On mag-nesium phase diagrams it is pretty common to see peritectic and eutectic systems, having intermetallicparticles such as Al, Mn, Ag, Zr, Ce etc. The most frequent alloys types are WE43, WE54, A356, AZ91and EZ54. The alloys’ types can also be categorized on wrought and cast products. The cast productshas as negative point its low corrosion resistant. This situation stated until 1925 when Manganese wasfirstly added at low portion, only 0.2 wt%. Moreover a second disadvantage is that the grains of mgalloys have a tendency to enlarge and to vary on size. This tendency degrades the microstructure of thealloy creating microporosity. By adding Zr as alloying element this tendency is limited because the ZRparticles have grain refining effect over the Mg ones. But the Zr particles tend to create compounds withthe AL and Mn elements. In order to counter that and to increase the mechanical properties of the alloyprecipitation hardening and ageing has become essential for every Mg alloy.

The average Mg production per year in the 90s in the western world was 250,000 tones and until now isincreasing rapidly reaching over 900,000 tones on world scale. The demand over the past decades increaseddue to its applications on automotive and aeronautic industries. Due to its continuously improvingproperties and its lower price the mg alloys will be able to compared with the stainless steel etc. In 1808,Humphry Davy successfully extracted Mg in its pure form. The most typical form of unprocessed Mg iscalled dolomite. The most common forms are the arbonates dolomite (MgCO3 CaCO3), oxide mineralbrucite (MgO +H2O) and carnalite chloride (MgCl2 + KCl +6H2 2O) and magnesite (MgCO3). Themost common production method is the electrolytic reduction of magnesium chloride.

Figure 1: Graph of creep resistance as a function of operational temperatures for automotive components[3].

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Table 1: Mechanical Properties of the WE43 [8].

Tensile Strength [MPa] Yield Strength [MPa] Elongation [%] HV3230 178 7 85

1.2 Medical Applications

The most interesting part on magnesium is that it can be used in medical devices due to the biocompat-ibility and if it is tailored enough it can become biodegradable in the human body and eventually can bereplaced by natural tissue or by healed bone . One of its applications in medicine is as orthopedic implantdue to its load baring properties. Other applications on the human body is as compression screws, asvascular stents and as bone fixation devices [4, 5]. Metals are better candidate materials for load bearingapplications combined with biocompatibility compared to the ceramics and polymers due not only tomechanical properties but to their superior fracture toughness [5] . Moreover its ability to sustain withprecision, initially low and afterward higher dissolution rate in the human body [6] offer to the patientsa higher standard of living because it relieves them from the risk of complications form an extra surgery.The most common alloy for biomedical uses, especially for osteosynthesis, is the WE43(Electron WE43Castings) Mg alloy [7] The need of a light, relative stiff and creep resistant materials in auto industrieslead to the creation of the alloys Mg-Y-Re-Zr series [8]. At elevated temperatures, up to 250 ◦C presentspretty adequate mechanical properties and an increase in ductility and creep resistance is also noticed[8].

1.3 Alloying elements

Mg- REThe magnesium can form solid solutions with a variety of raw material elements. Furthermore themicroporosity can be suppressed by the grain boundaries. Moreover, after ageing, precipitation can beperformed inside of the grains leading also to improved mechanical properties even further. The zirconiumcombined with RE decreases the size of the grains , increasing the strength but limiting the elongation [9].

Mg- YttriumAlloys like WE43 which includes Y, Nb and Zr present good solubility with the Mg. the drawback ofusing Y is its high cost and that Y forms compounds with MgCl2. In order to hinter the latter formationthe alloys are treated in Ar and SF6 at high temperatures. Moreover alloying elements like Gadoliniumand Erbium are added in order to decrease the need Y, sustaining Y’s impact on mechanical properties.

