Mg-based bulk metallic glasses: A review of recent developments

Shi Jie Bryn Bin, Ki Soon Fong, Beng Wh Chu, Mnoj Gupt,*

a Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore

b Singapore Institute of Manufacturing Technology, 73 Nanyang Drive, Singapore 637662, Singapore

Abstract Metallic biomaterials have been widely used in the fiel of medical implants for replacement purposes and/or for regeneration of tissue.Metals such as stainless steel (316 L), cobalt-chromium alloys and titanium alloys (Ti-6Al-4 V) are widely used as metallic implants today.However, they often exhibit unsatisfactory results such as stress shielding, the release of toxic ions and are often permanent and invasive -where a second surgery is required to remove the implant once the bone is fully healed.Magnesium as a biomaterial have attracted much attention recently due to its excellent biocompatibility, similar mechanical properties to bone and biodegradability.Unlike other metals and bio ceramics, the ability for magnesium alloys to undergo biodegradation eliminates the requirement for a second surgery to remove the implant.Additionally, the degradation of magnesium releases Mg2+ ions, which stimulates metabolism as they are a cofactor in numerous numbers of enzymes.Despite the advantages of magnesium alloys, the rapid degradation of magnesium proved to be challenging as the implant is unable to retain its structural integrity sufficientl enough to act as an implant.To improve the corrosion resistance of magnesium alloys,researchers have been working on the synthesis and characterization of Mg-based bulk metallic glasses, which can significantl improve the corrosion resistance of Mg-based alloys.This paper is a comprehensive review that compiles, analyzes and critically discusses the recent literature on the latest understanding of the processing, mechanical and biological characteristics of Mg-based bulk metallic glasses.

? 2021 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer review under responsibility of Chongqing University

Keywords: Magnesium; Bulk metallic glass; Amorphous; Biomedical applications.

1.Introduction

In recent years, a large number of studies reported on the development of magnesium-based materials for a wide spectrum of engineering and biomedical applications [1,2].In this regard, bulk metallic glasses are a relative novel class of materials.They are commonly based on Zn,Au,Mg,or Fe-based multicomponent systems.These bulk metallic glasses are different from other metallic alloys in the sense that they exist in the amorphous form and exhibit excellent yield strength[3].Besides, they can be processed above the glass transition temperature through the process of thermoplastic forming [4].Although bulk metallic glasses are a new class of materials, they have great potential in biomedical sector as they offer a great combination of properties such as yield strength, ductility, toughness, and ease of fabrication [5,6].As compared to other materials, magnesium is exceptionally light in weight with a density of around 1.74 g/cm3and has an elastic modulus that is closest to bone - making it suitable for biomedical implant applications [7].In addition to the aforementioned properties, the development of the Mgbased bulk metallic glasses (BMGs) is of great economic interest as magnesium is abundantly available in land and seawater [8].

The firs research on Mg-based BMGs was conducted by Inoue et al., in which they developed Mg-Cu-Y system with amorphous characteristics with excellent glass-forming ability [9].Inoue showed that the glass-forming ability of the Mg-bulk metallic alloy can be tailored by the substitution or addition of the external elements in the Mg-based bulk glasses matrix [9].

In recent years, Mg-based BMGs have attracted the attention of researchers as a promising candidate for biodegradable implants, not only because of their efficien mechanical properties but also due to their excellent biocompatibility and degradation characteristics [10].This leads to the intensifi cation of the research around the development of Mg-based BMGs using non-toxic alloying elements for the fabrication of different implants such as stents, pins, and screws.Magnesium based metallic glasses reported in recent years exhibited more enhanced mechanical properties and improved corrosion resistance both in thevitroandvivocondition [11,12]

Though magnesium is a very interesting candidate for biomedical applications with many advantages, there is one major limitation associated with the use of crystalline Mg in the biomedical application,which is its rapid degradation.The corrosion resistance of magnesium is very poor which causes the degradation of the Mg-based implants before the complete healing of the bone [10].Ramya et al.[13]in their research work stated that along with the problem of rapid degradation in the Mg-based implants, another major issue is the hydrogen evolution and solution alkalinization which is the major cause of tissue necrosis.Scientists are therefore inclining on the development of Mg-based Bulk metallic glasses due to their capability to display an excellent combination of biological, mechanical and corrosion properties when compared to the crystalline metallic Mg-based implants [6,11].

The use of Mg-based metallic glasses is not just limited to the biomedical field owing to the low-density feature, it can also be used as a lightweight component in defense,aerospace and automotive industries as well[14].Yet,one thing that limits its extensive use in these industries is that Mg-based BMG glasses are very brittle and would most probably fracture into pieces before the yielding point [14,15].Pei et al.discussed the ways to improve the mechanical properties of the Mgbased BMG for different structural applications, along with the great improvement in their corrosion resistance [16].One of the approaches discussed [16]was to apply the Zr-based thin metallic fil coating which results in the enhancement of the natural weak point of the Mg-based BMG, thus resulting in improvement of mechanical and corrosion properties.The use of coating or development of composite structure proved to be an effective approach for the improvement of properties of Mg-based BMG for different structural applications.

The most conventional approach for the development of bulk metallic glasses is through the rapid cooling technique.Examples of methods involved in rapid cooling are liquid quenching methods such as injection casting, tilt casting, and melt spinning [17].The working principle of these processes is that rapid quenching causes the freezing of atoms and does not allow rearrangement and diffusion of an atom.Thus, materials can maintain the glassy structure because of no order or short range order as rapid quenching freezes the liquid like atomic arrangement [18].Cooling rate and degree of crystallization have an important role in dictating the microstructure of the Mg-based BMGs.Wong et al.[19]mentioned that Mg-based biodegradable BMGs have unstable microstructure occasionally because of the combination of crystal nuclei and amorphous matrix which may cause high degradation rate.To address these challenges of unstable structure, Song [20]developed core-shell structure with crystalline core and amorphous shell.Through detailed analysis,it was found that coreshell structures have a low degradation rate and can maintain the amorphous structure even after the six weeks of the process of degradation.Thus, it can be said that the development of the core-shell structure is one excellent approach proposed by the researchers to overcome the effects of cooling rate during the synthesis of Mg-based bulk BMG and it provides a new way to enhance the characteristics of Mg-based BMG for biomedical applications.

2.Structure of Mg-based metallic glasses

Bulk metallic glasses attracted attention of researchers worldwide since after their discovery over fift years back and continue to do so till present time.As highlighted earlier,they are of utmost importance because of their exceptional chemical, mechanical and physical properties.Bulk metallic glasses are amorphous in structure which means that they do not follow any regular pattern, like crystalline metallic alloys [21].Their amorphous structure can be realized from a wide range of glassy compositions because of the absence of specifi stoichiometries.This is also favourable for the microscopic tuning of properties under a certain range through the optimization of the glass transition composition [22].

