Effect of Mn Content on Microstructure, Mechanical Properties, and Corrosion Behavior of Mg-Zn-Gd-Y Alloy

Abstract

Based upon its excellent biocompatibility, manganese (Mn) was used as an alloying element in extruded Mg-2Zn-0.5Gd-1Y Mg alloy for biomedical applications. Besides microstructural evaluation, mechanical and corrosion properties and biocompatibility of the Mg-Zn-Gd-Y-Mn alloys have been investigated by microscopic, tensile and corrosion analyses. Microscopic analysis has revealed the effect of Mn addition prior to extrusion of Mg-2Zn-0.5Gd-1Y grains, which was observed to be refined. Furthermore, the mechanical testing indicated the enhancement of tensile strength up to 265 MPa with 20.5% elongation. The chemical degradation analyses indicated that the Mn addition has effectively reduced the corrosion rate by increasing the corrosion potential of Mg alloys in Hank’s solution, which mainly refers to the inhibitory effect of Mn on the reduced precipitation of the Mg-2Zn-0.5Gd-1Y phase. As a result, the formation of Mg (OH)₂, calcium and magnesium phosphates with small amounts of Zn, Y and Mg salts, generated after corrosion, form a dense protective layer on the surface of the Mg-alloy, which further protects from surface erosion. The cytotoxicity test has shown that magnesium alloy did not have cell toxicity, showing good cytocompatibility.

1 Introduction

Magnesium (Mg) and its alloys have received significant attention as biodegradable surgical implant materials due to their excellent biocompatibility, low density (1.75 g/cm³), and mechanical properties comparable to bone (Ref 1,2,3,4). The implantation of Mg alloy can effectively avoid the phenomenon of "stress shielding" between other implant materials and bone tissues (Ref 5). In the early 20th century, Mg was studied as a resorbable bone implant, but it was abandoned based on its rapid corrosion and poor mechanical properties in a chloride-ionized water environment. With the evolution of Mg alloying technology, the corrosion resistance and mechanical properties of Mg alloys have been significantly improved, and Mg alloys have been further progressed as bone implant materials.

Mg-Zn binary alloy has enhanced biocompatibility with suitable mechanical properties for bio-implants; however, due to the coarseness of its microstructure, the mechanical integrity of the alloy is rapidly lost at the early stages of the corrosion test, which is detrimental to the application requirements (Ref 6, 7). Rare earth metal elements tend to improve the casting properties of Mg alloys and refine the grains; in addition, rare earth elements can dissolve in the corrosion product layer, increasing its stability and further improving the corrosion resistance of the alloy (Ref 8, 9). Considering the cost-effectiveness and cytotoxicity (Ref 10), Y and Gd have been studied extensively as alloying component. Many studies have shown that Y can stabilize the corrosion product layer, which exists in the form of Y₂O₃ and can reduce the galvanic corrosion in Mg alloys (Ref 8, 11). The corrosion potential of Gd and Mg is close to (− 2.4 V), which can reduce the potential of the second phase and suppress the galvanic corrosion effect of the alloy. Meanwhile, Gd³⁺ has the tendency to replace Mg²⁺ within the corrosion product layer as the corrosion progresses, weakening the penetration effect of Cl^- and improving the corrosion resistance of Mg alloy (Ref 9). Mainly, Y and Gd are involved in the formation of the second phase in the solid solution of the Mg-Zn alloy; besides, with the decrease of the Zn/RE ratio, the composition of the second phase tends to be a phase I (Mg₃Zn₆RE), phase W (Mg₃Zn₃RE₂) and long period stacking ordered (LPSO) structure (Ref 12,13,14). Furthermore, Y and Gd can also form second phases, such as Mg₂₄Y₅ and Mg₅Gd with Mg alone (Ref 15,16,17). The role of the second phase in the corrosion process is peculiar; the second phase can improve the corrosion resistance of the alloy by reducing the galvanic coupling corrosion, while the Mg₂₄Y₅ phase leads to accelerated micro galvanic corrosion (Ref 18, 19).