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2 Manufacturing

2.1 Magnesium Alloys Manufacturing

To fabricate any magnesium alloy, first the pure magnesium has to be extracted from the raw form itexists in nature. There are several methods of extraction:

• Calcination

• Pidgeon Process

• Dow Process

Calcination is based on heating up the raw MgCO3 to produce MgO. After that the product is mixedwith petroleum coke to reduce it to magnesium. Pidgeon process, on the other hand, uses siliconthermicreduction. A ferrosilicon is mixed with MgO to be heated up under vacuum to obtain magnesiumvapour. Which condenses into magnesium. The last process, Dow process, is an electrolysis process. Themagnesium in raw form is treated with HCl to produce MgCl2. MgCl2 is later electrolysed to obtain, byreduction, magnesium at the cathode of the setup. [10]

Alloys are produced by melting the magnesium and adding alloying elements to the molten magnesium.Everything is stirred and given time for the dross and impurities to settle at the bottom. The samplesare next taken to check the proper compostition of the alloy and ingots are made. [11]

Regarding manufacturing processes, magnesium can be fabricated by casting or by metal working. Threemethods of casting are available:

• Sand Casting

• Die Casting

• Thixo-casting

For metal working magnesium can be fabricated by:

• Rolling

• Extrusion

• Forging

The typical microstructures after different manufacturing processes can be seen in the following figures.[11]

In the Figure 2, a casted billets are seen. The upper left microstructure shows very coarse grains, dueto the slow cooling rate. Sometimes, if the alloy contains larger amounts of alloying elements, a bigintermetallic phases can form, as seen in the upper right microsturcture, in this case a magnesium-aluminum compound. If direct chill casting is used, the grains are becoming finer, as seen in the lowerleft microstructure. If heat treatment is applied, alloying elements can precipitate and increase themechanical properties on the alloy, as seen in the lower right microstructure.

In the Figure 3 microstructures after sand casting are shown. Upper left microstructure is after typicalsand casting, upper right after additional solution heat treatment, lower left - solution heat treatmentand aging.

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Figure 2: Microstructures after casting. [11]

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Figure 3: Microstructures after sand casting. [11]

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Figure 4: Microstructures after metal working. [11]6


Figure 4 shows microstructures after metal working. Upper left is after extruding and ageing, upper rightafter rolling and annealing, lower left after hard rolling, and lower right after rolling and annealing adifferent alloy.

2.2 WE43 Fabrication

The WE43 alloy is commercially available for sale in a wrought form as extruded or forged parts. Theschematic of fabrication of a WE43 part can be seen in the Figure 5. Those steps consists of, first ofall, melting the magnesium and alloying with the alloying elements, and next direct chill casting. To getrid of the residual stresses after casting and to prevent crack formation, the part is annealed. Then thesurface is scalped as a preparation for hot rolling. The part is preheated and rolled. The final step isheat treatment. Typical heat treatments of WE43 alloy are T6 - solutionized and artificially aged or T5- artificially aged. This gives the optimal mechanical properties and the part is ready to be cut for thefinal form. [12]

Figure 5: Fabrication of WE43 alloy process chain. [12]

2.3 Joining Processes

There are several ways of magnesium joining, like welding, riveting or other mechanical fasteners, oradhesive bonding. Regarding welding, it can be done by arc welding, resistance welding or friction stirwelding. The most common methods are gas tungsten arc welding (GTAW) and gas metal arc welding(GMAW). They can be used also for repair welding and even defect removal. Magnesium alloys showgood weldability by friction stir welding, a solid state joining technique, resulting in low porosity, goodmechanical properties and minimal distortion. [12]

Some researches were conducted on GTAW welding of WE43 alloy, e.g. by A. Turowska and J. Adamiec.[3] The resulting microstructure of the weld can be seen in the Figure 6. In the picture (c) we can observenew grains strat to grow, and in the picture (d) a refined grains. As concluded in this research paperthe welds are sufficient to fulfill the requirements for the part’s operating conditions. However, if thefracture may occur it would happen in the intermetallic phase as many voids form there and on the grainboundaries. [3]

2.4 Creep

Kierzek and Adamiec (2012) performed creep test for weld joint of WE43 comparing the surface qualityof samples that had been heat treated and not. On the weld non heat treatable samples it can be seentheir poor surface quality (Figure 7) . Many cracks are located in grain boundaries, presenting highcreep rates. On contrary the samples that were heat treated present the opposite result. Due to the finedispersion of the Mg12NdY in the Mg matrix, the dislocation are hindered, so the creep resistance isrisen. According to the Table 2 the samples that heat-treated by the T6 process have deformed less. Asexpected, the strain values are higher on elevated temperatures while on the HT samples the strain canbe four times smaller.