Gulenko [23]in the reported work, investigated the atomic structure of fi e different types of Mg-based bulk metallic glasses by combining two different methodologies which are neutron diffraction and classical molecular simulation.They suggested that bond length does not change so significantl with the change in composition.Similarly, Babilas et al.[24]also investigated the structure of the multicomponent Mg-based metallic glasses by combining different characterization methods such as high-resolution transmission electron microscope, neutron diffraction, and reverse Monte Carlo modeling.In the research, the bulk metallic glass under consideration was Mg65Cu20Y10Ni5.The calculated model showed random packing of the structure with the indication of some ordered regions.The researcher also described the amorphous structure using the peak values and coordination numbers and correlation functions, which shows some sort of clustering in the structure [24].The main aim of the experiment was to describe the effect of annealing on the structure from both experimental and modeling results of Mgbased BMG i.e., Mg65Cu25Y10.Fig.1 shows the structure of Mg-based metallic glass (Mg65Cu25Y10) obtained through the Monte Carlo modeling in a research work conducted by Babilas et al.[24]

Christie [25]in his work claimed that he had used the accurate method of molecular dynamic simulation to investigate the structure of two very important Mg-based BMGs used in biomedical applications i.e., Mg72Zn23Ca5and Mg60Zn35Ca5.Both materials exhibited different characteristics when used in implant applications, but through simulation-based structural analysis.It was found that thereis no significan difference in the structure of both Mg-based BMGs.The presence of icosahedral structural motifs is common in the bulk metallic glasses of the same composition which means that their presence is directly related to the increase in the glass-forming ability of the bulk metallic glasses [26,27].

Fig.1.Schematic illustration of the Mg-based BMGs (Mg65Cu20Y10Ni5)developed using the reverse Monte Carlo modeling,(a)shows simulation box,(b) elemental distribution (c,d) SRO region formation because of clustering[24].

There are a lot of recent research done which focus on the atomic structure model simulation of the metallic glasses and the use of a different combination of methods for the atomic structure analysis of the Mg-based bulk material glasses.However, initial research was mainly based on the computation study using the concept of tight-binding interatomic potential which proved to be the most common way adopted for the microstructure analysis of the metallic glasses[28].To investigate the compositional dependency of the bulk metallic glasses, there is still a need to investigate the bigger picture and to work on larger models for the atomic structure of the bulk metallic glasses.

One of the common issues highlighted earlier is that ductility is one of the major limitations of Mg-based bulk metallic glasses [29].Many works have been reported to improve the ductility of the Mg-based bulk metallic glasses through the enhancement in the plastic deformation ability of the bulk metallic glasses [30-32].One major approach adopted by most of the researchers is to reinforce the bulk metallic amorphous phase with a crystalline phase, but very limited research is done where the crystalline phase is reinforced with the non-crystalline/ amorphous phase.In 2020,Liu et al.developed very fine-graine Mg-based bulk metallic glass with the quasi-amorphous phase [33].They used the process of twin-roll casting for the development of this novel Mg-based bulk metallic glass and later investigated it using experimental and simulation data.Fig.2 shows the microstructure of the newly developed quasi phased Mgbased BMGs, where the as-cast strip (Fig.2a) shows fin dendrites and fin equiaxed grains in the microstructure.Fig.2b shows that the microstructure of these newly developed BMGs has a quasi-amorphous structure.The results also show that the novel paddle-like quasi amorphous structure(Fig.2c) results in the improvement of the tensile properties and this can be highly beneficia for downstream processing applications.

Fig.2.(a) Microstructure that look like small puddles (b) Microstructure showing quasi amorphous phases(c)schematic of the puddle like microstructure [33].

Guo et al.in their research work attempted to address the brittleness of the Mg-based bulk metallic glasses by introducing NiTi shape memory alloy particles [34].The resulting materials showed improvement in both mechanical and corrosion properties.Results showed that with the introduction of NiTi particles, there is a remarkable improvement in the compressive strength.The corrosion behavior was assessed in Hank's solution, and it depicts enhancement of anti-corrosive properties as compared to its monolithic glassy counterpart[34].Thus, we can say that microstructure tailoring is a great way to optimize the properties of Mg-based bulk metallic glasses.

Though metallic glasses are great candidates for structural applications, one of the limitations suppressing their extensive use in a structural application is that they display poor fatigue properties [35].Wang et al.proposed a solution to improve the fatigue properties of the bulk metallic glass materials by tailoring the microstructure of the material through suppressing shear band formation [36].Though their work does not specificall address the Mg-based bulk metallic glasses,it proved to be the new way to expand the use of Mg-based bulk metallic glasses in structural applications for future work by improving fatigue properties.

3.Development of Mg-based BMGs

Various studies were carried out and concluded that BMGs exhibit the three most important forming criteria which are[37]:

(1) Requiring a multi-component alloy with three or more elements.

(2) Exhibit mismatch of atomic size ratio greater than 12%among major constituents.

(3) Having a negative heat of mixing between major constituents.

These criteria are widely accepted as general guides for the development and alloying of BMGs.It is also well-established that the width of the supercooled liquid region,ΔT, that is the difference between the glass transition temperature Tgand crystallization temperature Tx,governs the glass forming ability of a particular alloy system.A largerΔTprovides a larger temperature range for processing in the amorphous state without the presence of crystallization.Described by Zhang et al.,the appearance of a wide supercooled liquid region can be attributed to the increase in amorphous-crystalline interfacial energy [38].For magnesium, this increase in energy, which was discussed by Inoue, is attributed to the formation of a densely packed atomic structure consisting of three or more elements with a relatively large negative heat of mixing and significantl different atomic size [38,39].

Apart from having a large negative heat of mixing and significan difference in atomic size, it has been proposed that an empirical criterion based on Hume-Rothery mechanism plays a part in identifying alloys that have the potential to exist in the metastable amorphous state.In this empirical criterion, Wang et al.discussed that with the use of electrons/atom-constant (e/a-constant) rule in the phase diagram in a given alloy system, the predicted composition with largest glass forming ability lie on this e/a-constant line [40].

