Influence of Noble Metals on the Microstructure and Properties of Biodegradable Mg-Nd-Zr Alloy

Abstract

In this work, the approach to improve the mechanical properties of a biodegradable Mg-Nd-Zr alloy through modification with noble metals (Ag and Au) was proposed. The separate and combined influence of silver and gold on the macro- and microstructure of the alloy was studied. A qualitative and quantitative assessment of the structural components of the alloy was carried out. It was shown that when modifying the melt, noble metals form the complex intermetallic phases which served as additional crystallization centers. It has been established that adding 0.05 to 0.1 wt.% of noble metals to Mg-based alloy increase the volume fraction of intermetallic compounds by ~1.5 times, shifting them towards smaller size groups with the simultaneous formation of spherical intermetallic compounds. The latter are located in the center of the grain proving them to be the additional nucleation sites. It was shown that complex modification (0.1% Ag + 0.1% Au) of a Mg-based alloy refined its structural components by ~1.5 times, increasing the strength by ~20%, and ductility by ~2 times due to the formation of the intermetallic compounds. The proposed technology for modifying cast biodegradable Mg-based alloys is feasible to be used for the manufacture of implants for osteosynthesis.

1. Introduction

Currently, various metals and alloys are widely used in orthopedics and traumatology for osteosynthesis of skeletal segments bearing increased load. Metal implants, which are mainly used today, are made from stainless steel, Co-based and Ti-based alloys [1,2,3,4].

In recent decades, many complaints have arisen regarding the materials from which fixators for the osteosynthesis of bone fragments are manufactured. The main problems associated with the use of metal implants are metal allergy, aseptic inflammation and metallosis. Also, a limitation of the use of metallic materials is the release of toxic ions or metal particles due to corrosion or wear, which leads to an inflammatory process [4,5,6,7]. In addition, the elastic moduli of these alloys differ significantly from bone tissue. As a result of the stress shielding effect, the formation of newly formed bone tissue is reduced and negative remodeling is enhanced, which leads to a degradation of the implant stability [8,9,10]. The use of bioinert metal fixators for osteosynthesis requires repeated surgery to remove the implant. As a rule, repeated surgery is no less traumatic than osteosynthesis itself. This entails an increase in the overall length of hospital treatment and temporary disability of patients. In this regard, there is a constant ongoing search for materials that could dissolve in the implantation area with synchronous replacement with bone tissue, which would not require the removal of fixing devices.

One of these materials may be magnesium, which is a natural element in the body, and its bioresorption products are characterized by high biocompatibility [11,12,13]. These properties make magnesium attractive for use in orthopedics and traumatology [14,15,16]. However, the main limitation of its use is low mechanical properties [3,17,18]. The solution to the problem is the use of Mg-based alloys, which have a higher level of mechanical properties and an elastic modulus comparable to that of bone tissue [19]. In this regard, the research and development of biodegradable Mg-based alloys is a promising direction in biomaterials engineering. The use of implants made of biodegradable Mg-based alloys eliminates the additional operations for removing them and the complications associated with the long-term presence of an implant in the bone. When choosing the chemical composition of a biodegradable alloy, it is important that the biocorrosion products are metabolized in the body, bioabsorbed and not toxic.

Previous studies [16] established the requirements for the development of a biodegradable Mg-based alloy intended for implants in osteosynthesis:

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Required level of mechanical properties is as follows: ultimate tensile strength (UTS) ≥150 MPa, total elongation (TE) ≥ 3.1%. This is comparable to the properties of bone tissue and ensures its reliable fusion up to complete consolidation of the fracture (3 months);

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No toxic effects on the body.

According to [20], the most widely used alloying elements in magnesium-based compositions are zinc, calcium, zirconium, manganese, silver, and rare earth elements. Zinc decreases the corrosive effects of some common impurities in Mg-based alloys such as iron and nickel. Additionally, a deficiency of zinc in the human body can result in serious illnesses; therefore, Zn also acts as a bioactive element in degradable magnesium alloys. Manganese improves mechanical properties and alloy viscosity at quantities below 2 wt.%. Calcium possesses grain refining effect and strengthening of grain boundaries. Due to Laves-type phases with Mg₂Ca composition, creep resistance of magnesium alloys can be improved [21]. The addition of Zr and rare earth elements, namely yttrium, neodymium and cerium, also leads to grain refinement and increases the strength of magnesium alloys [22].