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Figure 6: Welded joint of WE43 alloy: (a) macrostructure, (b) microstructure of the base material, (c)microstructure in the heat affected zone, (d) grain refinement in the weld area. [3]

Table 2: Creep test with and without T6 heat treatment [8]

Figure 7: Microstructure of weld σ=90 MPa ,T= 250oC a) cracks in weld , b) no cracks in heat treatedsample [8]

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2.5 Heat Treatment

In order to increase the creep resistance, heat treatments can occur. According to the manufacturer’srecommendation [13] the T6 HT is the one appropriate. T6 includes the process of solution heat treatmentfor 8 hours at 525oC and then air ageing at 250oC for 16 hours. The results are grain growth anddissolution of the precipitation phases as it is clear from the Figure 8.

Figure 8: a) Structure of WE43 without heat treatment b) Structure of WE43 with T6 heat treatment[8]

2.6 Surface Treatments

The surface treatment techniques aim to protect the material form the environment and to decrease thecorrosion rate. In order to fulfill the biodegradable role the WE43 the formed coating must be has slowcorrosion rate at early stage which will be increasing progressively by the time in order to be absorbedby the human body.

The oxide layers which protect the surface from the corrosion are MgO and crystalline Y2O3 [7]. Theinitial low corrosion rate is resulted due to the progressively penetration of the protective layer. Thecorrosion rate increases gradually where more and more parts of the surface are removed. The corrosionresistance is depended from the layer thickness. The yttrium is depleted below the oxide layer andimproves the corrosion resistance of the material. The negative of point of the alloying is the decrease ofthe ductility which can be countered by the strengthening techniques.

2.7 Thermal Treatments

Thermal treatments like recrystallization and ageing h.t. can increase the ductility in terms of elongationto fracture up to 25%, but the yield stress will be minor decreased. The latters are result of the smallgrain growth and to the easier dislocation propagation under the recrystallization process.

The high strength of the alloy WE43 is owed to the precipitation hardening and the T6 heat treatment.The precipitation hardening happens at 210oC during the artificial ageing and express the precipitationof β” to β’.Worth noticing is that there are 4 precipitates starting from the solid solution [14]. The givingphases are β”, β’, β1 and β. β” is a metastable phase, Hexagonal structure and coherent with the -Mgmatrix. β’ is also a metastable phase but semi coherent with the matrix, having chemical compositionMg12Nd. β1 is an intermediate phase having face centered structure and chemical composition Mg3Y.Finally the last precipitate is Mg14Nd2Y and has FCC structure. I.Peter and C.Castella et al. [14] carriedout additional heat treatments with varying ageing temperature and ageing time, keeping the 8h/5250C

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from T6 HT, adding a quenching stage (water ,60oC) among the two stages (solution treatment andartificial ageing) and discovered that the highest hardening values can be obtained for the 210oC ageing.

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3 Microstructure

3.1 Magnesium and WE43 Alloy

In this chapter mechanical properties of Magnesium and more specifically WE43 alloy based on themicrostructure are considered. The structure of magnesium is described as well as mechanisms of defor-mation and methods how to increase the strength of the alloys.

3.2 Mechanical Properties

Magnesium and its alloy have rather high specific strength, while at the same time the weight is relativelylow. The density of magnesium is 1.7 g/cm3 and elastic modulus is 45 GPa. [15] This makes themagnesium alloys difficult to deform at low temperatures. Magnesium has a hexagonal closed-packed(HCP) structure with low number of independent slip systems. A slip system is a combination of slipplane and a slip direction. In HCP there are only basal slips available in the (0001) and {101̄0} planein 〈112̄0〉 direction, and the total number of slip systems is 3. [15, 16] According to the Taylor criterion,which requires at least five independent slip system for homogeneous plastic flow, the HCP structuredoes not comply. [17] At elevated temperatures, additional dislocation planes are thermally activated -the pyramidal plane {101̄1} in the 〈112̄0〉 direction. The hexagonal structure with indicated planes anddirections of dislocations can be seen in the Figure 9. [12] All this properties makes the manufacturingof magnesium and its alloys rather difficult. The best way to increase the strength and ductility ofmagnesium alloys is to achieve a fine microstructure. The tensile ductility increases with smaller sizesof grains. [18] As can be observed in the Figure 10, tensile ductility drastically increases with really finegrains of a size smaller than 10 µm. [16, 18]