The width of the supercooled liquid region is also reported to be closely correlated to bond parameters, specificall its atomic size and electronegativity for Mg-based bulk metallic glasses.Fang et al.[41]made an interesting observation that there is a clear trend which demonstrates the direct proportionate relationship in width of the supercooled liquid region with electronegativity and atomic size parameters as shown in Figs.3 and 4.By combining both parameters, the width of the supercooled liquid region and consequently the forming ability of a Mg-based bulk metallic glass can then be predicted for tertiary Mg-based alloy systems.

Using a linear regression analysis, the ability for any Mgbased alloys to form bulk metallic glass can be modeled by[41]:

113.35386(Δx)+264.37921(δ)＞13.5224

Fig.3.Relationship between electronegativity difference on the width of supercooled liquid region for Mg-based bulk metallic glasses [41].

Fig.4.Relationship between atomic size difference on the width of supercooled liquid region of Mg-based bulk metallic glasses [41].

Fig.5.A comparison between calculated and theoretical width of supercooled liquid region of Mg-based bulk metallic glasses [41].

Where (Δx) represents the electronegativity difference while (δ) refers to the atomic size parameters.Their deduced equation was found to follow a similar trend with experimental values as shown in Fig.5.Because of the simplicity in calculating both bond parameters, Fang et al.recommended that the prediction can also be used for Mg-based quaternary as well as quinary alloy systems.However, no experimental data is available to support the claim.In addition, the atomic radius used in calculating bond parameters differ slightly from different alloy systems due to variation in coordination number and bond type.As such, predicting the width of the supercooled liquid region may not be accurate.

4.Synthesis of Mg-based BMGs

The synthesis of bulk metallic glasses is a very difficul and demanding task as in the case of metallic glasses, suppression of the crystalline phase to form amorphous phase demands very high cooling rates.The conventional manufacturing routes are very time-consuming and need a lot of resources to complete the fabrication process.To bypass the difficultie in the fabrication of bulk metallic glasses, different novel fabrication techniques have been reported recently with great throughput nature, alongside the ability to produce the single-component system with the optimized size of bulk metallic glasses.Table 1 shows some of the novel fabrication techniques for the development of bulk metallic glasses reported in the literature.

Table 1Novel synthesis routes for developments of Mg-based BMGs.

Gu et al.[57]synthesized Mg-Zn-Ca-based bulk metallic glasses with the combination of three metallic elements in one bulk metallic alloy and later, the same alloy with the critical diameter of 5 mm was fabricated using the process of conventional casting [58,59].Wang et al.researched on the synthesis of Mg-based bulk metallic glasses, where he fabricated the Fe reinforced Mg-based bulk metallic glasses composite material by using the industrial raw materials through the process of injection moulding [55].It has been reported that fabrication of this Mg-based bulk metallic glass composite resulted in significan improvement in corrosion resistance as compared to the pure Mg [60-62].

Considering the degradation capacity of the Mg-based bulk metallic glasses, Wang et al.reported that Mg73Zn21.5Ca5.5metallic glass formed by the process of ball milling was more effective in degrading Azo dyes when compared to the commercially available Fe based powders [63].These results demonstrated superior catalytic and chemical properties of Mg-based bulk metallic glasses.In 2021, Shamlaye published work where he developed Mg-Ag-Y and Mg-Ag-Y-Cu metallic glasses [64].The process used for the fabrication of the alloy was tilt casting in which the alloy with high purity metal content was tilt cast into a wedge-shaped mould made of Cu where the temperature is kept in the range of 100-150 °C[64].Significan improvement in the properties of Mg-based bulk metallic glasses was observed with the incorporation of silver content, as compared to the previously reported same class of materials [64].

In the last few decades,a lot of work have been reported to enhance the glass-forming ability of the Mg-based bulk metallic glasses by tailoring the synthesis, fabrication, and processing techniques.Most of the reported techniques to enhance the glass-forming ability of BMGs are through the synthesis of multi-component alloy based on the Mg-Cu-X-X1system[65-68].Ma et al.successfully synthesized Mg-based bulk metallic glass of 25 mm diameter using the copper mould casting technique [61].

There are some other types of synthesis techniques reported in the literature as well for the development of Mgbased bulk metallic glasses.Some traditional methods are induction heating, copper mould casting, high pressure diecasting, suction casting, and melt spinning techniques.Shanthi et al.successfully synthesized Mg67Zn28Ca5bulk metallic glass reinforced with 0.66-1.5 vol% nano-alumina particles using the disintegrated melt glass deposition method [69].They reported reasonable distribution of nano alumina powder within the Mg-based BMGs which have an excellent combination of strength, ductility, and hardness.They also related the presence and uniform distribution of nano alumina powder within the Mg-based BMGs to the enhancement in the ductility [47].

Though multicomponent bulk metallic glasses are of great importance in terms of their applications in different fields the development of such a system is very resource-consuming and time-consuming.Normally, the composition design of the bulk metallic glasses is based on hit and trial methods which are tedious and take a long time.Ding et al.proposed the high throughput method for the simultaneous development of Mg-Cu-Y metallic glass [47].The research characterized the alloy metallic glass system developed by parallel blow forming for thermoplastic formability.By doing so, it opened a novel pathway to future researchers and scientists to use the same strategy for the development of multi-componentMg-based bulk metallic glasses.The researchers believe that high thermal formality can be achieved in the multicomponent glass-forming Mg-Cu-Y alloy system [47].

Fig..6.The SEM images showing the BMGs morphology during the process of milling [72].

Although copper mould casting is an effective method for the development of BMGs, the main limitation associated with this technology is that close composition control is difficul and size of the cast product is limited to a few millimetres.Additionally, rapid solidificatio techniques also need very complex experimental conditions and are not so favourable for Mg and Zn because of their strong reactive nature with the oxygen.In literature, different works have been reported about the development of the Mg-Y-Cu [70,71]and Mg-Ni-Si [65]systems through the process of mechanical alloying and using the consolidation techniques.Recently,Manne et al.developed Mg-based bulk metallic glass, using the process of solid-state amorphization through ball milling[72].The results revealed that the use of this process results in an effective removal of crystalline phase because of the prolonged milling of powders which causes high throughput of amorphous phase in Mg-based bulk metallic glass system[72].Fig.6 shows the SEM micrographs of the particles during the milling process.

While traditional methods such as metal spinning and copper mould casting are quite effective in bulk metallic glasses synthesis, the small dimension of end products sometimes restricts its application for orthopaedic implants [73].To address this, Zhang et al.fabricated the Mg65Zn30Ca5based crystalline/amorphous layer composite materials using laser remelting process [74].The results showed that with the use of laser remelting technique, the obtained Mg-based bulk metallic glasses exhibited good corrosion resistance properties.Subsequently,Zhang et al.also conducted microstructural analysis of Mg-based bulk metallic glasses and used finit elemental model and simulation to investigate microstructural evolution [75].They suggested that laser remelting can be very effective in dimensional control and yield better throughput when compared to traditional moulding techniques.

