Mechanical properties and corrosion behavior of biodegradable Mg-Cu alloys

Abstract

Mechanical properties and in vitro corrosion behavior of Mg-Cu alloys in the simulated body fluid solution were investigated. Copper addition led to grain refinement and formation of α–Mg/Mg₂Cu eutectics, resulting in the best combination of mechanical properties for the as-cast Mg-0.5Cu alloy, for which the values of the yield stress, ultimate tensile strength, total elongation, and tensile toughness were obtained as 71 MPa, 164 MPa, 5.8%, and 7.5 MJ/m³, respectively. Hot deformation by extrusion led to fine grain sizes via dynamic recrystallization and breaking up the eutectics. Accordingly, the extruded Mg-0.1Cu alloy showed tensile toughness of ∼30 MJ/m³ and good corrosion resistance; while Mg-1.2Cu alloy with a distinct protective surface layer was also considered as a viable alternative for biomedical applications.

Keywords

magnesium biomaterials thermomechanical processing grain refinement mechanical properties corrosion behavior

Introduction

Mg alloys are increasingly used as engineering materials as well as biodegradable materials for biomedical applications.^1,2 However, the biocompatibility issues as well as too rapid degradation within the physiological environments and loss of strength are among the drawbacks of many available Mg alloys.^3,4 For instance, the presence of Al as an alloying element has been implicated in neurotoxicity and the impediment of cellular proliferation.⁵ Additionally, the incorporation of rare earth elements, which are not naturally found within the human body, has raised concerns regarding their long-term biological effects.⁶ Therefore, introducing new Mg alloys for biodegradable purposes is usually needed.

Copper exhibits a good functionality relevant to bone tissue engineering. Notably, Cu possesses antibacterial properties.^7,8 Additionally, Cu serves as an essential trace element, playing a critical role in immune system function.⁹ In bone metabolism, Cu contributes to restoring normal rates of bone resorption.¹⁰ These collective properties render Cu an attractive alloying element for addition to biodegradable Mg alloys.

Copper effect on the corrosion resistance of Mg alloys has extensively been studied. The alteration of corrosion behavior is attributed to the formation of Mg₂Cu intermetallic compound,^11–13 which acts as a protective barrier, hindering the diffusion of corrosive ions (e.g., Cl⁻) and oxygen towards the magnesium matrix, thereby retarding the overall corrosion process.¹¹ However, excessive Cu content can be detrimental due to the emergence of micro-galvanic corrosion between the Mg matrix and the Mg₂Cu phase.¹⁴ Copper additions can also improve the mechanical performance of Mg alloys, which is attributed to grain refinement and the formation of secondary phases.^15,16

While there are some reports on the effect of Cu addition to commercial Mg alloys,^15–19 the effect of Cu addition on the mechanical properties of as-cast and wrought binary Mg-Cu alloys^14,20 needs more investigation. Therefore, systematic investigation of the effect of thermomechanical processing and changing the amount of Cu on the mechanical and corrosion properties of binary Mg-Cu alloys needs more attention. Accordingly, the present work is dedicated to these subjects.

Experimental details

Materials and processing

The Mg - 20 wt% Cu master alloy and commercially pure Mg were utilized to create Mg-xCu alloys/composites, where x represents the weight percentage of Cu and the x values are 0, 0.1, 0.5 and 1.2. The raw materials were melted in an induction furnace under the protection of a CO₂ - 5% SF₆ gas and poured from ∼750 °C into a metallic mold, as shown in Figure 1(a). Since the positive effects of heat treatment have been emphasized,²¹ the homogenization treatment was conducted at 400 °C for 24 h to mitigate any chemical composition variations that may have arisen during the solidification process. Subsequently, the hot extrusion process was conducted with an extrusion ratio of 1:12 (Figure 1(b)), following a 1 h soaking of the billet at 350 °C. These temperatures were selected based on the Mg-Cu phase diagram²² (redrawn in Figure 1(c)) and previous research works on copper-containing Mg alloys.^12,15