The toxicological and physiological properties are other important issues regarding magnesium itself and the main alloying elements for biodegradable magnesium alloys. According to data summarized in [20,23], the distinctive properties of each element are as follows.

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Magnesium has a positive influence on metabolism, synthesis of protein, and cell proliferation. It promotes the stabilization of RNA and DNA, regulates protein activity and also affects cellular reactions;

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Zinc is important for the immune system.

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Calcium is mainly concentrated in bones. It regulates renal, intestinal and skeletal homeostases;

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Zirconium possesses low toxicity and acceptable biocompatibility;

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Rare earth elements have anticancer properties.

It has been established that the most suitable alloying system is Mg-Zr-Nd, which ensures the non-toxicity of alloying elements and the possibility of structural strengthening of the alloy [24,25]. The optimal chemical composition of a biodegradable Mg-based alloy has been determined as follows: 0.8–1.1 wt.% Zr, 2.4–2.8 wt.% Nd, 0.5–0.7 wt.% Zn. This composition ensures the following mechanical properties: UTS = 266–271 MPa; TE= 4.3–4.6%. After three months of immersion and biodegradation in artificial blood substitutes, the alloy retained mechanical properties close to the properties of bone tissue: UTS of 150 MPa and TE of 3.2% [26].

Installation of various fixing screws during osteosynthesis involves certain preparations. Using a drill and a guide, a canal is formed in the bone. In the formed channel, using a threaded tap, a thread is cut and a screw is fixed. Reliable fixation of bone tissue fragments without prior preparation with a tap is impossible because of the high possibility of damage to the implant. Thus, the preparatory formation of the canal and threading complicates the process of surgical intervention and lengthens the operation time. Therefore, an important task is to increase the mechanical properties of magnesium alloy to eliminate the specified preparatory operations.

One of the methods for increasing the mechanical properties of an alloy is modification of the microstructure. There are a number of requirements for modifying elements, one of which is non-toxicity. Noble metals, particularly silver and gold, are promising modifiers for Mg-based alloys. Gold has been known since ancient times for its bioinertness towards living organisms [27]. Silver, according to many literary sources, has an antibacterial effect [28]. An analysis of the phase diagrams of binary systems of silver and gold with magnesium showed that they can form solid solutions, as well as new intermetallic phases. These phases have higher melting points compared to magnesium, therefore they act as additional crystallization centers during the initial period of crystallization, thus refining the grain and positively affecting the properties of the metal [29].

Therefore, the development of new Mg-based alloys modified with noble metals with an improved structure and increased mechanical properties is a challenging task of biomaterials science. In this study, the separate and combined effects of silver and gold on the structure formation and properties of a biodegradable Mg-based magnesium alloy were studied.

2. Materials and Methods

The biodegradable Mg-based alloy of the following chemical composition was studied: 0.9 wt.% Zr, 2.6 wt.% Nd, 0.6 wt.% Zn, Mg is a balance. The alloy was smelted in a crucible induction furnace of 0.5-ton capacity and 140 kW power and also in a gas-protected furnace of 150 kg capacity. Preheated charge materials were loaded into the crucible furnace and, after melting, the melt was poured into removable crucibles at a temperature of 650–730 °C. Removable crucibles were installed in holding furnaces, in which the chemical composition of the alloy was adjusted followed by the flux refining (38–46 wt.% MgCl₂, 32–40 wt.% KCl, 5–8 wt.% BaCl₂, 3–5 wt.% CaF₂) at 740–760 °C. After this, Ag and Au additives were added to the melt (0; 0.05; 0.1; 0.2 wt.%—as calculated), with subsequent heating of the melt, holding and pouring it at 730 °C into molds to obtain samples and workpieces for mechanical testing and metallographic control.

Heat treatment of cast billets was carried out in a 112 kW Bellevue-type thermal shaft furnace and also in a thermal furnace PAP-4M type. The metal was examined after heat treatment according to the following regime: heating to 540 ± 5 °C, holding for 15 h, air cooling to room temperature and aging at 200 ± 5 °C for 8 h with subsequent air cooling.