Currently, there are reported researches about the use of additive manufacturing for the development of bulk metallic glasses[76].While this technique is still in the research phase and there is not much recent studies about the development of Mg-based bulk metallic using additive manufacturing, it provides an alternative pathway for the future research for the fabrication of Mg-based bulk metallic glasses [77,78].

Because of the inherited corrosion behavior of Mg alloys,it is essential to protect them from the corrosive environment.Therefore, after the development of the Mg-based bulk metallic glasses, their surface treatment is also very important, and it is widely addressed in different research works.The most preferred methods for the surface treatment of conventional Mg-based alloys are micro-arc surface treatment [79,80]and anodic surface treatment [81-83], which are based on the application of a protective layer on the surface of Mg alloys.The main disadvantage of these processes is the involvement of alkaline solution which entails the concern of pollution.Hence, the dry processes for the corrosion protection of the Mg-based alloys, such as the sputter coating technique which is considered as a green and environmentally friendly surface treatment process for the Mg-based alloys, are still mostly preferred [84].Pei-Hua et al.in their work uses the sputtering technology to deposit the layer of ((Zr53Cu30Ni9Al8)99.5Si0.5)on the surface of Mg-based bulk metallic glasses to enhance their mechanical and corrosion properties to make it more favourable to be used in structural and non-structural applications.

Wang observed the local surface crystallization in the Mgbased bulk metallic glasses during their synthesis through the copper surface moulding technique[85].The researchers were of the view that there exists a crystalline phase on the ascast surface of Mg bulk metallic glasses fabricated through the copper moulding technique, but there is no indication of these crystalline phases in the internal structure of the Mg-based BMG.The main reason for this abnormal behavior was attributed to the overheating phenomena and copper mould which induced nucleation on the surface[85].Liu et al.opted for a novel approach of the twin-roll casting for the development of Mg-based metallic glass displaying a quasiamorphous structure [33].The researchers optimized the processing parameters with the microstructural evolution for the newly developed Mg-based BMGs.It was observed that with an increase in the thickness of the strip, there is a decrease in the critical casting speed and the use of the twin-roll casting approach resulted in the paddle-like structure of the fina Mg-based BMGs with the quasi-amorphous phases [33].

A critical review of the literature revealed that just like other materials,the properties and structure of the bulk metallic glasses are highly dependent on the synthesis techniques adopted to fabricate these materials.Lee et al.in 2021, reported thein-situdevelopment of Mg-based BMGs which is composite with theα-Mg primary phases [86].The researchers showed that as compared to the monolithic Mgbased BMGs, their proposed composite materials displayed improved properties in terms of plastic strain, serrated plastic fl w and yielding.Thus, we can say that most recent research focus on the development of Mg-based bulk metallic glasses with multicomponent phases rather than monolithicbulk metallic glasses.For the use of Mg-based BMGs in structural applications, it is always preferred to use the particulate reinforced Mg-based BMGs composite.Li et al.synthesized the bulk metallic Mg glass reinforced with SiC (10-20μm)[87].This combination resulted in the formation of composite materials with no change in glass-forming ability and thermal stability of the parent bulk Mg glass, but it offers great improvement in the compressive fracture toughness [87].The work conducted so far indicated that apart from the effect of synthesis and fabrication techniques on the fina properties of Mg bases bulk metallic glasses, they also have a significan effect on the fina diameter.All the reported research showed that processing method,materials selection,material composition, mould and casting conditions have a great impact on the size of the fina product i.e., Mg-based bulk metallic glass.Table 2 summarizes some of the fabrication techniques for the development of Mg-based bulk metallic glasses for both biomedical and non-biomedical applications.It must be noted that size constraint is very critical in the case of biomedical applications.For the biomedical application, other than possessing good mechanical properties of Mg-based metallic glasses, it is also important that they should exhibit good degradation properties to prevent early failure of the implant.

Table 2Summary of reported synthesis method of Mg-based BMGs.

5.Properties/characteristics of Mg-based BMGs

Mg-based metallic glasses are excellent candidates for different applications because they are light in weight and can be used for weight-critical components for different applications such as in biomedical, aerospace, automotive, and 3C industries.In the fiel of biomedical, Mg-based bulk metallic glasses proved to be efficien candidates because they offer a good combination of properties such as strength, ductility,toughness, corrosion resistance and ease of fabrication using the novel current fabrication techniques as discussed earlier.

5.1.Mechanical properties of mg-based bulk metallic glasses

Fig.7.Comparison of Young's Modulus and Tensile strength of crystalline and amorphous materials [96].

As amorphous materials do not have an ordered arrangement in the structure, most of them lack the periodicity which is common in most crystalline materials.Therefore,amorphous materials often offer an interesting combination of properties which include high strength, elasticity, toughness and fatigue properties [92,93].Although there are many amorphous materials reported,Mg-based bulk metallic materials have gained significan attention because of their attractive strength to weight ratio along with their processing capability at low glass transition temperature [94,95].Fig.4 illustrates the comparison of mechanical properties of crystalline and amorphous metallic materials including magnesium.

Fig.7 reveals that there is a huge improvement in mechanical properties of Mg-based bulk metallic glasses as compared to the crystalline Mg alloys [96].The investigation of the mechanical properties of Mg-based bulk metallic glasses highlighted three important observations as follows:

(1) With the Mg-based BMGs, the improvement in the tensile strength is almost three times more than the crystalline Mg alloy.

(2) With the same tensile strength, Young's modulus of the Mg-based BMGs is almost three times more than the Mg alloy.

Fig.8.Stress-strain curve showing the change in comprehensive properties of Mg-based BMGs with the addition of Zn [101].

(3) There exists a linear relationship between the Young's modulus and tensile strength of the bulk metallic glass like the metallic alloys.

Liu et al.developed Mg-Li-based BMGs using the process of copper mould injection casting to investigate the change in mechanical properties as compared to the conventional Mg-Li alloys [97].The results showed the newly synthesized alloy possessed excellent mechanical properties such as Vickers hardness which was reported to be＞2 GPa, with high compressive strength of about 729 MPa.Overall, the researchers noted that there was a great improvement in mechanical properties of the amorphous Mg-Li BMGs when compared to its crystalline alloy [97].