Characterization of microstructure and mechanical properties

The microstructures were revealed using a MIRA3 TESCAN scanning electron microscope (SEM) after etching with an acetic picral solution)5 ml acetic acid, 6 g picric acid, 10 ml water, 100 ml ethanol). The average grain size was measured by the intercept method using an image analysis method. Elemental analysis was performed using an energy dispersive spectroscopy (EDS) detector of the SEM instrument. Phase analysis was carried out using a PHILIPS X-ray diffractometer with Cu-kα radiation at a scan speed of 2°/min with a step interval of 0.5°. Room-temperature tensile tests were conducted on subsize ASTM E8 specimens with a gauge length of 16 mm (Figure 1(d)) at the constant cross-head speed of 0.1 mm/min. The tensile tests were repeated three times to ensure the reproducibility of the results.

Characterization of corrosion resistance

For the electrochemical corrosion test, circular specimens were molded into epoxy resin, exposing only one side with an area of 60 mm². The working surface was grind using SiC papers up to 1000 grit and then polished. The electrochemical test was performed at 37 ± 1 °C in a beaker containing 100 ml of simulated body fluid (SBF) to stimulate the biological environment (Table 1). A Solartron potentiostat (Model SI 1287) with a standard three-electrode configuration was used, with a saturated calomel electrode as the reference, a platinum electrode as the counter, and the sample as the working electrode. In the potentiodynamic polarization test, the working electrode was initially immersed in the SBF solution for 5 min, followed by the measurement of the polarization curve using the potential range of open circuit potential ±200 mV at a scanning rate of 0.5 mV/s. After the test, SEM was used to inspect the surface of the samples. Moreover, surface elemental analysis was determined through the EDS detector.

Table 1.

Chemical composition of the SBF solution (g/L).

NaCl	NaHCO₃	KCl	K₂HPO₄.3H₂O	MgCl₂.H₂O	CaCl₂	Na₂SO₄	(CH₂OH)₃CNH₂	1 M HCl (mL)
8.035	0.355	0.225	0.231	0.311	0.292	0.072	6.118	Added until reaching pH ∼ 7.4

SBF, simulated body fluid.

Results and discussion

Microstructure of as-cast alloys

Figure 2 presents the microstructures and phases in the as-cast pure Mg and Mg-Cu alloys, utilizing optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The microstructure of pure Mg reveals a coarse grain structure with an average grain size of approximately 520 µm, as observed in Figure 2(a). This is consistent with the results of other investigations on pure Mg.²³ The corresponding XRD pattern in Figure 2(b) confirms the presence of the expected α-Mg phase. As shown in Figure 2(c), the introduction of 0.1 wt% Cu into the magnesium matrix demonstrably refines the grain size, where this figure illustrates a noticeable reduction in grain size to 380 µm on average. This grain refinement phenomenon aligns with the notion that the addition of alloying elements is quite useful for grain refinement.^24–26 Subsequent studies^11,12,27 have further corroborated the grain-refining effect of copper in Mg alloys. The SEM image of the Mg-0.1Cu sample in Figure 2(d) reveals the formation of secondary phases at grain boundaries. In this regard, the corresponding XRD pattern in Figure 2(e) depicts the presence of Mg₂Cu diffraction peaks. The formation of happens by the eutectic reaction, which can be verified using the Mg-Cu phase diagram of Figure 1(c).

Figure 2 also illustrates the influence of further Cu addition on the grain size. The addition of 0.5 wt% Cu resulted in a further reduction in the average grain size to 170 µm (Figure 2(f)). This finding corroborates a trend of continuous grain refinement with increasing copper content, compared to the effect observed with the 0.1 wt% Cu addition. Moreover, the increase in the amounts of Mg₂Cu compound (Figure 2(g)) and its diffraction intensity (Figure 2(h)) is evident. Furthermore, a finer average grain size (100 μm) was obtained following the introduction of 1.2 wt% copper (Figure 2(i)) and the formation of the Mg₂Cu compound is promoted (Figures 2(j) and 2(k)). Therefore, Cu addition leads to grain refinement and formation of Mg₂Cu compound, which might have a significant effect on the mechanical properties and corrosion behavior.