Mn is a commonly used alloying element in corrosion-resistant Mg alloys, mainly relying on reducing the content of impurity elements such as Fe to improve its corrosion resistance (Ref 20, 21). Among the known impurities in Mg alloy smelting, Fe plays a detrimental role in the corrosion resistance (Ref 22,23,24). Mn, in combination, with Fe in the smelting process to form Fe-Mn binary compounds with higher density (Ref 25), thus reducing the concentration of impurity Fe and improving the corrosion resistance. Xu et al. (Ref 26) have reported that the addition of small amounts of Mn to Mg alloys significantly affected the microstructure refinement of Mg alloys, which was beneficial for improving the mechanical properties and corrosion resistance of Mg alloys. It has been shown that the addition of Mn and zinc elements to Mg alloys can accelerate the formation of Mg-containing phosphates, and trace amounts of Mn oxides participate in the formation of corrosion products, thus providing better protection to the base alloy (Ref 27, 28). Mn can influence the corrosion resistance of Mg alloys by affecting the formation of beneficial precipitated phases (Ref 29). In addition, it can promote the formation of long-period stacking ordered phase (LPSO) by increasing the layer fault probability of the α-Mg matrix, and the area fraction of the LPSO phase increases with the addition of Mn (Ref 30, 31). For Mg alloys with a low Zn/RE ratio, which cannot form LPSO phases, the effects of Mn on the formation of the second phase are discussed in this work. Hence, Mn is used as a minor alloying element in Mg-2Zn-0.5Gd-1Y in order to develop a medical Bio-Mg alloy with good biocompatibility, suitable degradation rate, and good mechanical properties and to investigate the effect of Mn on the second phase of Mg alloy and its role in improving mechanical properties and corrosion mechanism.

2 Experimental

2.1 Material Preparation

Mg alloys were developed by melting high-purity Mg (99.94 wt.%), high-purity Zn (99.99 wt.%), Mg-30 wt.% Gd master alloy (30 wt.% Gd and Balance Mg), Mg-30 wt.% Y master alloy (30 wt.% Y and Balance Mg), and Mg-15 wt.% Mn master alloy (15 wt.% Mn and Balance Mg). The raw material was heated in a vacuum furnace to 750 °C, and held for 10 minutes. Ar gas was purged as a protective gas. After all the raw materials were melted, they were subjected to electromagnetic stirring for 5 minutes to homogenize the melt, which was then poured into a crucible to form an ingot. The extruded magnesium alloy bar with diameter of 15 mm was extruded at 420 °C with an extrusion ratio of 10:1 and an extrusion speed of 1.5 mm/s. The chemical compositions of the alloys were determined by chemical analysis on an ICP (Inductive Coupled Plasma Emission Spectrometer), as listed in Table 1.

Table 1 Chemical composition of alloys

2.2 Microstructure and Mechanical Analyses

Specimens were cut from ingots and extruded bars which were taken perpendicular to the extrusion direction. The microstructural characterization was performed by Leica optical microscope and scanning electronic microscopy (SEM). The phases of the cast and extruded Mg alloys were examined using x-ray diffraction analysis (XRD) (Rigaku Smart Lab). The samples for transmission electron microscopy (TEM) were prepared by mechanical thinning and punching to obtain φ3 $\times$ 0.05 mm discs, which were thinned using a double spray solution mixed with 2% perchloric acid and 98% ethanol by volume fraction, under a test voltage of 25 V and temperature of − 40 °C. The specimens were thinned by ion polishing for 30 min to obtain TEM samples, which were immediately subjected to TEM analysis to investigate the effect of Mn and second phase particles in Mg alloy.

Further, for mechanical testing, dog-bone-shaped samples were cut to perform the tensile test at room temperature on universal material testing machine CMT5105, dimensions of the tensile sample: length 18 mm, width 4 mm and thickness 2 mm at the rate of 1 mm/min. An extension meter with a gauge length of 10 mm was used to measure the elongation. The average value of the three samples was obtained.

2.3 Corrosion Testing

The Versa STAT 3 electrochemical workstation was used for electrochemical tests. A standard three-electrode configuration was used: saturated calomel as a reference, platinum as the counter, and sample as the working electrode. The alloy samples were polished with 3000-grit sandpaper and subjected to polarization tests in Hank's simulated body fluid at 37 ± 1 °C. An open-circuit potential test of the 1800 s was first performed to ensure that the potential reached a stable value before the latter test. The test potential for the dynamic potential polarization curve was scanned from − 1.8 V relative to the open circuit potential to − 1 V at a rate of 1 mV/s. The electrochemical test was performed in three replicated experiments. The composition of Hank's simulated body fluid is shown in Table 2.

Table 2 Comparison of Hank's simulated body fluid and human body fluid in ion concentration (mmol L^-1)

A square specimen of 10 mm × 10 mm × 4 mm was weighed with 1000# ~ 3000# sandpaper after water abrasion and weighed the mass m_0, and immersed in Hank's solution at 37 ± 1 °C for in vitro immersion weight loss test. According to the ASTMG31-72 standard, the ratio of solution volume to surface area was kept at 30 ml/cm², and the solution was replaced every 24 h with a test cycle of 14 days. After soaking, the surface corrosion products were washed off with boiling chromic acid and weighed as m₁. The average corrosion rate (Pi, mm/a) was calculated from the average of three parallel experiments according to the following equation:

$$\mathrm{Pi}=\frac{3650({m}_{0}-{m}_{1})}{{\rho }_{\mathrm{mg}}AT}$$

(1)

where ρ_mg is the material density, g/cm³; A is the surface area, cm²; T is the immersion time, day.