In literature, it has been reported that the most favourable way to enhance the mechanical properties of the bulk metallic glasses is throughex-situapproach by introducing various types of second phase particles such as WC [70], TiB2[98],SiC [87]and Ti [99]in the matrix of bulk metallic glasses.Hsieh et al.in their research evaluated the mechanical properties of Mg58Cu29.5Y6Nd5Ag1.5bulk metallic glass and found this alloy to be brittle [100].To address this issue, they added 20-30% volume fraction of Mo powder in the base material to form the composite of Mg-Mo-based bulk metallic glass.The results showed that the mechanical properties of Mgbased BMGs improved with the addition of Mo.The fracture strength was reported to be 1124 MPa along with 29%plastic strain [100].They reasoned that the presence of Mo particles in the base material hinders the propagation of shear bands which resulted in the formation of multiple shear bands within the system and acts as a strengthening mechanism for the improvement of mechanical properties [100].

Liu et al.investigated the influenc of the addition of Zn on the mechanical properties and glass formability of the Mg65-xZnxCu25Gd10BMGs [101].The results are presented in Fig.8 [101].

Fig.8 shows that with the addition of Zn of 3- 5% in the base materials, the compressive fracture strength improved from 648 MPa to 698 MPa, but when the Zn content is further increased beyond 8% it results in the decrease in compressive fracture strength which is because of the formation of crystalline phases during the fast cooling in alloy metals[101].Thus, results revealed that optimum composition modificatio is very important to achieve the required increase in mechanical properties.

Soubeyroux et al.proposed the idea of the development of bulk metallic glass nanocomposite to enhance mechanical properties [102].The researchers added 3% of Fe nanocrystal into the Mg65Cu25Gd10 bulk metallic glasses.The resulting composite materials were investigated through different characterization techniques and the results show favourable increase in the fracture stress with the addition of Fe nanocrystal.No signs of partial crystallization was reported in the Mg-based BMG under consideration [102].In 2021, Shamlaye et al.designed a novel Mg-Ag- Y-Cu and Mg-Ag- Y BMGs system with extended ductility as compared to other Mg-based BMGs [64].Fig.9a shows the stress-strain curves of the newly developed bulk metallic glasses whereas Fig.9b shows the fractograph of the least ductile Mg-based BMGs and Fig.6c shows the fractograph of most ductile Mg-based BMG.The results show that minor addition of the Cu in the BMG results in the improvement of the ductility.The researchers indicated this as a pathway for the further investigation of the relationship between the positive mixing heat and electronic bonding for the future design of Mg-based bulk metallic glasses with improved ductility [64].

Although Mg-based BMGs offers good glass-forming ability and specifi strength as compared to the other bulk metallic glasses, its significantl lower ductile behavior remains amajor issue.Laws et al.in their study presented a range of new Mg-based bulk metallic glasses from the ternary alloys system with the magnesium content greater than 67% [103].The study was based on fabricating newly developed ductile Mg-based BMGs by utilizing the concepts of two major models: atomic packing mode and atomic bond-band theory.They concluded based on the theoretical analysis and observation that ductility can be significantl improved in the Mg-based BMGs if thefanddelectron interaction can be constrained in the alloys system through metallurgical or mechanical processing.Table 3 summarizes the mechanical properties of recently reported magnesium-based BMGs.

Table 3Mechanical properties of recently reported magnesium-based BMGs.

Fig.9.a) Compression curve of Mg65Cu25-xAgxY10 (uniaxial) with their quaternary counterparts, b) fractography of least ductile Mg65Cu25-xAgxY10 BMG and c) fractography of Mg65Cu25-xAgxY10 BMG [64].

5.2.Corrosion behavior of Mg-based bulk metallic glasses

With growing interest in the application of Mg-based bulk metallic glasses in the field of engineering and biomedical,degradation or corrosion rate remains an important factor for material's selection.Generally, Mg-based alloys exhibit poor corrosion resistance.As compared to crystalline Mg-based alloys, Mg-based BMGs have gained great attention because of their corrosion-resistant properties.Conventionally, Mg-based alloys are made corrosion resistant by the application of a protective layer on the surface of the material [62,104,105].Gebert investigated the corrosion behavior of the Mg-based bulk metallic glasses and showed that new Mg-based BMGs offer low corrosion reactivity and increase passivity as compared to the crystalline BMGs [62].While there is an improvement in corrosion resistance, the authors also mentioned that in the case of Mg-based BMGs, filifor and pitting corrosion can be major problems due to the amorphous structure.

Babilas et al.studied the corrosion properties of Mg-based BMGs in NaCl solution [106].The electrochemical and immersion results showed that the corrosion rate of Mg-based metallic glasses is lower than that of pure magnesium in the crystalline phase.The reason for the decrease in the corrosion reactivity was attributed to the glassy microstructure, where the single phase structure of the amorphous region resulted in higher polarisation measurements, indicating lower corrosion current density [106].Babilas et al.proposed to enhance the corrosion properties by opting for surface modificatio techniques such as a biocompatible coating as illustrated by Chen[106,107].In their paper, Chen et al.showed that a simple strontium phosphate coating on magnesium not only maintains its integrity thereby significantl reducing corrosion butit is also biocompatible and permit cell proliferation to a level similar to that of pure magnesium [107].

Fig.10.Comparative analysis of Mg-based BMGs (e Mg-Zn-Ca-Sr) with other reported BMGs and crystalline Mg-based BMGs used for biomedical applications [108].

Li et al.also showed that Mg-based alloys with suitable corrosion and mechanical properties can be used for biodegradable applications, most preferably for implant applications [108].Their study revolved around the development of two important Mg-based bulk metallic glasses i.e.Mg-Zn-Ca and Mg-Zn-Ca-Sr.Comparing to Mg-Zn-Ca, Mg-Zn-Ca-Sr has improved corrosion and mechanical properties with the enhancement in the glass formability because of the incorporation of Sr in minor amounts.Fig.10 illustrates the comparison of Mg-based BMGs with the Mg-based crystalline alloys and other reported BMGs for biodegradable applications [108].

Various researchers have used the concept of coating methodologies to reduce the hydrogen evolution in the bioresorbable Mg-based implants.Their results indicated that coatings help to maintain the mechanical properties by controlling the onset of degradation.David et al.proposed the model for the development of phosphate conversion coating on the Mgbased metallic glasses along with a detailed investigation of the effect of coating on the corrosion properties of the Mgbased BMGs [107].Their results suggested that corrosion behavior of the ternary Mg-Zn-Ca BMGs can be improved by order of magnitudes if the anodic kinetics is significantl reduced in the simulated flui body [107].