The formation of the Mg₂Cu compound by the eutectic reaction can be confirmed based on Figure 3, where the provided SEM image at high magnification reveals the morphology of a typical intergranular eutectic structure, for which the EDS analysis of Point A reveals the presence of 31.13 wt% Cu. Based on the Mg-Cu phase diagram of Figure 1(c), the eutectic point is located at 33.24 wt% Cu, which is consistent with the EDS analysis of Point A. Therefore, the observed intergranular structure is the α-Mg/Mg₂Cu eutectic structure. The EDS maps of Mg-Cu alloys are also depicted in Figure 3, revealing the enrichment of Cu (blue color) at the intergranular areas, related to the copper-rich Mg₂Cu phase in these areas in the form of the eutectic structure.

Mechanical properties of as-cast alloys

Figure 4(a) illustrates the tensile stress-strain curves and a summary of mechanical properties for the as-cast alloys is depicted in Figure 4(b). Pure Mg exhibits yield stress (YS) and ultimate tensile strength (UTS) of 27 MPa and 98 MPa, respectively, with an elongation to failure of 7.5%. The addition of 0.1 wt% and 0.5 wt% copper leads to a significant increase in YS to 31 MPa and 71 MPa, respectively. Moreover, the UTS value is enhanced to 104 and 164 MPa, respectively. These enhancements can be attributed to both grain refinement and formation of the hard particles,²⁸ which are related to the Mg₂Cu compound in this work. However, the introduction of copper also results in a decrease in elongation due to the formation of the α-Mg/Mg₂Cu eutectic structure. Accordingly, the elongation to failure decreased from 7.5% for pure Mg to 7.2 and 5.8 for Mg-0.1Cu and Mg-0.5Cu alloys, respectively.

A comparison of the tensile properties for Mg-0.5Cu and Mg-1.2Cu alloys reveals a negligible enhancement in the YS but a reduction in UTS for the latter, accompanied by a decrease in the total elongation. The enhancement of YS can be attributed to both grain refinement and an increase in the amount of Mg₂Cu compound. However, since the intergranular eutectic structures can act as easy crack growth paths during tensile deformation,²⁹ the work-hardening ability is impaired, leading to the decrease in UTS and total elongation. Accordingly, the improvement of tensile has been saturated at ∼ 0.5 wt% Cu, which can be clearly seen in Figure 4(b). This suggests that 0.5 wt% copper content represents the optimal balance between strength and ductility. Notably, Mg-0.5Cu achieves the highest UTS, nearly doubling the value observed in pure Mg. However, its total elongation is lower than pure Mg. Despite the decrease in elongation, the strength-ductility balance of Mg-0.5Cu alloy is quite superior, as can be evaluated by measuring the tensile toughness (the area under the stress-strain curve),^30–32 which has been reported in Figure 4(b). It can be seen that the tensile toughness of Mg-0.5Cu alloy is the highest among the studied alloys, depicting the positive effect of Cu addition at the optimum amount. However, high Cu addition (e.g., 1.2 wt%) tends to decrease the tensile toughness.

Microstructure of extruded alloys

Following hot extrusion, the microstructure of each alloy exhibits a remarkable structural refinement, as illustrated in Figure 5. The absence of elongated grains, coupled with a grain size refinement, signifies a substantial reduction compared to the as-cast counterparts. The measured grain sizes are also summarized in Figure 5, revealing this effect. These observations suggest the occurrence of dynamic recrystallization (DRX) triggered by the combined effects of mechanical working at hot working condition.^33–35 Another notable effect can be observed in the provided SEM images of Figure 5, revealing the breaking up of the eutectic structure toward fragmentation and dispersion of the Mg₂Cu particles. The presence of these particles also contributes to the reduction in the grain size during DRX,²⁸ and hence, the grain size of extruded Mg-Cu alloys is finer compared to the extruded pure Mg, as summarized in the grain size graph in Figure 5. On the other hand, the microstructure of extruded pure Mg reveals the presence of deformation twins; while these twin boundaries cannot be seen in the microstructure of extruded Mg-Cu alloys. This reveals the positive effect of Cu alloying in the activation of slip systems³⁶ and the formation of a more homogeneous microstructure with equiaxed morphology.