The change in the pH value of the solution was also measured during the immersion test for the first 120 h. The pH values were averaged over three sets of measurements.

2.4 Microstructural Analysis

XRD was used to examine the phases of the corrosion layer on the surface of the soaked sample, and x-ray photoelectron spectroscopy (XPS) was used to analyze the elemental distribution of the soaked sample along the thickness direction. SEM was used to observe the post-corrosion morphology, combined with EDS to analyze the elemental composition of the corrosion products.

2.5 Cytotoxicity Assessments

Cytotoxicity experiments were performed using mouse MC3T3-E1 osteoblasts and the CCK-8 method on cast Mg-2Zn-0.5Gd-1Y-0.5Mn Mg alloy. The metal components were firstly sterilized by UV light and then immersed in an α-MEM medium with 10% FBS according to the ratio of the surface area of metal materials to leaching solution of 1.25 cm²/ml according to ISO101993Pat12 standard and placed in a thermostat at 37 °C with 5% CO₂ and 95% relative humidity for 24 h to obtain the leaching solution.

The cells were placed in 24-well plates with only α-MEM medium as the negative control and incubated in the medium at the ratio of 6 $\times$ 10³/well inoculation for 24 h and 72 h, respectively. The medium was removed, each well was washed three times with PBS, and the cells were added to the medium containing 10% CCK-8 at 100L/well and incubated in a constant temperature incubator at 5% CO₂ and 37 °C for 2 h. The absorbance value at 450 nm (OD) was detected by the enzyme marker. And the relative cell proliferation value was calculated by OD results:

$${\mathrm{RCGR}=\mathrm{OD}}_{\mathrm{experimental\, group}}/{\mathrm{OD}}_{\mathrm{negative \,conteol \,group}}\times 100\%$$

(2)

The experiment was repeated three times. The t-test for statistical analysis of samples was used to determine whether there was a significant difference in the cytotoxicity test. Statistical significance was P < 0.05.

3 Results

3.1 Microstructure Analysis

The quaternary alloy containing Zn, Gd and Y exhibited mostly equiaxed grain structure with traces of dendrites in the cast condition, while with Mn addition, refines grains with a clear dendritic structure was observed as shown in Fig. 1(a), (b) and (c). The addition of alloying elements to Mg alloys has a tendency to form equiaxed grain structure with traces of dendrites, which is due to the growing solid/liquid interface separating low concentrations of alloying elements, resulting in a lowering of nucleation rate (Ref 32, 33). In as-cast Mg alloys, the average grain size without Mn is 248.1 μm, as shown in Table 3. The addition of Mn effectively refined the grain, at 0.5% and 1%, the grain size was significantly reduced by 10.8% and 16.9%, respectively. Further, after extrusion, the grains are greatly refined, as shown in Fig. 1(d), (e) and (f). During the extrusion process, the dendritic structure was broken and uniformly distributed in the Mg alloy. Compared to the extruded Mg alloy without Mn addition, the grain size was reduced by 22 and 33% with the addition of 0.5 and 1%, respectively. Compared with the as-cast state, the grain refining effect of the addition of Mn in the extruded state became stronger, which is mainly due to the extremely low solid solution degree of Mn in the matrix Mg (3.4 wt.%) during the hot extrusion and deformation of the alloy, the precipitated second phase Mn particles play a vital role, which can significantly impede the movement of dislocations and grain boundaries, thus inhibiting the growth of recrystallized grains, and the grain refining effect is greatly enhanced (Ref 34).

Fig. 1

Optical microstructure of Mg-2Zn-0.5Gd-1Y-$x$ Mn in cast (a)-(c) and extruded state (d-f): (a), (d)$x$=0; (b), (e)$x$=0.5; (c), (f)$x$=1

Table 3 Grain size (μm) of cast and extruded magnesium alloys

Figure 2 shows the XRD diffraction patterns of the extruded Mg alloy, where the α-Mg phase and Mg₃Y₂Zn₃ phase were detected in all three samples, and the Mg₂₄Y₅ phase was detected only in the sample without Mn addition. This might be attributed to the low volume fraction of the Mg₂₄Y₅ phase in the Mn-added Mg alloy. For further analysis of the intermetallic phases, an EDS mapping analysis was performed. Figure 3 shows the SEM images and the corresponding EDS elemental maps of Mg-2Zn-0.5Gd-1Y-$x$ Mn, respectively. The second phases of all three alloys are semi-continuously distributed, consisting of small broken particles. Most of the second phases in Fig. 3 are enriched with Mg, Zn, Y and Gd elements and are considered as Mg-Zn-RE phases. Only the separate Mg and Y elements are found enriched at the yellow dashed line in Fig. 3(a) and combined with the XRD diffraction pattern; the intermetallic phase here is the Mg₂₄Y₅ phase. The morphology of the Mg₂₄Y₅ phase was observed by magnifying the Mg₂₄Y₅ phase. The morphology of the Mg₂₄Y₅ phase was irregular, with an average size of 1 μm. Comparing with Fig. 3(b) and (c), after Mn addition, the separate Mg and Y enriched phases disappeared, indicating that the addition of Mn is not conducive to the precipitation of the Mg₂₄Y₅ phase. The distribution of Mn in the Mg alloy is neither affected by the second phase nor is involved in the formation of a uniform solid solution in the Mg matrix.