Haifei et al.investigated the evolution of Mg-based bulk metallic glasses for biodegradable application,considering the strategies to improve its corrosion and degradation properties[109].Haifei et al.conductedin vitroinvestigation of Mgbased bulk metallic glass for biomedical application, where the material composition used was Mg-Zn-Ca-Ag.The researchers used the process of copper mould casting for the synthesis of Mg-based BMGs with the addition of Ag.The researchers were of the view that the addition of the Ag in the Mg-Zn-Ca decreases the glass-forming ability of the metallic glass but does not have much effect on the microhardness of metallic glass [109].On the other hand, it contributes to the suppression of the hydrogen evolution and improvement in the corrosion resistance of the material to great extent.Thus, Mg-Zn-Ca-Ag proved to be a great option for future biodegradable applications.

In a separate work by Li et al., the corrosion fatigue characteristics of Mg66Zn30Ca3Sr1bulk metallic glass was studied[110].The work was based on the observations collected from a simulated physiological environment and in the air and they established that the corrosion in the material was responsible for the inferior fatigue resistance [110].

Hasannaeimi et al.reviewed the corrosion and electrochemical behavior of different types of bulk metallic glasses[111].Discussing about Mg-based BMGs, the researchers were of the view that Zn enriched Magnesium-based BMGs have better corrosion properties, such that an increasing Zn content results in better corrosion properties.While Zn enhances the corrosion resistance of Mg-based BMGs, it also results in the reduction of the glass-forming ability of magnesium.Fig.11 shows a schematic illustration of the Pourbaix diagram, indicating corrosion, passivation, and immunity zones for Mg66Zn30Ca4metallic glass where the vertical axis represents potential, V and the horizontal axis represents pH of the solution [111].

Thus, from the detailed review of the corrosion properties of Mg-based BMGs, it is quite clear that this area of research has received great attention in recent years.The importance of corrosion of Mg-based bulk metallic glasses is immense especially if it serves as an orthopaedic implant, as that will defin its suitability as an implant material.While the corrosion properties of Mg-based bulk metallic glasses are better than their crystalline counterpart, the degradation of metallic glasses can further be controlled by additional factors, which are discussed in forthcoming sections.

5.2.1.Effect of alloying element on corrosion response of Mg-based BMGs

Microalloying has a very important role in tailoring the corrosion resistance of Mg-based BMGs.The addition of Cr and Ti increases the corrosion resistance of the Mg-based BMGs yet at the same time, lowers the glass-forming ability of metallic glasses [112].In literature, there is much-reported work about the use of the microalloying concept to improve the mechanical and corrosion properties of Mg-based BMGs.Zhang and Chen demonstrated the concept of micro alloying to enhance the mechanical properties, thermal stability,and corrosion resistance of Mg65Cu20Y10Zn5[112].The researchers indicated that the superior corrosion resistance of the amorphous Mg BMGs sample is because of the formation of a high corrosion protective layer due to the formation of a passive oxide layer in the presence of Ti and Cr.

Babilas et al.added gold (Au) to Mg-Zn-Ca metallic glass using high pressure die casting [113].The corrosion resistance of the ternary Mg-Zn-Ca metallic glass was compared with the quaternary Mg-Zn-Ca-Au metallic glass by hydrogen evolution and electrochemical test,conducted both in Ringer's solution and simulated body flui (SBF)at 310 K after 1,3,5 and 7 h immersion.Surface morphology of the layers was observed using ZEISS SteREO Discovery V.12 light microscope.Corrosion activity was revealed to be lower with the additionof Au.This was evident both by significantl lower carbon and oxygen content (suggesting lesser formation of corrosion products of calcium hydroxide and calcium carbonate) on the surface of Mg-Zn-Ca-Au as compared to Mg-Zn-Ca metallic glasses, as well as lower corrosion current density,icorrvalue of 0.209 mA/cm2compared to 1.853 m/cm2[113].

Fig.11.Pourbaix diagram pH-Potential of Mg66Zn30Ca4 BMG [111].

The addition of rare earth elements has proven to be effective in increasing corrosion resistance.Yao et al.added Yttrium (Y) and Neodymium (Nd) to Mg-Cu and Mg-Ni metallic glasses, which was prepared via melt-spinning technique[114].The corrosion resistance of ternary Mg65Ni20Nd15and Mg65Cu25Y10bulk metallic glasses were compared with their respective binary Mg81Ni18and Mg79Cu21crystalline alloys via hydrogen evolution and potentiodynamic polarization tests in 0.01 M Na(OH) at pH 12.Their finding revealed signifi cant improvement in corrosion resistance, which was evident from the shifting of corrosion potentials to more positive values by approximately +1.0 V.Surface morphology suggested that the anodic formation of Nd and Y oxides in alkaline condition was thermodynamically favoured over Mg, Ni and Cu oxides.As a result, the presence of rare earth oxides resulted in the formation of a denser and more chemically inert passive fil which provided better protection against oxidation compared to the typical magnesium hydroxide layer formed in this electrolyte [114].

Chen et al.worked on a novel approach of using computational intelligence model for the development of new Mgbased glassy compositions [115].The model presented by them incorporated artificia intelligence and machine learning approach.By adding the input values of alloying composition,the model can predict the inherent properties of Mg-based BMGs.It also influence the use of minor alloying elements to enhance the glass-forming ability and other properties of the Mg-based BMGs.This model not only serves as a research option for the development of Mg-based BMGs but is also applicable to other types of metallic glass as well [115].

5.2.2.Effect of microstructure on corrosion of Mg-based BMGs

The relatively poor corrosion resistance of magnesium and its alloys stems from the high reactivity of magnesium as well as the existence of second phases and impurities which serve as localized cathodes, thereby causing galvanic corrosion [62].Numerous post-processing methods such as severe plastic deformation and ball milling have been utilized to improve corrosion resistance.These methods seek to produce fine and more homogenous microstructure by creating more nucleation sites for the formation of the adherent magnesium hydroxide fil (MgOH2).

Ball milling for instance, was studied by Grosjean et al.on magnesium to improve corrosion resistance of magnesium in alkaline solution [116].With high-energy ball milling, grain size was reduced to the nanoscale.In addition, high defect and grain boundary density was also present.As such, the number of Mg(OH)2nucleation sites increased, leading to the formation of a denser Mg(OH)2fil formed on the surface,thereby retarding the corrosion process.