Figure 9: HCP planes and direction of dislocation slips. [12]

While decreasing the size of grains, the yield strength increases, because of the grain boundary strength-ening. The effect was discovered by Hall and Petch in 1950s. The grain boundary strengthening orHall-Petch strengthening is based on the dislocation movement obstruction by the grain boundaries. Thegrain boundaries pin the dislocations, which accumulate causing further dislocations in neighbouringgrains. With small sizes of grains the dislocations are not being piled-up in one point, but disperse themat multiple boundaries. This requires more stress levels to cause deformation. [16] It is worth remember-ing that the inverse Hall-Petch relation exists for very small grain sizes. [19] The Figure 11 shows valuesfor yield strength for different grain sizes in magnesium alloys.

The mechanism for deformation in HCP materials, with limited slip systems, like magnesium is twinning.This can be observed in the Figure 12. [20] The twinning is indicated with blue lines. For pure magnesiumif the stress is increased the number of twinning also increases. According to Stanford et al. [21] theprecipitate particles in magnesium alloys prevent the formation of twinning as the twin growth requires

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Figure 10: Grain size relation to elongation in a pure Mg at room temperature. [16]

(a) From [16] (b) From [20]

Figure 11: Hall-Petch relation for different grain sizes.

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the twinning dislocation to propagate along the twin interface along the twinning direction. The presentprecipitate particles obstruct this movement.

Figure 12: Deformed pure magnesium samples with an applied strain of (a) 1% and (b) 4%. [20]

Another way to strengthen magnesium is through work hardening. This process increases the densityof the dislocations inside the magnesium structure. Multiple dislocations can act on each other and pinthemselves and obstruct the dislocation movement, which in result increase the strength. [16] One way toaccomplish that it to be done by equal-channel angular pressing. [22] The idea of this process is to press amagnesium sample through an L-shaped die, while the strain is applied with no change in cross-sectionaldimensions. This method also reduces the size of the grains through recrystallization during pressing.The results of the experiments with equal-channel angular pressing are shown in the Figure 13. [22] Workhardening both reduces the size of the grains in magnesium and increases the ductility as well as thestrength.

(a) Grain size reduction after equal-channel angularpressing.

(b) Stress - strain curve for different number of press-ings.

Figure 13: Results of equal-channel angular pressing. [22]

The last way to strengthen magnesium and its alloy is through solid solution strengthening and precip-itation hardening. Both methods works on similar principle of introducing internal stress hindering the

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movement of the dislocations. In solid solution strengthening a stress is generated by a size mismatchcompared to the regular crystal structure of interstitial or substitutional point defects. In precipitationhardening, the precipitates (e.g. alloying atoms) act as a pinning points. [16]

3.3 WE43 Alloy

The WE43 magnesium alloy is a suitable alloy to use as a material for implants used in orthopedics.The main area of use is osteosynthesis, which is stabilization of a fracture of a bone with the implants.WE 43 is characterized by strength, corrosion resistance and non-toxicity. [16] The alloy consists ofmagnesium, 3.7 - 4.3 wt.% of yttrium, 2.4 - 4.4 wt.% of rare earth elements like neodymium, ytterbium,erbium, dysprosium, and ∼0.4 wt.% of zirconium. [23] What makes this alloy ideal for implant use is thecorrosion properties that will be explained in detail in further chapter. Overall, WE34 corrodes relativelyfast at the beginning until a protective layer forms on the surface. [16] The high strength of WE43 alloyand similarity of the value of the Young’s modulus to the one of human bone is what makes this alloya perfect fit for osteosynthesis. The comparison of mechanical properties of the WE43 alloy to humantissues can be seen in the Figure 14. After the bone is healed magnesium dissolves in the body and thereis no need for another operation to remove the fixture. [24]