Fig. 2

X-ray diffraction pattern of Mg-2Zn-0.5Gd-1Y-$x$ Mn alloy

Fig. 3

The EDS images of Mg alloy and the elemental distribution of corresponding elements Mg, Zn, Gd and Y: (a) Mg-2Zn-0.5Gd-1Y; (b) Mg-2Zn-0.5Gd-1Y-0.5Mn; and (c) Mg-2Zn-0.5Gd-1Y-1Mn

Figure 4 shows the SEM microstructure and EDX energy spectrum of the extruded Mg alloy, and Table 4 corresponds to the results of elemental analysis of the second phase of the Mg alloy at each position, respectively. In Fig. 4(a) in position 1, 2 and the corresponding elemental analysis results found that, except for the Mg matrix, the elemental Y content is extremely high, while the Zn and Gd content is very low. Comparing the energy spectra in Fig. 4(d), element Y energy spectrum is blue, and the phase is observed to be blue at positions 1 and 2. Combining the XRD in Fig. 2 and the EDS in Fig. 3, the phase here is considered to be Mg₂₄Y₅ phase. In Fig. 4, the second phase was observed to be broken after extrusion with a semi-continuous distribution. The elemental analysis results of the second phase hit points show Zn/RE ratios ranging from 1.4 to 2.3, and the second phase here is mainly the Mg₃Zn₃RE₂ phase. In addition, it is noted that the content of Y is much larger than that of Gd when composing Mg₃Zn₃RE₂ phase, because the solid solubility of Gd in Mg (23.5 wt.%) far exceeds that of Y in Mg (12.3 wt.%), Gd exists more in solid solution form, and the Y content is also higher than that of Gd in the design of Mg alloy composition, so the Gd content in the second phase is lower (Ref 9, 35, 36).

Fig. 4

SEM and EDS microstructures of extruded Mg alloy: (a), (d) Mg-2Zn-0.5Gd-1Y; (b), (e) Mg-2Zn-0.5Gd-1Y-0.5Mn; and (c), (f) Mg-2Zn-0.5Gd-1Y-1Mn

Table 4 EDS results of the areas marked in Fig. 4 for Mg-2Zn-0.5Gd-1Y-$x$ Mn alloys

Figure 4 shows the SEM microstructure and EDX energy spectrum of the extruded Mg alloy, and Table 4 represents the results of elemental analysis of the second phase of the Mg alloy at each position, respectively. In Fig. 4(a), SEM analyses indicate that at 1, 2 points in corresponding EDX micrographs, apart from the Mg matrix, the elemental Y content is extremely high, while the Zn and Gd content is very low. Comparing the energy spectra in Fig. 4(d), the element Y energy spectrum is blue, and the phase is observed to be blue at positions 1 and 2. Combining the XRD in Fig. 2 and the EDS in Fig. 3, the corresponding phase is considered to be the Mg₂₄Y₅ phase. In Fig. 4, the second phase was observed to be broken after extrusion with a non-uniform distribution. The elemental analysis results of the second phase marked points show Zn/RE ratios ranging from 1.4 to 2.3, and the second phase at these points is mainly the Mg₃Zn₃RE₂ phase. In addition, it was observed that the content of Y is much higher than that of Gd when composing the Mg₃Zn₃RE₂ phase because the solid solubility of Gd in Mg (23.5 wt.%) exceeds that of Y in Mg (12.3 wt.%), Gd exists majorly in solid solution form, and the Y content is also higher than that of Gd in the design of Mg alloy, so the Gd content in the second phase is lower (Ref 9, 35, 36).

To confirm the presence of Mn and the second phase in the Mg alloy, was observed by TEM. Figure 5 shows the TEM bright field images, selected electron diffraction spots and selected EDX elemental distribution of the precipitates of Mg-2Zn-0.5Gd-1Y-0.5Mn Mg alloy. In Fig. 5(a), position A contains Mn and Mg elements, and the electron diffraction spot at this point exhibits a body-centered cubic (BCC) crystal structure, and the precipitated phase corresponds to α-Mn. In Fig. 5(b), the EDX element distribution at position B is composed of a large amount of Mn and a small amount of Zn and Y in addition to the Mg matrix, and the size of the α-Mn singlet precipitated here is tens of nanometers. A large amount of precipitated α-Mn monomers aggregated into spherical rods with a size up to 0.5 μm. The solid solution of Mn in Mg alloy is limited, and the excessive Mn will be phased/precipitated out as α-Mn. The Mg₃Zn₃RE₂ phase, composed of elements Zn, Y and Mg matrix at position C in Fig. 5(c), has a particle size of about 0.1 μm.