Mechanical properties of extruded alloys

Figure 6(a) illustrates the tensile stress-strain curves and a summary of the mechanical properties for the extruded alloys is depicted in Figure 6(b). The trends observed for YS and UTS mirror those of the cast counterparts, exhibiting an initial rise followed by a plateau. Notably, the strength levels nearly doubled compared to the as-cast state, reaching approximately 250 MPa for UTS in Mg-0.5Cu and Mg-1.2Cu alloys. This UTS level is significantly higher than the corresponding level for the as-cast alloys.

In contrast to the strength behavior, the changes in the total elongation displayed a distinct trend compared to the as-cast condition. Interestingly, these changes were relatively insignificant when compared to the as-cast state. This observation may be attributed to the enhanced grain homogeneity achieved through hot extrusion, particularly for alloys with higher copper content. Consequently, the elongation is more than doubled compared to those of the as-cast materials.

These findings reinforce the notion that hot extrusion serves as a viable technique for improving the mechanical properties of alloys.^37–39 This enhancement primarily stems from the refinement of the grain structure facilitated by the extrusion process as well as the breaking up of the eutectic structure toward fragmentation and dispersion of the Mg₂Cu particles. As a result, the tensile toughness values of the extruded Mg-Cu alloys are on the order of ∼ 30 MJ/m³, which is much higher compared to the highest value obtained for the as-cast counterparts, i.e., ∼ 7.5 MJ/m³ for the as-cast Mg-0.5Cu alloy.

As established by the Hall-Petch equation,^40–42 the average grain diameter (D) plays a critical role in influencing the yield strength of polycrystalline metallic materials. This equation can be represented as YS = YS₀+ K_YS/√D, where YS₀ is the intrinsic strength and K_YS is the Hall-Petch slope and represents the magnitude of the grain size effect on YS. The YS data for the studied alloys are plotted against 1/√D in Figure 7, which can be represented by a Hall-Petch line of

YS = 653 / \sqrt D + 4.13

(1)

It can be seen that the data reasonably follow a single Hall-Petch line. However, some deviations can also be seen, which might be related to the presence of the Mg₂Cu phase and texture changes. Anyway, the good correlation coefficient of (R²= 0.95) reveals that the yield stress of the alloys is mainly controlled by the grain size, which can be quantified by Equation 1.

Corrosion resistance of extruded alloys in SBF solution

Since the extruded alloys showed much better mechanical behavior, these alloys were considered for the evaluation of corrosion resistance in the SBF solution. The objective was to identify the chemical composition that exhibits an optimal synergy between mechanical performance and resistance to corrosion. Figure 8(a) presents the polarization diagram comparing extruded pure Mg with extruded Mg-Cu alloys and Figure 8(b) illustrates the correlation between the obtained corrosion current density (i_Corr) and corrosion potential (E_Corr) by the Tafel extrapolation method with the copper content in the alloys.

As the concentration of copper reaches 0.1 wt%, there is no notable alteration in E_Corr. Due to the restricted solubility of the majority of elements in crystalline magnesium, there are only slight modifications to the electrochemical potential of the magnesium phase. However, adjustments in the anodic area have resulted in a decrease in i_Corr. The Tafel slope in the anodic area (B_a) for pure Mg is measured at 431.39 mV, while for Mg-0.1Cu is recorded at 323.29 mV. Gusieva and colleagues have additionally documented the influence of copper on the anodic region.⁴³ The anodic polarization curve is postulated to correspond with magnesium dissolution. An increase in copper content is hypothesized to lead to an augmented quantity of the secondary phase (Mg₂Cu), with the majority distributed along grain boundaries. The presence of this secondary phase is anticipated to exert an influence on the corrosion rate and overall corrosion behavior of the material. Furthermore, it is postulated that the secondary phase may exhibit a dualistic role in the material's corrosion behavior:

The formation of a hydroxide layer on the surface: In accordance to Chen et al.,¹¹ the initial stages of corrosion are characterized by a rapid rise in pH due to the evolution of hydrogen gas (H₂) at the surface, which consequently leads to an accumulation of hydroxyl ions (OH⁻). Subsequently, a magnesium hydroxide (Mg(OH)₂) film rapidly forms during this immersion period. This protective film serves to hinder the further progression of corrosion. The literature suggests that a finely and continuously distributed second phase can more effectively impede the corrosion process.^44,45 On the other hand, a poorly dispersed secondary phase may worsen the process of corrosion.