Figure 14: Mechanical properties of different tissues compared to WE43 alloy. [10]

According to K. Kubota et al., the properties of thermo-mechanical treated WE43 are as seen in theFigure ??. The tensile strength and elongation to failure is shown in regard to the temperature. WE43possesses high strength, high ductility, high creep resistance and high strain rate superplasticity, thanksto the fine precipitates and small grain size of about 1 µm. [25]

Regarding the phases of the WE43 alloy, the manufacturing method matter. Considering casting thealloy consists of a solid solution α-Mg matrix with percipitates of intermetallic phases located both atgrain boundaries and grain interiors. [26] The X-ray diffraction shows α-Mg matrix, toghther with twoother phases: Mg24Y5 and Mg41Nd5. The micrograph and diffraction profile can be seen in the Figure16. The binary phase diagram of magnesium and yttrium can be found in the Appendix A.1.

As concluded by T. Rzychoń and A. Kiełbus the microstructure of a casted WE43 alloy, beside theaforementioned α-Mg matrix, consists of irregular shaped precipitates of Mg41Nd5, rectangular particlesof MgY, particles of Mg24Y5 and longitudinal percipitates of β (Mg14Nd2Y). [26]

3.4 Ultra Fine Grain Refinement

The Mg alloy WE43 has higher mechanical properties compared to the pure Mg but with cyclic defor-mation by equal channel pressing followed by annealing at 360oC for one hour ultra-fine grains with sizesmaller than 1 µm can be produced, as seen in the Figure 17.

Fine grain structures can offer smaller grain sizes which increase the strength by 30% and the elongationby 40% [5]. According to the Table 2 the mechanical properties of the alloy are increased. Worth noticing

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Figure 15: Tensile strength and elongation to failure of WE43 as a function of temperature. [25]

(a) Micrograph of a casted WE43 alloy. (b) Diffraction profile of a casted WE43 alloy.

Figure 16: Phases of a WE43 alloy. [26]

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Figure 17: Microstructure of ultra-grained WE43 (grain size < 1µm)

is that the refined samples is that the corrosion rate is increased. The ultra-fine grained alloys is morevulnerable to corrosion by 33% material loss in a period of 7 days compared to the industrial alloy whichpresented only 17%. faster than the coarse grained due to the larger area of the grain and the interphasesboundaries in the ultra-fined samples. [27]

Table 3: Comparison of industrial WE43 and WE43 which has ultra-fine grain size below 1 µm [5]

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4 Biodegradability

4.1 Biodegradable properties of WE43

Mg alloys is one of the most common choice for the osteosynthesis as implant material. This selection isdown to three main abilities that characterize this alloy. Firstly, it is compatible to the tissue and to thebone, characterized by similar mechanical properties (tensile strength) (Table 4), secondly by tailoringits corrosion rate can be fully biodegradable to the human body (7.8pH), taking into consideration thatit is not toxic or harmful in any way. The last ability of the alloy, which makes it a very favorable choiceis its ability to stimulate the bone formation [28].

Table 4: Young’s modulus for the most common orthopedic implants, worth noticing is that the Mgalloys are closer to the bone’s modulus [16]

Implant Material Young’s Modulus [MPa]Mg alloy 41-45

Ti 110-120Co-Cr alloy 230Human bone 3-20

In order to achieve a material loss for biomedical application , a control degradation of the surfaceis needed. At early stages the degradation rate must be slow in order to assist the damaged bond andsupport it; and afterwards to be higher in order to be vanished from the body. This can be accomplished byapplying surface coating techniques [29]. Moreover for the fully bone’s recovery the mechanical propertiesof the alloy must be taken into account beside the corrosion ones. It is clearly understandable that highcorrosion rate equals to lower mechanical properties.