Fig. 5

TEM results of Mn nanophase in Mg-2Zn-0.5Gd-1Y-0.4Mn alloy: (a) bright field phase A; (b) bright field phase B; (c) bright field phase C; (d) EDX element distribution at point A; (e) EDX element distribution at point B; (f) EDX element distribution at point C

3.2 Mechanical Properties

Figure 6 shows the typical stress–strain curves for the extruded state Mg-2Zn-0.5Gd-1Y-xMn alloy. The stresses of the three alloys increase slightly with increasing strain, indicating that Mg alloys do not exhibit work hardening in the extruded state. The tensile strength, yield strength and percentage elongation of the Mg alloys are listed in Table 5. Compared with the extruded Mg-2Zn-0.5Gd-1Y Mg alloy, the addition of 0.5%Mn reduced the grain size by 22% and increased the tensile strength, yield strength and elongation by 11.9, 4.3 and 24.4%, respectively. The addition of 1%Mn reduced the grain size by 33% and increased the tensile strength, yield strength and elongation by 18.4, 8.7 and 30.5%, respectively. The addition of Mn can enhance the mechanical properties of Mg combined through grain size strengthening and solid solution strengthening.

Fig. 6

Stress–strain curve of Mg-2Zn-0.5Gd-1Y-$x$ Mn at room temperature

Table 5 Tensile strength, yield strength and elongation of Mg alloy

Figure 7 shows the SEM morphology of the ductile fracture after tensile testing. The fracture of Mg alloy without Mn addition (Fig. 7a) showed a river-like pattern which showed mixed ductile and brittle fracture behavior. However, upon addition of Mn, the Mg alloy underwent purely ductile fracture, evidenced by large-sized dimples, which are more prominent in the 1%Mn sample compared with 0.5%Mn, as shown in Fig. 7(b) and (c).

Fig. 7

Tensile fracture morphology: (a) Mg-2Zn-0.5Gd-1Y; (b) Mg-2Zn-0.5Gd-1Y-0.5Mn; and (c) Mg-2Zn-0.5Gd-1Y-1Mn

3.3 Corrosion Investigation

Figure 8(a) shows the dynamic potential polarization curves of the extruded Mg alloy; all three alloys showed a plateau area in the anodic region, which indicates that the Mg alloy formed a passivation film on the surface during the corrosion process to protect the Mg substrate from further erosion. The Mg alloy with 0.5%Mn had the shortest passivation cycle and the best protection of the passivation film to the Mg substrate. The electrochemical parameters of the polarization tests are listed in Table 6, showing that the addition of Mn has shifted the corrosion potential of the alloy in the positive direction, reducing the corrosion current density and resulting in improved corrosion resistance. The Mg alloy with 0.5%Mn has the highest corrosion potential and the lowest corrosion current density and exhibits the best corrosion resistance. Figure 8(b) shows the Nyquist plot of the polarization resistance of Mg alloys. The shape of the Nyquist diagram is the same for all three alloys, which means the same corrosion mechanism/behavior. The radius of the impedance curve is linearly related to the corrosion resistance, as depicted in Fig. 8. The polarization resistance (R_p) of 0.5%Mn alloy is the largest, and the corrosion resistance is the best, which is consistent with the conclusion of the electrochemical data.

Fig. 8

Mg-2Zn-0.5Gd-1Y-$x$ Mn alloy at 37°C under Hank's simulated body fluid: (a) polarization curve; (b) Nyquist diagrams

Table 6 Electrochemical parameters of the dynamic potential polarization curve of Mg-2Zn-0.5Gd-1Y-$x$ Mn at 37 °C in Hank's solution

Figure 9(a) shows the pH variation of the Mg alloy immersed in Hank's solution for 120 h. In the first few hs, the Mg substrate was corroded to generate a large amount of OH^- resulting in a rapid increase in the pH of the solution. Subsequently, the pH growth rate slowed due to the formation of corrosion film on the surface. The pH growth rate and the final pH from a certain degree to reflect the corrosion of Mg alloy, from the curve shows that the addition of 0.5%Mn Mg alloy pH gradually decreases, the final pH is much lower than the other two alloys, showing the best/highest corrosion resistance. Figure 9(b) shows the degradation rate of Mg alloy in Hank's solution at 37 °C. The addition of Mn resulted in a lower degradation rate and improved corrosion resistance of the Mg alloy. The addition of 0.5%Mn showed the best corrosion resistance, which is consistent with the electrochemical and pH results.