Microgalvanic corrosion between Mg and Mg₂Cu: At low copper concentrations, the formation of a protective hydroxide layer outpaces microgalvanic corrosion between the Mg matrix and Mg₂Cu intermetallic phase, leading to enhanced corrosion resistance compared to pure Mg. However, as the copper content increases and the volume fraction of the Mg₂Cu phase grows, microgalvanic corrosion intensifies. This results in a significant increase in corrosion current, ultimately exceeding the protective capabilities of the hydroxide film and deteriorating the overall corrosion behavior of the Mg-Cu alloy relative to pure Mg. This trend is further corroborated by the observations presented in Figure 8. The analysis of current and corrosion potential trends reveals an initial enhancement in corrosion resistance with increasing copper content, followed by deterioration at higher copper concentrations. Researchers have reported different amounts for the optimal copper content. These observations are consistent with the findings reported by Chen et al.¹¹ and Liu et al.¹³

Microscopic analysis of the post-corrosion specimens (Figure 9) revealed extensive pitting on the pure Mg surface, indicative of significant material degradation. EDS data in Table 2 revealed the presence of calcium, phosphorus, and chlorine on the corroded surfaces, along with their corresponding molar ratios. Notably, the low concentrations of calcium and phosphorus, reflected in a Ca/P ratio of 0.49, suggest the formation of a tenuous corrosion film on the pure Mg. This ratio falls considerably short of those reported for established corrosion-resistant ones like tricalcium phosphate (Ca₃(PO₄)₂) and hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂).⁴⁶ Moreover, the observed needle-like morphology of this layer, consistent with findings by Chen et al.,¹¹ suggests an incomplete formation process, indicative of an embryonic stage and limited protective capability. These observations collectively point towards the weak corrosion resistance of pure Mg and the formation of an ineffective magnesium hydroxide (Mg(OH)₂) film. A comparative analysis of the surface morphology (Figure 9) and elemental composition (Table 2) for magnesium alloys with varying copper content revealed a progressive enhancement in corrosion resistance. Pure Mg exhibited a poorly developed surface layer, evidenced by the presence of large pits and a low Ca/P ratio. However, the introduction of copper into the magnesium matrix demonstrated a beneficial effect. The Mg-0.1Cu alloy displayed an improvement compared to pure Mg, with a decrease in the detrimental surface chlorine content. Additionally, the Ca/P ratio showed an increase, which is quite desirable. However, high-magnification electron microscopy did not reveal a distinct surface layer. These observations are also valid for the Mg-0.5Cu alloy.

Table 2.

Elemental analysis of corroded surfaces of extruded alloys.

Sample	Ca (wt%)	P (wt%)	Cl (wt%)	Molar ratio of Ca/P
Pure Mg	2.43	5.17	26.74	0.49
Mg-0.1Cu	1.83	2.85	2.46	0.85
Mg-0.5Cu	8.62	13.06	2.21	0.88
Mg-1.2Cu	17.24	20.27	2.63	1.13