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5 Corrosion and Surface Treatment

5.1 Introduction

At room temperature the electrode potential for the Mg is – 2.30V. According to the electrochemicalseries its position is on the highest series making it one of least noble metals. Worth noticing is thatthe mg solution potential in dilute chloride solution is less than -1.7V thanks to the polarization thathappens in the surface with a Mg(OH)2 film. The manufacture method plays a heavily role because duringthe casting a lot of impurities can be casted which act as cathodes and will be able to create galvaniccorrosion. The impurities act as cathodic cells on the anodic magnesium matrix leading to a rise of thecorrosion rate. The Ni, Cu and the iron particles are the most common impurities in casting Mg alloys.In order to eliminate the latter problem, one solution is the addition of Manganese particles which createintermetallic compounds with the iron particles positioning them in the bottom of the melt. Moreover,by adding zirconium particles the results are similar. One other approach to the corrosion problem is toage threaten the alloy according to T6 process. Furthermore the creation of Zr-alloys and casting insteadof wrought products can also minimize the corrosion development. The corrosion is an electrochemicalprocess which is characterized by the loss of material usually starting from the surface. An electrolytein liquid form, specifically the blood consist the environment of the material. Corrosion tends to happenwhen the surrounding environment is too aggressive for the Mg alloy. Then the elements of the materialwhich form a protective –passive layer, are not in sufficiently levels in order to hinder the corrosion rate.As a result, the passive film cracks and a material loss is happening. In order to characterize a materialas corrosion resistant must has a corrosion rate less than 0.1 mm/year. Concerning the We43 whichis biodegradable the corrosion rate must be tailored with high precision, offering low corrosion rate atthe initial stage in order to assist the healing and strengthening phase of the bone and afterwards athigher rates in order to be displaced from the body. The problem emanates from the difficulty of themanipulation process because the corrosion rate can be increased dramatically during a wrong period.The most common types of corrosion that affects the WE43 are the pitting and crevice corrosion, thestress corrosion cracking and the intergranular one.

5.2 Corrosion in Biodegradability

On specific pH level, the corrosion rate of the alloy must be on a certain level in order to be degradedfully on the pH level of the human body. This can be achieved by alloying with various elements or bycoating the mg surfaces. Magnesium can be dissolved in the human body with no complications becauseon average the body contains up to 28 grams of Mg. In case of high corrosion rate the implant canbe degraded without fulfilling its purpose which is to mechanically support the broken bone. Moreoverduring the dissolution of Mg, hydrogen is released and can create frustration on the damaged tissue.The most significant application of a degradable implant is the non-required removal operation, whichby its avoidance can have numerous advantages in a patient’s life. Concerning the competitive implant’smaterials, the Mg alloys present higher strengths and creep resistance compared to the polymers andextremely closer values to the human bone’s Young modulus compared with other biocompatible metalslie Titanium, stainless Steel and d Co-Cr alloys. Moreover the latter materials, which are considered as‘permanent implants’, after a period of time can release toxic debris due to the wear.

5.3 Corrosion

The corrosion type that is subjected the Mg alloys are mainly galvanic. The corrosion in aqueous solutionis happening according to:

Mg(s) + 2H2O(aq) → Mg(OH)2(s) + H2(g)

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Mg → Mg2+ + 2e−2H2O + 2e− → H2 + 2OH−

Mg2+ + 2OH− → Mg(OH)2

The formation of the magnesium hydroxide is key factor for the corrosion protection because it is themain component of the protective film. In the pH of the human body which ranges around 7.35, the Mgis actively dissolved so any formation is easy to happen. For pH values lower than 11.5, the Mg formatsMg(OH)2 a passive film which protects the bulk part (Figure 18).

Figure 18: Pourbaix diagram of Mg. Worth noticing is the passivation starts at 11.5 [16].

5.4 Galvanic Corrosion

Low solubility of Mg- galvanic cells This type of corrosion is happening when the Mg is in contact withother, less noble metal or due the existence of matrix’s impurities. On pure Mg impurities such as Fe,Nietc. can create corrosion even in extremely small portions. Moreover the low solubility of the Mg favorsthe formation of micro galvanic cells causing internal galvanic corrosion. The impurities act as cathodesat small areas, leading to dissolution of Mg [16]. One other influential factor is the grain boundaries,where acting as cathodic regions compared to the grain interior. Moreover the dislocations that exist candrop the equilibrium potential, accelerating the dissolution of Mg [16].