High-pressure-torsion treatment as a severe plastic deformation technique on Mg-1Ca was also studied by Parfenov et al.and it led to better corrosion resistance (disintegration time of 30 days compared to 4 days) when immersed in Ringer's solution [117].The degradation mechanism discussed was based on the balance between anodic dissolution of Ca-containing secondary phase and the precipitation of hydroxide compounds that contributes toward the retardation of corrosion [117].This in turn is dependent on the size and distribution of the Ca-containing secondary phase.Similar results were also observed in ZK60 magnesium alloy processed by two-step multi axial isothermal forging (MIF) [118]with a corrosion rate of 4.6 ± 0.9 mm/year for ZK60 (cast) compared to 2.6 ± 0.5 mm/year in ZK60 MIF.However, there are opposing arguments that suggest that fine- ain sizes tend to have higher corrosion rate due to presence of localizedcorrosion.For example, Bin et al.discussed the effect of severe plastic deformation of AZ31 magnesium alloy in Hanks'solution-a solution that simulates body fluid Despite achieving grain sizes in nanoscale, high defect and grain boundary density, corrosion resistance was poorer for the as-processed conditions as shown in Fig.12 [119].This was a result from the attack on the adherent Mg(OH)2layer by chloride ions in the Hanks' solution, exposing a fresh substrate beneath the oxide layer thus, accelerating the corrosion rate [119].

Fig.12.Comparison of corrosion rate of AZ31 between as-received and asprocessed conditions [119].

Metallic glasses, on the other hand, have proven to show significantl better corrosion resistance compared to their crystalline counterparts.The lack of grain boundaries and dislocations inhibit galvanic couples which prevent intergranular corrosion [120].In addition, the lack of structural defects can retard ion diffusion.Many studies have indicated that metallic glasses have higher corrosion resistance in comparison to their crystalline counterparts [62,108,121-123].

The corrosion mechanism of Mg-Zn-Ca metallic glass differs from its crystalline parts.Uniform corrosion was observed to minimize galvanic corrosion and this was attributed to the absence of second phase in Mg-Zn-Ca metallic glass as reported by Gu et al.[124].In their proposed corrosion mechanism of Mg-Zn-Ca BMGs, Gu et al.discussed that the corrosion of Mg-based BMGs undergoes a three-step process[124]:

(1) The anodic dissolution of Mg and formation of Mg(OH)2on the surface of the substrate which is susceptible to attack by aggressive chloride ions resulting in the formation of soluble MgCl2precipitate and OH-ions, exposing the surface to further attack by the aggressive medium.

Mg(OH)2(s)+ Cl-(aq)→MgCl2(aq)+ OH-(aq)

(2) Further attack on the exposed surface led to the dissolution of Mg2+and Zn2+ions.The formation of Zn(OH)2is preferred over Mg(OH)2due to the lower solubility product of Zn(OH)2, which provides better surface protection compared to Mg(OH)2.Zn(OH)2also aids to facilitate the repair of surface defects due to the dissolution of Mg(OH)2.

(3) As corrosion proceeds, Zn(OH)2layer spreads along the surface evenly, providing favourable sites for apatite nucleation which in turn aids in corrosion resistance.

The difference between corrosion resistance of a Mg-based metallic glass and their crystalline counterpart was also studied by Zhang et al.[125]In their study, the corrosion resistance of ternary Mg65Cu20Y10Zn5BMG was compared with AZ31 using potentiodynamic polarization tests in Hanks' solutions at 310 K while the surface morphology was characterized using scanning electron microscopy.Significan improvement in corrosion resistance was observed.This was evident from a shift in corrosion potential to more positive values and significantl lower corrosion current density, with anicorrvalue of 0.017 (mA/cm2) compared to the crystalline AZ31 with anicorrvalue 1.326 (mA/cm2) [125].Their find ings also revealed that pits with large size were distributed non-uniformly across the surface of AZ31, while pits were absent on the surface of Mg65Cu20Y10Zn5BMG - evidently displaying uniform corrosion.This improvement in corrosion resistance for Mg-based metallic glasses was also confirme by one of the earlier works on Mg-based bulk metallic glasses carried out by Nowosielski et al., showing better corrosion resistance with Mg65Zn30Ca5metallic glasses as compared with Mg64Zn32Ca4alloy [126].

Bulk metallic glasses exhibit different corrosion characteristics at different pH levels.Gebert et al.discussed the effect of pH on the dissolution rate of various Mg-based BMGs[127].Their finding revealed that by decreasing pH levels, the corrosion reactivity of Mg65Cu7.5Ni7.5Ag5Zn5Gd5Y5in borate buffer solution increases.This was evident by a shift of the corrosion potential, Ecorr,to more negative values and an increase in corrosion current density,icorr[128].Mg and Mg-based alloys are very stable at pH 13 with very low current densities of ≤1μA/cm2[128]due to their spontaneous passivation ability in highly alkaline environment.Despite an increase in corrosion activity at lower pH levels, passivation still exists in Mg65Cu7.5Ni7.5Ag5Zn5Gd5Y5up to pH 7, suggesting the existence of some form of anodic protection.On the other hand, pure Mg and AZ31 were found to be highly destabilizing at pH 8, with extremely high corrosion current density values reached, indicating a dominating dissolution process.In contrast, the increase in corrosion current density in Mg65Cu7.5Ni7.5Ag5Zn5Gd5Y5was less pronounced, in the range of 10-20μA/cm2at pH 7.For pH values＜6, corrosion current density continued to rise, leading to a tendency for dissolution.

A separate study conducted by Subba Rao et al.suggested that Mg65Y10Cu15Ag10displayed stable anodic surface layers up to pH 6 in borate buffer solution[129].The difference in passivation characteristics between Mg65Cu7.5Ni7.5Ag5Zn5Gd5Y5and Mg65Y10Cu15Ag10suggest that alloying elements play a significan role in determining passivity.As suggested, the absence of Zn inMg65Y10Cu15Ag10could possibly aid in passivation because Zn can be quite reactive in the presence of noble elements[128].

Certain Mg-based metallic glasses are also known to absorb hydrogen gas.As a result, the stability of the amorphous structure under cathodic polarization, dominated by hydrogen evolution, can be of importance in determining its effect on degradation.For example, melt-spun Mg65Cu25Y10 ribbons during cathodic polarization at room temperature in 0.1 M NaOH with current densities at -1 and -10 mA/cm2showed the ability to be electrochemically charged up to 3.7 wt.%hydrogen [130].Consequently, the increasing hydrogen absorption resulted in a change from a single-state amorphous state to a very fin nanocrystalline structure [130].The effect of hydrogen absorption on the corrosion rate of these alloys has not been studied.However, the de-stabilization of magnesium brought about by this change in state should be considered in future works.