The quality of cast samples and workpieces made of Mg-based alloys was determined by X-ray control. Optical microscopy was used for the observation of the macro- and microstructure of samples (OLYMPUS IX 70). The grain size was measured according to the intercept method using the built-in software for the optical microscope. The specimens for microstructure analysis were mirror polished according to standard procedure and etched by the reagent consisting of 1% nitric acid, 20% acetic acid, 19% distilled water and 60% ethylene glycol. The microhardness of the structural components of the alloys was measured with Buehler and LM-700AT microhardness testers at a 0.1 N load.

The fractured surface of the samples was observed using a Jeol JSM-6360LA scanning electron microscope. Local chemical analysis of the structural components was performed using REMMA 202M and REM 16I scanning electron microscopes equipped with an energy-dispersive microanalyzer.

The mechanical properties of the samples were determined using an INSTRON 2801 tensile testing machine. The tensile testing was performed for as-prepared samples and for samples held in an artificial blood substitute for one, two and three months at 36 ± 1 °C. The temperature steadiness was regulated by the thermostat UT-15. The artificial blood substitute was a Gelofusine^® solution. Before immersion in blood substitute, the samples were degreased by ethanol. After immersion, the samples were cleaned by chromic anhydride at 18–25 °C for 3 min to delete the corrosion products and, afterwards, they were washed in distilled water and air dried.

3. Results and Discussion

3.1. Microstructure Examination

The microstructure of the alloy under study without modification was the δ-solid solution, δ + γ eutectoid in the form of spherical formations and individual intermetallic inclusions of the γ-phase. The addition of silver and gold into the alloy refined its grains. At the same time, the addition of silver contributed to a stronger grain refinement than the addition of gold. Joint modification of the alloy (Ag + Au) contributed to the formation of finer grains (Figure 1).

Macrofractographic study of fractures of as-cast samples of the original alloy of the Mg-Zr-Nd system without modification showed the presence of a coarse-crystalline structure. The separate introduction of increasing additives of silver and gold into the alloy under study refined the cast metal structure. At the same time, their combined influence enhanced the refining effect and the nature of the fracture became matte and finely crystalline.

Increasing Au and Ag additives contributed to a decrease in the grain size and the distance between the second-order axes (

Table 1. Parameters of the structure of magnesium alloy with different contents of Ag and Au.

Modifier Content, wt.% (as Calculated)	Grain Size, μm	Secondary Dendrite Arm Spacing, SDAS, μm	Microhardness of Metal Matrix HV, MPa
			Before Heat Treatment	After Heat Treatment
-	250	24	1113	1228
0.05% Au	177	20	1125	1234
0.10% Au	125	19	1136	1268
0.20% Au	123	18	1141	1285
0.05% Ag	88	19	1145	1241
0.10% Ag	62	18	1137	1262
0.20% Ag	61	16	1155	1273
0.10% Au + 0.10% Ag	44	15	1232	1294

). Heat treatment contributed to an increase in the homogeneity of the structure and an increase in the microhardness of the matrix. With an increase in the amount of silver and gold in the metal, the microhardness of the matrix increased. At that, the microhardness of the eutectic was ~1.5 times higher than the microhardness of the δ-solid solution.

The dependence of the grain size on the content of Au and Ag can be expressed by the following regressions, respectively:

$$\begin{gathered} l=-599.4·x+221.2;R^2=0.74, \\ l=-37.9·\ln \left(x\right)-15.7;R^2=0.98, \end{gathered}$$

where

• l—average grain size, µm;
• x—modifier content, wt.%.

When constructing the dependence for silver, zero Ag content was supposed as 0.001% Ag for the sake of correct taking the logarithm.

As follows from the EDX analysis of samples after heat treatment, the intermetallic phases contained both Ag and Au (Figure 2 and Figure 3).

According to compositions obtained for IMCs (Tables in Figure 2 and Figure 3), only two elements (Mg and Nb) are present in relatively stable quantities. Therefore, it can be assumed that the compositions under study are solid solutions on the Mg-Nb intermetallic base. In the Mg-Nd alloy system, the following five intermetallic compounds, namely Mg₁₂Nd, Mg₄₁Nd₅, Mg₃Nd, Mg₂Nd and MgNd can be formed [30]. According to the mass ratio, the closest composition is Mg₁₂Nd.