5.5 Stress Corrosion and Cracking

The SCC is usually affected and resulted by the hydrogen embrittlement, which tends to infiltrate tothe Mg matrix, degrading it from the inside. Hydrogen can also be diffused to the crack tips and create

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magnesium hydride leading to brittle fracture due to hydrogen embrittlement [16].Due to the load bearingapplication of the implants, stress corrosion cracking (SCC) consist one more corrosion type that can behazardous for the structure. For that reason the Zr and other RE are used as alloying elements in orderto limit the SCC effect [16].

5.6 Pitting Corrosion

The pitting corrosion is a highly localized type of corrosion that is resulted by the existence of halogenidesin the blood like chlorides which can break down the passive film. The pitting corrosion can characterizethe degradation process where the attack happens at a significant depth on the bulk material (high aspectratio). On the other hand, the crevice is defined by a narrow area where within it the corrosion happens.

5.7 Intergranular Cracking

The main cause of intergranular cracking is due to precipitation of chlorium carbides in the grain bound-aries. The most common corrosion attacked areas are the stress concentration points and regions close tothe cathodic precipitates. Also crack propagation on the film surface is able to create transgranular stresscorrosion cracking. Brittle fracture can occur due to electrochemical attacks which form pits and notches,which by their turn will assist the further dissolution of the Mg alloy [16]. In order to minimize the riskof this corrosion type a stabilizing element like Nb must be added (niobium carbide does not precipitate).For the above reason the WE43 has extremely low possibility to get degraded by intergranular corrosion.

5.8 Coating and Surface Treatment

The corrosion resistance of pure Mg can be improved at high temperatures by adding alloying elementslike Zr, like the WE43 alloy. Further improvement of corrosion resistance can be achieved by coating-surface treatment techniques [6]. Coating techniques on the Mg alloy are used broadly in order to tailorthe corrosion rate of the alloy due the formation of a protective surface oxide layer. One of them is tocover with Parylene film the mg alloy. One other and more typical approach is to immerse the alloyto hydrofluoric acid. The reaction will produce MgF2 which form a protective layer. By applying highvoltage in range of 200 to 400 Volts, a anodic film containing Mg oxide and hydroxide will be formed.The minimum corrosion rate (higher corrosion resistance) is achieved by the creation of an outer layer byoxide fusion. The process parameters for the number and the size of the pores are the anodizing current,the electrochemical potential and the immersion time. V.Birs et al. (2004) applying 10 mA/cm2 achievedto create pores with diameter 0.5 µm. At high voltages the size of the pores become too large whichresult to failure because the thin film cracked making it vulnerable to corrosion attacks.t Thus the highestresistance emanates from the small sizes and numbers of the pores. In case of cracks appeared, localizedattacks will occur, they will decrease the polarization resistance (Polarization resistance: Rp behavesas a resistor, meaning that high Rp equlas to high corrosion resistance of a metal) and undermine theoxide layer over time [7]. Moreover on WE43 particles of Y2O3 will appear and they will try to slow thedegradation (increase corrosion resistance) by increasing the polarization resistance. Coating techniqueslike covers by siloxane, phosphorylcholine (PC) and sulfobetaine (SB) are developed in order to manipulatethe corrosion rate. These macromolecules act as surface agents and can increase the biocompatibility ordecrease the corrosion rate [30].

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6 Conclusion

Summarizing, properties of magnesium alloys with a focus put on WE43 alloy were studied and deemedas suitable for medical use in orthopedics and osteosynthesis. Hexagonal closed-packed structure ofmagnesium and its microstructure were researched based on the literature review. The state-of-the-artresearch papers were studied to determine mechanical properties and strengthening methods of WE43alloy. The most important feature of WE43 alloy was investigated - corrosion. Mechanisms of corrosionand its influence on medical use were described. All in all, magnesium alloy WE43 was considered asa ideal representative of the magnesium alloys to be used in medicine, and especially in orthopedicsas an implant ore bone fracture fixture, as the second operation is eliminated because of the non-toxicdissolution in the human body.

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A Appendix

A.1 Mg-Y Phase diagram

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Documents

Use of Magnesium in Orthopedics