Fig. 9

Mg-2Zn-0.5Gd-1Y-$x$ Mn alloy continuously immersed in Hank's simulated body solution at 37°C: (a) pH value change curve at 120 h; (b) corrosion rate

Figure 10 shows the SEM morphology and EDS of the sample surface after 14 days of immersion. Both the surface flocculent and surface rough surface layer are corrosion products, and the components are mainly Mg, O, Ca and P. The phosphorus-to-calcium ratio (P/Ca) is about 1.4, which is close to 1.5 (Ca₃(PO₄)₂). The corrosion product coverage shows that the addition of 0.5%Mn corrosion product film is the densest and the least cracked, and the other two have severe cracking and partial exfoliation. The XRD of corrosion products in Fig. 11 shows that Mg(OH)₂ and (Ca₃(PO₄)₂) etc. accumulate on the surface of Mg-2Zn-0.5Gd-1Y-0.5Mn Mg alloy. A strong background and many widened/broader peaks are observed in Fig. 11, indicating that some complex compounds and some amorphous phases may be present in the corrosion products. Combined with the study of Kuwahara (Ref 37), the corrosion product on the surface of Mg in Hank’s solution shows amorphous nature (Ca_0.86Mg_0.14)₁₀(PO₄)₆(OH)₂, including other amorphous calcium and Mg phosphates observed after immersion in this test.

Fig. 10

SEM morphology and corresponding EDS spectra after 14 days of immersion: (a), (b) Mg-2Zn-0.5Gd-1Y; (c), (d) Mg-2Zn-0.5Gd-1Y-0.5Mn; (e), (f) Mg-2Zn-0.5Gd-1Y-1Mn

Fig. 11

XRD image of Mg-2Zn-0.5Gd-1Y-0.5Mn alloy after 14 days of immersion

Figure 12 shows the map of XPS elements on the corroded alloy surface. In Fig. 12(a), XPS full spectrum can be seen that the surface of the specimen after corrosion contains Mg, Zn, O, Ca, P and Y. Mg(OH)₂ and MgO are detected in the fine spectrum of Mg elements, where MgO is the result of oxidation of the exposed Mg matrix after immersion. Combined with the XRD results, the corrosion products mainly consisted of Mg(OH)₂, Calcium-Mg phosphate and a small amount of Zn and Y salts.

Fig. 12

XPS analysis of Mg-2Zn-0.5Gd-1Y-0.5Mn alloy after 14 days of immersion: (a) full spectrum of Mg alloy; (b)binding energy of Mg 1s

3.4 Cytotoxicity Assessments

Figure 13 shows the proliferation rate of MC3T3-E1 cells cultured in Mg-2Zn-0.5Gd-1Y-0.5Mn alloy-containing extract after 1 and 3 days of culture. Compared with the negative control group, the cell proliferation rate of the extract was 85.3 and 78.7% on days 1 and 3, respectively. According to ISO10993-5, when the cell proliferation rate is 75-99%, the cytotoxicity of the material is level 1, indicating that the material has superb biocompatibility.

Fig. 13

RGR of MC3T3-E1 cells after 1 and 3 days incubation. *P > 0.05

4 Discussions

4.1 Microstructure

Zinc is a common additive element to Mg alloys for its solid solution strengthening effect, and rare earth elements can significantly enhance the mechanical properties and corrosion resistance of Mg alloys. Nabila et al. (Ref 38) presented the relationship between phase formation in Zn/Y and Mg-Zn-Y alloy. When the Zn/Y ratio is 10, the phases are α-Mg, Mg-Zn phase, and I phase (Mg₃Zn₆Y). When the Zn/Y ratio is 0.94, the main phase in the alloy is the W phase (Mg₃Zn₃Y₂). The results show that when the Zn/Y ratio is greater than 4.38, the requirement for the complete formation of the I phase is satisfied, and as the Zn/Y ratio decreases, the W phase will gradually develop, leading to W and I binary phases. The rare earth elements Y and Gd are often involved in the formation of the above phases simultaneously, so Mg₃RE₂Zn₃ is used to refer as, Mg₃(Y, Gd)₂Zn₃. The second phase elemental analysis results are presented in Table 3, the Zn/RE ratio is around 2.2, the second phase of Mg alloy is mainly the Mg-Zn-RE phase (Mg-Zn-RE phase consists of most of the W phase (Mg₃Zn₃RE₂) and a small amount of I-phase (Mg₃Zn₆RE) is mixed). The Mg₂₄Y₅ phase was detected in the Mg alloy, but not the Mg₅Gd phase, which is because the solid solution degree of Gd in Mg is much higher than that of Y, because Y is more likely to undergo compositional segregation at the grain boundaries, the compositional range of Mg₂₄Y₅ is wider than that of Mg₅Gd and the Y content in the Mg alloy is higher than that of Gd. Gd mainly strengthens the Mg alloy in solid solution form, and a small amount is involved in the formation of Mg-Zn-RE second phase, while Y is heavily involved in the formation of the second phase. After adding Mn, the Mg₂₄Y₅ phase disappears, and Mn is not conducive to the precipitation of the Mg₂₄Y₅ phase; at this time, the second phase of Mg alloy is the Mg-Zn-RE phase. Mn is not involved in the formation of the second phase, only solid solution in the Mg matrix; due to the small solid solution degree of Mn in Mg (3.4 wt.%), the excess Mn will be precipitated in the form of α-Mn monolithic particles, as shown in Fig. 5.