3.2. Quantitative Metallographic Analysis

The microscopic observation revealed the presence of lamellar and spherical intermetallic phases in the studied alloys. Lamellar intermetallic compounds were located predominantly along the grain boundaries, and spherical ones were located in the center of the grain. Spherical intermetallic phases can serve as additional crystallization centers and contribute to the refinement of grains and structural components of the alloy. Metallographic analysis showed that the volume fraction of the intermetallic phase increases with increasing content of alloying elements in the alloy (

Table 2. Volume fraction of intermetallic compounds (V) and their distribution by size in the alloy of the Mg-Nd-Zr system depending on Au and Ag contents.

Modifier	Content, wt.%	Distribution of Intermetallides (V·10−3, vol. %) by Size Groups, µm
		<2	2.0…3.9	4.0…7.9	8.0…11.5	11.6…15.0	15.1…19.0	Total
No alloying with Au/Ag		7/0	19/53	37/29	31/11	31/12	19/0	144/105
Au	0.05	40/1	82/75	21/43	11/25	31/0	19/1	204/145
	0.10	65/2	91/155	43/43	11/19	19/1	7/1	236/221
	0.20	76/79	160/129	42/25	6/7	18/1	0/1	302/242
Ag	0.05	47/1	48/110	31/43	25/37	35/7	19/1	205/199
	0.10	91/19	29/116	47/44	19/32	19/7	16/1	221/219
	0.20	147/62	25/116	73/34	19/26	11/7	5/0	280/245
0.1 wt.% Au + 0.1 wt.% Ag		143/112	111/119	69/71	9/14	0/0	0/0	232/316

Note: the numerator—the volume percentage of lamellar intermetallides, the denominator—the volume percentage of spherical intermetallides.

). When 0.05...0.10% modifiers were added to the alloy, the volume fraction of spherical intermetallic compounds increased to a greater extent compared to lamellar ones. A further increase in the alloying content (up to 0.2%) led to a slight increase in the volume fraction of spherical inclusions located inside the grain and an intensive increase in the number of lamellar inclusions.

Analysis of the distribution of intermetallides by size showed that lamellar ones were predominated in the original alloy, most of which were 4–15 μm in size. Spherical intermetallides were mainly 2.0–7.9 μm in size. Additives of Au and Ag contributed to the refinement of the intermetallic phase. The number of intermetallides smaller than 2 μm increased, but the number of coarse ones (>11.6 μm) decreased.

With an Au and Ag content of 0.05...0.1 wt.%, there was an intensive increase in the volume percentage of spherical intermetallides located inside the grain, the grain was refined and the plasticity of the alloys increased. A further increase in the content of elements in the alloy to 0.2 wt.% increased the volume percentage of lamellar and spherical intermetallic compounds and increased the tensile strength.

Thus, gold and silver, positively changing the morphology and topology of intermetallic compounds, refined the macro- and micrograins in the alloy and contributed to an increase in its mechanical properties. At the same time, the highest indicators of strength and ductility of the alloy were achieved with complex modification with noble metals (

Table 3. Mechanical properties of the Mg-based alloy depending on Ag and Au contents (after heat treatment).

Modifier Content, wt.% (as Calculated)	Mechanical Properties
	UTS, MPa	TE, %
-	232	3.6
0.05% Au	236	4.0
0.10% Au	250	5.7
0.20% Au	254	4.9
0.05% Ag	239	4.7
0.10% Ag	252	5.9
0.20% Ag	256	5.4
0.10% Au + 0.10% Ag	262	6.2

). IMS containing Ag and Au presumably possess higher melting temperatures than Mg. During cooling of the melt, IMSc with Ag and Au solidify first in the form of spherical particles. They act as crystallization sites refining the grains and improving mechanical properties.

The dependence of UTS on the content of Au and Ag can be expressed by the following regression:

$$UTS=119.7·x+233.4;R^2=0.88,$$

where

• x—modifier content, wt.%.

The effect of strengthening metal matrix with intermetallic or other second-phase inclusions is analogous for different groups of alloys including aluminum [31], iron [32] and other metal-based compositions.