This trend continued with increasing copper content. The Mg-1.2Cu alloy exhibited a significant rise in both calcium and phosphorus elements, accompanied by the absence of chlorine enrichment and a high Ca/P ratio. This elevated Ca/P ratio holds particular significance in the context of biomaterial applications, as a high Ca/P ratio indicates a good surface film. In fact, the formation of Ca₃(PO₄)₂ and hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) by increasing the Ca/P ratio is quite effective for biomedical Mg alloys.^47,48 Furthermore, electron microscopy confirmed the formation of a well-defined surface layer for the Mg-1.2Cu alloy. The observed improvement can potentially be attributed to the copper-induced reduction in grain size within the magnesium matrix. Grain refinement is known to enhance the surface energy of materials. This phenomenon translates to increased hydrophilicity, bioactivity, and improved wettability of the surface.^49–51 Grain boundaries, acting as high-energy regions, serve as preferential nucleation sites, thereby promoting the formation of a calcium phosphate (CaP) layer. Notably, the CaP layer observed on the fine-grained Mg-1.2Cu alloy in Figure 9 exhibits a more uniform morphology compared to those formed on coarser-grained alloys.

The extruded Mg-0.1Cu alloy has the lowest i_Corr (Figure 8(b)) and the highest tensile toughness value (Figure 6(b)). Therefore, it shows the best combination of mechanical properties and corrosion resistance. However, it does not form a distinct protective layer (Figure 9) with a high Ca/P ratio. Accordingly, the extruded Mg-1.2Cu alloy with a distinct protective surface layer can also be considered as a viable alternative for this purpose.

The extruded Mg-0.1Cu alloy has the lowest i_Corr of ∼30 μA/cm² among the studied Mg-Cu alloys. This value is lower than that obtained for the competitive extruded Mg-1Zn-0.5Ca-0.5Zr alloy (∼90 μA/cm²); while the latter alloy has been proven to be a viable biodegradable Mg alloy.⁴⁸ Moreover, the i_Corr value of 27 μA/cm² has been reported for the biodegradable Mg-0.24Sn-0.04Mn alloy,⁵² which is comparable to the obtained value for the Mg-0.1Cu alloy. Furthermore, the i_Corr value of 409 μA/cm² has been reported for WE43 alloy,⁵³ as one of the most famous rare earth containing Mg alloy, depicting the potential of the Mg-0.1Cu alloy in the present work.

It is also noteworthy that for evaluating the corrosion resistance of medical magnesium alloys, besides the electrochemical experiments, supplementing the immersion experiments is quite useful to illustrate the corrosion performance in the actual condition for a long time. Therefore, it is suggested to evaluate the applicability of Mg-Cu alloys by immersion experiments.

Conclusions

Mechanical properties and corrosion behavior of biodegradable Mg-Cu alloys were investigated. The following conclusions can be drawn:

In the as-cast condition, Cu addition led to grain refinement as well as the formation of α–Mg/Mg₂Cu eutectic structure, which resulted in the improvement of yield stress. However, upon Cu addition, the UTS initially increased but then decreased at high Cu concentrations and also led to a decrease in total elongation due to the formation of the intergranular eutectic structure. The interaction of these effects led to the achievement of best mechanical properties for the as-cast Mg-0.5Cu alloy, for which the values of YS, UTS, total elongation, and tensile toughness were obtained as 71 MPa, 164 MPa, 5.8%, and 7.5 MJ/m³, respectively.

Hot extrusion resulted in a remarkable grain refinement via the occurrence of DRX as well as the breaking up of the eutectic structure toward fragmentation and dispersion of the Mg₂Cu particles. This resulted in the significant improvement of mechanical properties and high tensile toughness values (on the order of ∼ 30 MJ/m³) for all Mg-Cu alloys.

The yield stress of the studied alloys was found to be mainly controlled by the grain size, for which the Hall-Petch equation of YS = 653/√D + 4.13 was proposed.

The extruded Mg-0.1Cu showed the best combination of mechanical properties and corrosion resistance. However, it did not form a distinct protective layer with high Ca/P ratio. On the other hand, the extruded Mg-1.2Cu alloy with a distinct protective surface layer was also considered as a viable alternative for biomedical applications.

Footnotes

Data availability

The authors stated that the processed data required to reproduce these findings were available in this manuscript.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethical statement

The manuscript has been prepared by the contribution of all authors, it is the original authors work, it has not been published before, it has been solely submitted to this journal, and if accepted, it will not be submitted to any other journal in any language.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Hamed Mirzadeh

Massoud Emamy

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