4.2 Mechanical Properties

Table 7 summarizes the mechanical properties of a variety of Mg alloys similar to those studied here. Mg alloys possess a low modulus of elasticity, around 40 GPa. When only Zn is added, the tensile and yield strengths of the Mg alloys increase with increasing Zn content and elongation decreases, and the mechanical properties of the combined Mg alloys are enhanced after extrusion treatment. The extruded state Mg-1Zn-2.5Gd has the highest yield strength of 386 MPa, and the high strength originates from the highly dispersed and bent LPO organization and the refinement of the a-Mg matrix grains (Ref 42). However, its elongation is too low, and its plasticity is poor. The addition of Zr in the extruded state Mg-1.8Zn-1.74Gd-0.5Y-0.4Zr can greatly refine the Mg alloy grains, and its mechanical properties are excellent. The extruded Mn-containing Mg alloy studied in this paper/work has high tensile strength, yield strength and excellent elongation, with optimal overall mechanical properties.

Table 7 Mechanical properties of different Mg alloys

The mechanical properties of Mg alloy are enhanced mainly by grain size strengthening mechanism, solid solution strengthening mechanism and second phase strengthening mechanism. The added Zinc (Zn), Yttrium (Y), Gadolinium (Gd) and Mn atoms can form solid solutions in Mg alloy, among which Gd and Y atoms have the best solid solution effect owing to their better solubility. Gd has a large solid solution degree in Mg, which can significantly improve the room temperature and high-temperature mechanical properties of Mg alloys (Ref 44, 45) and enhances the age-hardening and solid solution-strengthening effect of Mg alloy (Ref 35). Y has a solid solution degree up to 12.3 wt.% in Mg and has the same crystal structure and similar lattice constant as Mg (Ref 46), which can play a vital role in aging precipitation and solid solution strengthening. The second phase of extruded Mg alloy is mainly the Mg-Zn-RE phase. In its composition, phase I and α-Mg matrix have high bonding strength and excellent strengthening effect (Ref 47, 48). Generally, the W phase is not considered an effective strengthening phase for Mg alloy (Ref 49, 50), but the strength and elongation of Mg alloy can be effectively improved by controlling the size and dispersion of W phase particles (Ref 51, 52). Extrusion can effectively refine the grain and evenly distribute the second phase. The mixed distribution of the W phase and I phase can greatly improve the mechanical properties of Mg alloy (Ref 53). Therefore, Mg-Zn-RE greatly contributes to the improvement of the mechanical properties of Mg alloys. Mg₂₄Y₅ phase has very strong stability (Ref 54); breaking during the fracture process is difficult to trigger stress concentration to produce cracks, which can rapidly expand along the grain and grain boundary Fig. 6. In the fracture morphology, the fracture pits of Mg alloy without the addition of Mn is less, can be observed in the lower left corner with a river-like pattern, showing the mixed fracture characteristics of brittle and ductile fracture, low elongation. With the addition of Mn, limited precipitation of Mg₂₄Y₅ was observed, the volume fraction of the second phase increased, and Mn its solid solution strengthening effect (Ref 55). The fracture morphology, the fracture is covered by pits, showing an obvious ductile fracture, the strength and elongation of Mg alloy are/were enhanced.

4.3 Corrosion Properties

Corrosion of Mg alloys in Hank's simulated body fluid is an electrochemical process, and the role of second-phase particles has been shown to significantly affect the overall corrosion behavior of the Mg alloy system (Ref 56). The main second phase in the Mg alloy is the Mg-Zn-RE phase; the Mg-Zn-RE phase is close to the Mg matrix potential and uniformly dispersed, which does not produce a strong galvanic corrosion effect. Mg₂₄Y₅ meets the Mg matrix potential difference, which produces a galvanic corrosion effect and becomes filamentary corrosion propagation after the local corrosion starts (Ref 19, 20). Mg₂₄Y₅ phase microcell test shows that it supports a higher cathodic reaction than Mg higher cathodic reaction rate while showing similar anodic reaction kinetics as compared with Mg, placing the corrosion process of Mg alloy under cathodic control (Ref 37). Mg is prone to corrosion with a corrosion potential as high as − 2.37 V. The electrochemical reaction in the simulated body fluid proceeds according to the following equation.