3.3. Mechanical Properties after Biocorrosion

The biocorrosion was studied by keeping the tensile testing samples (diameters of the head and working part were 10 mm and 5 mm, respectively) in Gelofusine^® for 1, 2 and 3 months, followed by tensile testing of the corroded samples. Visual inspection of such samples after tensile testing showed the presence of multiple sites of corrosion initiation in the form of craters with the presence of crumbling white deposits from biodegradation products (Figure 4).

Microstructural study of cross sections of the heads of tensile samples has shown that the depth of the corrosion damage craters after 3 months of aging in Gelofusine^® for all variants of the alloy was approximately at the same level and amounted to ~2.0 mm (Figure 5a). At the same time, corrosion damage spread throughout the body of grains of the δ-solid solution and strengthening phases (Figure 5b).

The results of mechanical tests of the studied samples after biodegradation have shown a loss of strength of the alloy with various modification options. The lowest level of strength after biocorrosion was observed in the magnesium alloy without modification. Silver and gold contributed to increasing the tensile strength of the magnesium alloy. The samples with complex modifications of Au and Ag had the highest properties after 3 months of aging in Gelofusine^® (

Table 4. Mechanical properties of the Mg-based alloy depending on Ag and Au contents after different times of immersion in Gelofusine^®.

Modifier Content, wt.% (as Calculated)	UTS after Immersion in Gelofusine®, MPa
	1 Month	2 Months	3 Months
-	232	170	150
0.05% Au	236	200	190
0.10% Au	250	205	200
0.20% Au	254	195	185
0.05% Ag	239	201	200
0.10% Ag	252	208	205
0.20% Ag	256	205	202
0.10% Au + 0.10% Ag	262	215	210

).

The conducted studies showed the possibility of successfully modifying Mg-based alloys with noble metals in amounts from 0.05 to 0.2 wt.%. An analysis of data from the literature on the use of silver and gold in the manufacture of implants showed that the experiments were mainly related to coating or use in pure form [33,34]. Other studies have shown the positive effect of silver on the properties of a Mg-based alloys.

Modification of a Mg-Nd-Zr alloys with silver and gold, described in this article, makes it possible to increase the volume fraction of intermetallic phases by ~1.5 times, shifting them towards smaller size groups, with the simultaneous formation of spherical intermetallic compounds located in the center of the grains and acting as additional crystallization centers. The obtained results as to the refinement of the microstructure of the alloy are consistent with studies carried out by other authors [35].

The studies carried out showed the possibility of reducing the grain size and significant refinement of the structural components due to the modification carried out. In addition, the dependence of the separate and joint influence of noble metals on the mechanical properties of a magnesium alloy has been established. Thus, separate modification of the alloy under study provided the maximum level of its strength and ductility with the addition of 0.1 wt.% Ag or Au, which is consistent with previous studies describing modification with other elements [36].

The results obtained show that at complex modification (0.1 wt.% Au + 0.1 wt.% Ag) of magnesium alloy, the sizes of its structural components significantly decreased, the strength of the alloy increased by ~20% and ductility by ~2 times due to the formation of intermetallic phases of complex composition. The conducted research may be of interest to manufacturers of castings from magnesium alloys. In addition, it should be noted that the modifiers used are non-toxic to the body compared to the use of, for example, rare earth elements such as yttrium or scandium [37,38,39].

The use of a biodegradable Mg-based alloy with an increased set of properties makes it possible to eliminate the preparatory operation of forming a canal and cutting threads with a tap in osseous tissue during osteosynthesis, which reduces the risk of injury during the surgical intervention and significantly reduces the time of surgery.

The results obtained make it possible to develop improved technologies for modifying cast magnesium alloys with noble metals, improving the quality of castings and increasing the mechanical properties of products. The results of these studies may be useful for aviation, automotive and other industries [40].

4. Conclusions

It has been shown that modification of a magnesium alloy with silver and gold from 0.05 to 0.2 wt.% increases the volume percentage of intermetallic compounds by ~1.5 times, shifting them towards smaller sizes with the simultaneous formation of spherical intermetallic compounds located in the center of the grain and serving as additional crystallization centers. The separate and complex introduction of silver and gold into the alloy of the Mg-Zr-Nd system contributed to a decrease in micrograin size by ~2...4 times and increased mechanical properties of the alloys in the as-manufactured state and after biocorrosion.