5.3.Biocompatibility and biodegradability of Mg-based BMGs

The biocompatibility factor is important for Mg-based BMGs in biomedical applications.Not only the material used should be biocompatible, but its degradation products must also be non-toxic and biocompatible.These features are very important when the intention is to use the Mg-based BMGs for bioresorbable implants.In literature, various studies have reported the use of Ca and Zn as an alloying element in the development of Mg-based BMGs for biomedical applications because both elements are essential components of the human body [109,131,132].Dambatta et al.conducted a detailed review about the use of Mg-based BMGs for biodegradable application and investigated the potential of the materials in terms of their corrosion, mechanical and biodegradable properties [96].In their work, the researchers reported that Mg66Zn30Ca4and Mg70Zn25Ca5exhibited great cell viability as compared to the crystalline pure Mg material.Recently, Sun et al.investigated the behavior of Mg66Zn30Ca4by adding Sr element [133].Mg66Zn30Ca4is termed as a great biodegradable bulk metallic glass, and with the addition of Sr element, it appears to be an even more promising biodegradable material.Just like calcium, strontium (Sr) is also a great bone-seeking element, and it can stimulate protein synthesis and bone formation [133].Wong conducted the osteogenic capacity and biocompatibility study of Mg-Zn-Ca BMGs to be used for the Rabbit Bone Fixation [134].The researchers, through detailed analysis, found that Mg60Zn35Ca5bulk metallic glass offers an excellent biocompatibility required for implant applications and the materials also have great osteogenic potential to be used for bothin vitroandin vivoconditions [134].

6.Mg-based BMG composites

Fig.13.The schematic of the BMG composite preparation,proposed by Guo at al.[135].

In recent years, adding second phase particles in bulk metallic glasses has proved to be an efficien way to improve the properties of bulk metallic glasses, especially in terms of plasticity and corrosion resistance.In literature, two types of methods are reported for the preparation of BMG based composite - theex-situmethod and thein-situmethod.Most of the research work used composite development using theex-situprocess because of the difficult in dealing with thein-situmethods.Guo et al.addressed this issue by proposing a novel technique for the synthesis of Mg-based BMGs composite materials [135].Researchers used the process of selective phase leaching process for the introduction of thein-situNi-Ti-Nb reinforcing phase in the Mg-Ni-Gd-Ag amorphous BMG matrix [135].Fig.13 illustrates the methodology adopted by Guo et al.for thein-situsynthesis of Mgbased BMGs composites.The detailed investigation of the microstructure and mechanical properties shows that the addition of the reinforcing phase to develop a hybrid composite structure proved to be an effective approach in hindering the propagation of shear bands.This allows the formation of multiple shear bands which contributes to the work hardening of the materials that enable the enhancement in mechanical properties.

Similarly, Shao et al.developed Fe reinforced Mg-based BMGs composite material with the concept of dealloying the bulk melt [136].The addition of Fe in the form of reinforcement led to an increase in the fracture strength and plastic strain of the Mg-based BMGs composite material.The reported fracture toughness of 901 MPa and plastic strain of 5.6% was much improved as compared to the base material [136].The improvement in mechanical properties was attributed to the addition of Fe, inducing multiple initiations of the shear bands, which allows for strengthening within the materials because of the work hardening offered by reinforcement [136].

Song et al.established that Mg-based BMGs proved to be a promising candidate for orthopaedic implant fixatio because of their properties like low degradation rate, biocompatibility, and osteogenic potential [20].The researchers in this work developed a core-shell structure that exhibited lowdegradation rates because of the hybrid amorphous structure [20].Fig.14 illustrates the core-shell structure of Mg66Zn29Ca5 as synthesized by Song et al.It illustrates the degradation in the solid structure which is responsible for the high degradation rate.The newly developed core-shell structure offers to enhance corrosion resistance properties because of the unique composite structure.

Fig.14.Schematic illustration of the degradation of solid structure and core-shell Mg-based BMGs [20].

Fig.15.Effect of varying surface roughness on Mg66Zn29Ca5 BMGs for cell adhesion in implant application [139].

Recently, researchers in this fraternity have actively published on the development of Mg-based BMGs with composite phases rather than the fully amorphous phase.Wang et al.developed Mg-based BMGs composite, where SiC particles with size 10-20 μm used as the second phase were coated with Cu and then added in the base alloy composition,Mg54Cu26.5Ag8.5Gd11[137].The SiC particles were coated with the Cu using the process of electroless plating and later added to the base material in the amount of 5%.The results showed that with the addition of the coated reinforcement,there is a great improvement in the plastic deformation of Mg54Cu26.5Ag8.5Gd11bulk metallic glass [137].These results further validated that adding a secondary phase provides a novel technique for the optimization of mechanical properties and interface microstructure.Just like other field of materials, the development of a composite structure for the bulk metallic glasses is also an effective way to enhance properties.

Although Mg-based BMGs and Mg-based BMG composites are strong potential candidates in biomedical implant applications because of better corrosion resistance and biodegradability, there are some issues with cell adhesion,particularly in the earlier stages of implant implementation[138].This issue was addressed by Wong et al.in which they investigated the relationship between ontogenetic ability and surface roughness of Mg-based BMGs implants [139].Fig.15 shows the highlights of the relationship varying surface roughness demonstrated by them.The researchers believe that optimizing surface roughness can promote cell adhesion.By inducing gaps and crevices created by surface roughness,surface area for cell adhesion increases, allowing for more interaction between the cells and the implant.

7.Conclusion

This study presents a detailed discussion on the recently reported research on Mg-based BMGs.Mg-based BMGs can be seen as very promising candidates for current and future structural and non-structural applications in engineering and biomedical sectors.For biodegradable applications, they can be used as scaffold,bioresorbable implants and stents because they offer an excellent combination of mechanical properties,low degradation rate, and biocompatibility.For structural application, they can be used for automotive and aerospace industry as it offers high strength to weight ratio, improved mechanical properties, and corrosion resistance.The recent development of Mg-based BMGs is primarily focused through tailoring of microstructure and innovation in processing techniques.

Despite better resistance to degradation, more suitable mechanical properties and biocompatibility, the development of magnesium-based bulk metallic glasses for biomedical applications is still in the stage of infancy.Although initial degradation studies showed promising results compared to their crystalline counterparts, the mechanism behind theirin vivodegradation is not yet well understood.From the processing point of view, challenges remain in obtaining a bulk sample- that is a sample that is sufficientl large enough to be of use.

Magnesium-based bulk metallic glasses certainly are promising candidates for future biomedical applications.Breakthrough in processing methods to obtain a sufficientl large bulk metallic glass without compromising the amorphous structure should pave the way for accelerated research as a substitute for current biomaterials and in targeted engineering applications.