$$\begin{gathered} {\text{anodic}}{\mkern 1mu} {\text{ reaction}}\,\,~{\mkern 1mu} {\text{Mg}} \to {\text{Mg}}^{{2 + }} + 2{\text{e}}^{ - } \hfill \\ {\text{cathodic}}{\mkern 1mu} {\text{ reaction}}\,\,{\mkern 1mu} 2{\text{H}}_{2} {\text{O}} + 2{\text{e}}^{ - } \to {\text{H}}_{2} + 2{\text{OH}}^{ - } \hfill \\ {\text{Total}}{\mkern 1mu} {\text{ reaction:}}{\mkern 1mu} {\text{Mg}} + 2{\text{H}}_{2} {\text{O}} \to {\text{Mg}}\left( {{\text{OH}}} \right)_{2} + {\text{H}}_{2} \hfill \\ \end{gathered}$$

(3)

Figure 14 shows the corrosion mechanism of Mg alloys. The Mg₂₄Y₅ phase in the Mg alloy has a large potential difference with the Mg matrix and forms a micro battery with the surrounding Mg matrix, resulting in galvanic corrosion. The Mg matrix, as the anode, loses electrons and accelerates dissolution. The resulting Mg(OH)₂ is insoluble in water and partially adheres to the surface of the Mg matrix, preventing corrosion. However, the intense Mg(OH)₂ film is easily infiltrated by Cl⁻ and converted into water-soluble MgCl₂ (Reaction (1) in Fig. 14). At the same time, the presence of active ion Cl^- will induce pitting and is accompanied by the Cl^- autocatalytic mechanism, accelerating the corrosion rate and making the initial oxidation layer lose its protective effect. With the further progress of corrosion, more and more corrosion products are generated, and Ca²⁺, PO₄^3- constantly involved in the reaction (Reaction (2) in Fig. 14), resulting in the formation of corrosion products composed of Mg(OH)₂, Ca₃(PO₄)₂ and some complex amorphous Ca and Mg phosphate, which are deposited on the surface of Mg alloy in large quantities. A dense corrosion product film is formed to prevent the further dissolution of the Mg matrix and stabilize the corrosion. When Mn is added to Mg alloys, the Mg₂₄Y₅ phase precipitation is inhibited by Mn to reduce precipitation, thus reducing electrochemical corrosion. In addition, the addition of Mn also allows the Mg alloy to form denser corrosion products, preventing the solution of the matrix and improving corrosion resistance (Ref 27, 57, 58). However, too much Mn will precipitate as α-Mn particles, which will fall off and form cavities during corrosion, allowing further corrosion to proceed. Therefore, the corrosion rate increases with the addition of 1% Mn in the continuous immersion test compared to 0.5% Mn Mg alloy.

Fig. 14

Schematic diagram of the corrosion mechanism of Mg-2Zn-0.5Gd-1Y-$x$ Mn alloy. (a) Onset of corrosion; (b) Initial stage of corrosion; (c) Pitting corrosion deep; (d) Corrosion product film formation, corrosion mitigation

5 Conclusion

In this study, the effects of Mn content on the microstructure evolution, mechanical properties and corrosion resistance of extruded Mg-2Zn-0.5Gd-1Y Mg alloy were studied. The results are summarized as follows:

• 1. The addition of Mn can effectively refine the grain size and significantly improve the tensile strength and elongation of the Mg alloy. The microstructure observation shows that Mn does not participate in the formation of the second phase in the Mg alloy and is solidly dissolved in the Mg alloy matrix. Too much Mn is precipitated in the form of α-Mn particles.

• 2. Mg-Zn-Gd-Y-Mn alloy is mainly composed of α-Mg, Mg-Zn-RE phase, Mg₂₄Y₅ phase and Mn. The relative mechanical properties of Mg-Zn-RE are improved greatly, and the stress concentration of the Mg₂₄Y₅ phase will be induced during the tensile process, which will reduce the strength and elongation of Mg alloy.

• 3. Galvanic corrosion occurs mainly in Mg alloy, and the Mg₂₄Y₅ phase acts as the cathode of the microcell and puts the corrosion process of Mg alloy under cathode control. The addition of Mn can inhibit the Mg₂₄Y₅ phase to reduce precipitation, thus improving the mechanical properties and corrosion resistance of Mg alloys. The addition of appropriate Mn is an effective measure to improve the mechanical properties and corrosion resistance of Mg-2Zn-0.5Gd-1Y Mg alloy, and 0.5Mn is the best addition.